SWI-Prolog C-library
Jan Wielemaker
VU University of Amsterdam
The Netherlands
E-mail: J.Wielemaker@vu.nl
Abstract
This document describes commonly used foreign language extensions to SWI-Prolog distributed as a package known under the name clib. The package defines a number of Prolog libraries with accompagnying foreign libraries.

On Windows systems, the `library(unix)` library can only be used if the whole SWI-Prolog suite is compiled using Cygwin. The other libraries have been ported to native Windows.

# Table of Contents

## 1 Introduction

Many useful facilities offered by one or more of the operating systems supported by SWI-Prolog are not supported by the SWI-Prolog kernel distribution. Including these would enlarge the footprint and complicate portability matters while supporting only a limited part of the user-community.

This document describes `library(unix)` to deal with the Unix process API, `library(socket)` to deal with inet-domain TCP and UDP sockets, `library(cgi)` to deal with getting CGI form-data if SWI-Prolog is used as a CGI scripting language, `library(crypt)` to provide password encryption and verification, `library(sha)` providing cryptographic hash functions and `library(memfile)` providing in-memorty pseudo files.

## 2 library(process): Create processes and redirect I/O

Compatibility
  SICStus 4
To be done
  Implement detached option in process_create/3

The module `library(process)` implements interaction with child processes and unifies older interfaces such as shell/[1,2], `open(pipe(command), ...)` etc. This library is modelled after SICStus 4.

The main interface is formed by process_create/3. If the process id is requested the process must be waited for using process_wait/2. Otherwise the process resources are reclaimed automatically.

In addition to the predicates, this module defines a file search path (see user:file_search_path/2 and absolute_file_name/3) named `path` that locates files on the system's search path for executables. E.g. the following finds the executable for `ls`:

    ?- absolute_file_name(path(ls), Path, [access(execute)]).

Incompatibilities and current limitations

- Where SICStus distinguishes between an internal process id and the OS process id, this implementation does not make this distinction. This implies that is_process/1 is incomplete and unreliable.
- It is unclear what the `detached(true)` option is supposed to do. Disable signals in the child? Use `setsid()` to detach from the session? The current implementation uses `setsid()` on Unix systems.
- An extra option `env([Name=Value, ...])` is added to process_create/3. As of version 4.1 SICStus added `environment(List)` which modifies the environment. A compatible option was added to SWI-Prolog 7.7.23.

[det]process_create(+Exe, +Args:list, +Options)
  Create a new process running the file Exe and using arguments from the given list. Exe is a file specification as handed to absolute_file_name/3. Typically one use the `path` file alias to specify an executable file on the current PATH. Args is a list of arguments that are handed to the new process. On Unix systems, each element in the list becomes a separate argument in the new process. In Windows, the arguments are simply concatenated to form the commandline. Each argument itself is either a primitive or a list of primitives. A primitive is either atomic or a term `file(Spec)`. Using `file(Spec)`, the system inserts a filename using the OS filename conventions which is properly quoted if needed.

  Options:

  stdin(Spec)
  stdout(Spec)
  stderr(Spec)
    Bind the standard streams of the new process. Spec is one of the terms below. If `pipe(Pipe)` is used, the Prolog stream is a stream in text-mode using the encoding of the default locale. The encoding can be changed using set_stream/2, or by using the two-argument form of `pipe`, which accepts an `encoding(Encoding)` option. The options `stdout` and `stderr` may use the same stream, in which case both output streams are connected to the same Prolog stream.
    std
      Just share with the Prolog I/O streams. On Unix, if the `user_input`, etc. are bound to a file handle but not to 0,1,2 the process I/O is bound to the file handles of these streams.
    null
      Bind to a null stream. Reading from such a stream returns end-of-file, writing produces no output
    pipe(-Stream)
    pipe(-Stream, +StreamOptions)
      Attach input and/or output to a Prolog stream. The optional StreamOptions argument is a list of options that affect the stream. Currently only the options `type(+Type)` and `encoding(+Encoding)` are supported, which have the same meaning as the stream properties of the same name (see stream_property/2). StreamOptions is provided mainly for SICStus compatibility - the SWI-Prolog predicate set_stream/2 can be used for the same purpose.
    stream(+Stream)
      Attach input or output to an existing Prolog stream. This stream must be associated with an OS file handle (see stream_property/2, property `file_no`). This option is not provided by the SICStus implementation.
  cwd(+Directory)
    Run the new process in Directory. Directory can be a compound specification, which is converted using absolute_file_name/3. See also process_set_method/1.
  env(+List)
    As `environment(List)`, but only the specified variables are passed, i.e., no variables are inherited.
  environment(+List)
    Specify additional environment variables for the new process. List is a list of `Name=Value` terms, where Value is expanded the same way as the Args argument. If neither `env` nor `environment` is passed the environment is inherited from the Prolog process.
  process(-PID)
    Unify PID with the process id of the created process.
  detached(+Bool)
    In Unix: If `true`, detach the process from the terminal Currently mapped to `setsid()`; Also creates a new process group for the child In Windows: If `true`, detach the process from the current job via the CREATE_BREAKAWAY_FROM_JOB flag. In Vista and beyond, processes launched from the shell directly have the’compatibility assistant' attached to them automatically unless they have a UAC manifest embedded in them. This means that you will get a permission denied error if you try and assign the newly-created PID to a job you create yourself.
  window(+Bool)
    If `true`, create a window for the process (Windows only)
  priority(+Priority)
    In Unix: specifies the process priority for the newly created process. Priority must be an integer between -20 and 19. Positive values are nicer to others, and negative values are less so. The default is zero. Users are free to lower their own priority. Only the super-user may raise it to less-than zero.

  If the user specifies the `process(-PID)` option, he must call process_wait/2 to reclaim the process. Without this option, the system will wait for completion of the process after the last pipe stream is closed.

  If the process is not waited for, it must succeed with status 0. If not, an process_error is raised.

  Windows notes

  On Windows this call is an interface to the CreateProcess() API. The commandline consists of the basename of Exe and the arguments formed from Args. Arguments are separated by a single space. If all characters satisfy `iswalnum()` it is unquoted. If the argument contains a double-quote it is quoted using single quotes. If both single and double quotes appear a domain_error is raised, otherwise double-quote are used.

  The CreateProcess() API has many options. Currently only the `CREATE_NO_WINDOW` options is supported through the `window(+Bool)` option. If omitted, the default is to use this option if the application has no console. Future versions are likely to support more window specific options and replace win_exec/2.

  Examples

  First, a very simple example that behaves the same as `shell('ls -l')`, except for error handling:

      ?- process_create(path(ls), ['-l'], []).

  The following example uses grep to find all matching lines in a file.

      grep(File, Pattern, Lines) :-
              setup_call_cleanup(
                  process_create(path(grep), [ Pattern, file(File) ],
                                 [ stdout(pipe(Out))
                                 ]),
                  read_lines(Out, Lines),
                  close(Out)).

      read_lines(Out, Lines) :-
              read_line_to_codes(Out, Line1),
              read_lines(Line1, Out, Lines).

      read_lines(end_of_file, _, []) :- !.
      read_lines(Codes, Out, [Line|Lines]) :-
              atom_codes(Line, Codes),
              read_line_to_codes(Out, Line2),
              read_lines(Line2, Out, Lines).

  Errors
    `process_error(Exe, Status)` where Status is one of `exit(Code)` or `killed(Signal)`. Raised if the process is waited for (i.e., Options does not include `process(-PID)`), and does not exit with status 0.
  bug
    On Windows, `environment(List)` is handled as `env(List)`, i.e., the environment is not inherited.
[det]process_id(-PID)
  True if PID is the process id of the running Prolog process.
  deprecated
    Use `current_prolog_flag(pid, PID)`
[det]process_id(+Process, -PID)
  PID is the process id of Process. Given that they are united in SWI-Prolog, this is a simple unify.
[semidet]is_process(+PID)
  True if PID might be a process. Succeeds for any positive integer.
process_release(+PID)
  Release process handle. In this implementation this is the same as `process_wait(PID, _)`.
[det]process_wait(+PID, -Status)
[det]process_wait(+PID, -Status, +Options)
  True if PID completed with Status. This call normally blocks until the process is finished. Options:
  timeout(+Timeout)
    Default: `infinite`. If this option is a number, the waits for a maximum of Timeout seconds and unifies Status with `timeout` if the process does not terminate within Timeout. In this case PID is not invalidated. On Unix systems only timeout 0 and `infinite` are supported. A 0-value can be used to poll the status of the process.
  release(+Bool)
    Do/do not release the process. We do not support this flag and a domain_error is raised if `release(false)` is provided.

  Status | is one of `exit(Code)` or `killed(Signal)`, where Code and Signal are integers. If the `timeout` option is used Status is unified with `timeout` after the wait timed out.

[det]process_kill(+PID)
[det]process_kill(+PID, +Signal)
  Send signal to process PID. Default is `term`. Signal is an integer, Unix signal name (e.g. `SIGSTOP`) or the more Prolog friendly variation one gets after removing `SIG` and downcase the result: `stop`. On Windows systems, Signal is ignored and the process is terminated using the TerminateProcess() API. On Windows systems PID must be obtained from process_create/3, while any PID is allowed on Unix systems.
  Compatibility
    SICStus does not accept the prolog friendly version. We choose to do so for compatibility with on_signal/3.
[det]process_group_kill(+PID)
[det]process_group_kill(+PID, +Signal)
  Send signal to the group containing process PID. Default is `term`. See process_wait/1 for a description of signal handling. In Windows, the same restriction on PID applies: it must have been created from process_create/3, and the the group is terminated via the TerminateJobObject API.
[det]process_set_method(+Method)
  Determine how the process is created on Unix systems. Method is one of `spawn` (default), `fork` or `vfork`. If the method is `spawn` but this cannot be used because it is either not supported by the OS or the `cwd(Dir)` option is given `fork` is used.

  The problem is to be understood as follows. The official portable and safe method to create a process is using the `fork()` system call. This call however copies the process page tables and get seriously slow as the (Prolog) process is multiple giga bytes large. Alternatively, we may use `vfork()` which avoids copying the process space. But, the safe usage as guaranteed by the POSIX standard of `vfork()` is insufficient for our purposes. On practical systems your mileage may vary. Modern posix systems also provide `posix_spawn()`, which provides a safe and portable alternative for the `fork()` and `exec()` sequence that may be implemented using `fork()` or may use a fast but safe alternative. Unfortunately `posix_spawn()` doesn't support the option to specify the working directory for the child and we cannot use working_directory/2 as the working directory is shared between threads.

  Summarizing, the default is safe and tries to be as fast as possible. On some scenarios and on some OSes it is possible to do better. It is generally a good idea to avoid using the `cwd(Dir)` option of process_create/3 as without we can use `posix_spawn()`.

## 3 library(filesex): Extended operations on files

This module provides additional operations on files. This covers both more obscure and possible non-portable low-level operations and high-level utilities.

Using these Prolog primitives is typically to be preferred over using operating system primitives through shell/1 or process_create/3 because (1) there are no potential file name quoting issues, (2) there is no dependency on operating system commands and (3) using the implementations from this library is usually faster.

[det]set_time_file(+File, -OldTimes, +NewTimes)
  Query and set POSIX time attributes of a file. Both OldTimes and NewTimes are lists of option-terms. Times are represented in SWI-Prolog's standard floating point numbers. New times may be specified as `now` to indicate the current time. Defined options are:
  access(Time)
    Describes the time of last access of the file. This value can be read and written.
  modified(Time)
    Describes the time the contents of the file was last modified. This value can be read and written.
  changed(Time)
    Describes the time the file-structure itself was changed by adding (`link()`) or removing (`unlink()`) names.

  Below are some example queries. The first retrieves the access-time, while the second sets the last-modified time to the current time.

      ?- set_time_file(foo, [access(Access)], []).
      ?- set_time_file(foo, [], [modified(now)]).

[det]link_file(+OldPath, +NewPath, +Type)
  Create a link in the filesystem from NewPath to OldPath. Type defines the type of link and is one of `hard` or `symbolic`.

  With some limitations, these functions also work on Windows. First of all, the underlying filesystem must support links. This requires NTFS. Second, symbolic links are only supported in Vista and later.

  Errors
    `domain_error(link_type, Type)` if the requested link-type is unknown or not supported on the target OS.
[det]relative_file_name(+Path:atom, +RelToFile:atom, -RelPath:atom)
[det]relative_file_name(-Path:atom, +RelToFile:atom, +RelPath:atom)
  True when RelPath is Path, relative to the file RelToFile. Path and RelTo are first handed to absolute_file_name/2, which makes the absolute and canonical. Below are two examples:

      ?- relative_file_name('/home/janw/nice',
                            '/home/janw/deep/dir/file', Path).
      Path = '../../nice'.

      ?- relative_file_name(Path, '/home/janw/deep/dir/file', '../../nice').
      Path = '/home/janw/nice'.

  Add a terminating `/` to get a path relative to a directory, e.g.

      ?- relative_file_name('/home/janw/deep/dir/file', './', Path).
      Path = 'deep/dir/file'.

  All | paths must be in canonical POSIX notation, i.e., using / to separate segments in the path. See prolog_to_os_filename/2.

  bug
    It would probably have been cleaner to use a directory as second argument. We can not do such dynamically as this predicate is defined as a syntactical operation, which implies it may be used for non-existing paths and URLs.
[det]directory_file_path(+Directory, +File, -Path)
[det]directory_file_path(?Directory, ?File, +Path)
  True when Path is the full path-name for File in Dir. This is comparable to `atom_concat(Directory, File, Path)`, but it ensures there is exactly one / between the two parts. Notes:

  - In mode (+,+,-), if File is given and absolute, Path is unified to File.
  - Mode (-,-,+) uses file_directory_name/2 and file_base_name/2
[nondet]directory_member(+Directory, -Member, +Options)
  True when Member is a path inside Directory. Options defined are:
  recursive(+Boolean)
    If `true` (default `false`), recurse into subdirectories
  follow_links(+Boolean)
    If `true` (default), follow symbolic links.
  file_type(+Type)
    See absolute_file_name/3.
  extensions(+List)
    Only return entries whose extension appears in List.
  file_errors(+Errors)
    How to handle errors. One of `fail`, `warning` or `error`. Default is `warning`. Errors notably happen if a directory is unreadable or a link points nowhere.
  access(+Access)
    Only return entries with Access
  matches(+GlobPattern)
    Only return files that match GlobPattern.
  exclude(+GlobPattern)
    Exclude files matching GlobPattern.
  exclude_directory(+GlobPattern)
    Do not recurse into directories matching GlobPattern.
  hidden(+Boolean)
    If `true` (default), also return hidden files.

  This predicate is safe against cycles introduced by symbolic links to directories.

  The idea for a non-deterministic file search predicate comes from Nicos Angelopoulos.

[det]copy_file(+From, +To)
  Copy a file into a new file or directory. The data is copied as binary data.
[det]make_directory_path(+Dir)
  Create Dir and all required components (like mkdir -p). Can raise various file-specific exceptions.
[det]copy_directory(+From, +To)
  Copy the contents of the directory From to To (recursively). If To is the name of an existing directory, the contents of From are copied into To. I.e., no subdirectory using the basename of From is created.
[det]delete_directory_and_contents(+Dir)
  Recursively remove the directory Dir and its contents. If Dir is a symbolic link or symbolic links inside Dir are encountered, the links are removed rather than their content. Use with care!
[det]delete_directory_contents(+Dir)
  Remove all content from directory Dir, without removing Dir itself. Similar to delete_directory_and_contents/2, if symbolic links are encountered in Dir, the links are removed rather than their content.
[det]chmod(+File, +Spec)
  Set the mode of the target file. Spec is one of `+Mode`, `-Mode` or a plain Mode, which adds new permissions, revokes permissions or sets the exact permissions. Mode itself is an integer, a POSIX mode name or a list of POSIX mode names. Defines names are `suid`, `sgid`, `svtx` and all names defined by the regular expression `[ugo]*[rwx]*`. Specifying none of "ugo" is the same as specifying all of them. For example, to make a file executable for the owner (user) and group, we can use:

      ?- chmod(myfile, +ugx).

## 4 library(uid): User and group management on Unix systems

See also
  Please check the documentation of your OS for details on the semantics of this predicates.

This module provides and interface to user and group information on Posix systems. In addition, it allows for changing user and group ids. When changing user and group settings for the calling process, bear in mind that:

- Changing user and groups of the calling process requires permission.
- The functions `setgroups()` and `initgroups()` are not part of the POSIX standard and therefore the derived predicates may not be present.

[det]getuid(-UID)
  UID is the real user ID of the calling process.
[det]getgid(-GID)
  GID is the real group ID of the calling process.
[det]geteuid(-UID)
  UID is the effective user ID of the calling process.
[det]getegid(-GID)
  GID is the effective group ID of the calling process.
[det]getgroups(-GroupsIDs:list(integer))
  GroupsIDs is the set of supplementary group IDs of the calling process. Note that these are numeric identifiers. Use group_info/2 to obtain details on the returned group identifiers.
[det]user_info(+User, -UserData)
  UserData represent the passwd information for User. User is either a numeric UID or a user name. The predicate user_data/3 can be used to extract information from UserData.
user_data(?Field, ?UserData, ?Value)
  Value is the value for Field in UserData. Defined fields are:
  name
    Name of the user
  password
    Password hash of the user (or `x` if this is not accessible)
  uid
    Numeric user id of the user
  gid
    Numeric primary group id of the user
  comment
    The gecos field
  home
    Home directory of the user
  shell
    Default (login) shell of the user.
[det]group_info(+Group, -GroupData)
  GroupData represent the group information for Group. Group is either a numeric GID or a group name. The predicate group_data/3 can be used to extract information from GroupData.
group_data(?Field, ?GroupData, ?Value)
  Value is the value for Field GroupData. Defined fields are:
  name
    Name of the user
  password
    Password hash of the user (or `x` if this is not accessible)
  gid
    Numeric group id of the group
  members
    List of user-names that are member of this group.
setuid(+UID)
  Set the user id of the calling process.
seteuid(+UID)
  Set the effective user id of the calling process.
setgid(+GID)
  Set the group id of the calling process.
setegid(+GID)
  Set the effective group id of the calling process.
[det]initgroups(+User, +Group)
  Initialise the group access list of the calling process to the registered groups for User and the group Group. This predicate is only available if the underlying OS provides it.
[det]setgroups(+Groups:list(integer))
  Set the group access list of the caling process to the indicated groups. This predicate is only available if the underlying OS provides it.
[det]set_user_and_group(+User)
[det]set_user_and_group(+User, +Group)
  Set the UID and GID to the User. User is either a UID or a user name. If Group is not specified, the primary group of User is used. If initgroups/2 is available, the resulting group access list of the calling process consists of the registered groups for User and the specified Group.

## 5 library(syslog): Unix syslog interface

See also
  - detach_IO/1 to detach normal I/O of the process and remove it from the process group.
  - fork/1 to create a daemon process.
  - `library(uid)` to manage user identifiers (e.g., drop root privileges).

This library provides an interface to the Unix `syslog()` facility. The interface is an almost direct translation of the POSIX syslog API, with two additions:

- syslog/3 exploits format/3 to format syslog messages
- The library integrates into `library(debug)` using prolog:debug_print_hook/3, where debug topics are mapped to syslog priorities and remaining debug topics are mapped to the syslog priority `debug`.

Note that this interface makes no attempt to abstract over logging facilities of operating systems. We expect that such abstractions will be implemented at the Prolog level using multiple integrations into `library(debug)`.

[det]openlog(+Ident:atom, +Options:list(atom), +Facility:atom)
  Open system log. This predicate provides a direct interface into the `openlog()` library call. If the library call is successful, it runs `at_halt(closelog)` to ensure closing the system log on clean exit.

  Ident | prepended to every message, and is typically set to the program name.
  Options | is a list of options. Values are corresponding C options, after removing =LOG_= and translation to lower case: `cons`, `ndelay`, `nowait`, `odelay`, `perror`, `pid`.
  Facility | is one of `auth`, `authpriv`, `cron`, `daemon`, `ftp`, `kern`, `local0` ... `local7`, `lpr`, `mail`, `news`, `syslog`, `user` or `uucp`.

[det]syslog(+Priority, +Message)
  Send a message to the system log. Note that syslog/2 implicitly opens a connection to the system log if such a connection has not been opened explicitly using openlog/3.

  Priority | is one of `emerg`, `alert`, `crit`, `err`, `warning`, `notice`, `info` or `debug`.

[det]syslog(+Priority, +Format, +Args)
  Send a formatted message to the system log if system logging is opened using openlog/3. This predicate combined format/3 with syslog/2. If there is no open syslog connection, syslog/3 calls print_message/2.
[det]closelog
  Close the system log.
[semidet,multifile]prolog:debug_print_hook(+Topic, +Format, +Args)
  Integration of debug/3 with the syslog facility. If syslog is enabled, debug/3 is re-routed to use the syslog facilities. If the topic of the debug message matches one of the sylog priority values (see syslog/2), the message is sent with the corresponding syslog priority. Otherwise it it sent with the `debug` priority.

## 6 library(socket): Network socket (TCP and UDP) library

The `library(socket)` provides TCP and UDP inet-domain sockets from SWI-Prolog, both client and server-side communication. The interface of this library is very close to the Unix socket interface, also supported by the MS-Windows winsock API. SWI-Prolog applications that wish to communicate with multiple sources have three options:

- Use I/O multiplexing based on wait_for_input/3. On Windows systems this can only be used for sockets, not for general (device-) file handles.
- Use multiple threads, handling either a single blocking socket or a pool using I/O multiplexing as above.
- Using XPCE's class `socket` which synchronises socket events in the GUI event-loop.

### 6.1 Client applications

Using this library to establish a TCP connection to a server is as simple as opening a file. See also http_open/3.

    dump_swi_homepage :-
        setup_call_cleanup(
            tcp_connect(www.swi-prolog.org:http, Stream, []),
            ( format(Stream,
                     'GET / HTTP/1.1~n\c
                      Host: www.swi-prolog.org~n\c
                      Connection: close~n~n', []),
              flush_output(Stream),
              copy_stream_data(Stream, current_output)
            ),
            close(S)).

To deal with timeouts and multiple connections, threads, wait_for_input/3 and/or non-blocking streams (see tcp_fcntl/3) can be used.

### 6.2 Server applications

The typical sequence for generating a server application is given below. To close the server, use close/1 on AcceptFd.

    create_server(Port) :-
          tcp_socket(Socket),
          tcp_bind(Socket, Port),
          tcp_listen(Socket, 5),
          tcp_open_socket(Socket, AcceptFd, _),
          <dispatch>

There are various options for <dispatch>. The most commonly used option is to start a Prolog thread to handle the connection. Alternatively, input from multiple clients can be handled in a single thread by listening to these clients using wait_for_input/3. Finally, on Unix systems, we can use fork/1 to handle the connection in a new process. Note that fork/1 and threads do not cooperate well. Combinations can be realised but require good understanding of POSIX thread and fork-semantics.

Below is the typical example using a thread. Note the use of setup_call_cleanup/3 to guarantee that all resources are reclaimed, also in case of failure or exceptions.

    dispatch(AcceptFd) :-
            tcp_accept(AcceptFd, Socket, Peer),
            thread_create(process_client(Socket, Peer), _,
                          [ detached(true)
                          ]),
            dispatch(AcceptFd).

    process_client(Socket, Peer) :-
            setup_call_cleanup(
                tcp_open_socket(Socket, StreamPair),
                handle_service(StreamPair),
                close(StreamPair)).

    handle_service(StreamPair) :-
            ...

### 6.3 Socket exceptions

Errors that are trapped by the low-level library are mapped to an exception of the shape below. In this term, Code is a lower case atom that corresponds to the C macro name, e.g., `epipe` for a broken pipe. Message is the human readable string for the error code returned by the OS or the same as Code if the OS does not provide this functionality. Note that Code is derived from a static set of macros that may or may not be defines for the target OS. If the macro name is not known, Code is `ERROR_nnn`, where nnn is an integer.

    error(socket_error(Code, Message), _)

Note that on Windows Code is a `wsa*` code which makes it hard to write portable code that handles specific socket errors. Even on POSIX systems the exact set of errors produced by the network stack is not defined.

### 6.4 TCP socket predicates

[det]tcp_socket(-SocketId)
  Creates an INET-domain stream-socket and unifies an identifier to it with SocketId. On MS-Windows, if the socket library is not yet initialised, this will also initialise the library.
[det]tcp_close_socket(+SocketId)
  Closes the indicated socket, making SocketId invalid. Normally, sockets are closed by closing both stream handles returned by open_socket/3. There are two cases where tcp_close_socket/1 is used because there are no stream-handles:

  - If, after tcp_accept/3, the server uses fork/1 to handle the client in a sub-process. In this case the accepted socket is not longer needed from the main server and must be discarded using tcp_close_socket/1.
  - If, after discovering the connecting client with tcp_accept/3, the server does not want to accept the connection, it should discard the accepted socket immediately using tcp_close_socket/1.
[det]tcp_open_socket(+SocketId, -StreamPair)
  Create streams to communicate to SocketId. If SocketId is a master socket (see tcp_bind/2), StreamPair should be used for tcp_accept/3. If SocketId is a connected (see tcp_connect/2) or accepted socket (see tcp_accept/3), StreamPair is unified to a stream pair (see stream_pair/3) that can be used for reading and writing. The stream or pair must be closed with close/1, which also closes SocketId.
[det]tcp_open_socket(+SocketId, -InStream, -OutStream)
  Similar to tcp_open_socket/2, but creates two separate sockets where tcp_open_socket/2 would have created a stream pair.
  deprecated
    New code should use tcp_open_socket/2 because closing a stream pair is much easier to perform safely.
[det]tcp_bind(SocketId, ?Address)
  Bind the socket to Address on the current machine. This operation, together with tcp_listen/2 and tcp_accept/3 implement the server-side of the socket interface. Address is either an plain Port or a term HostPort. The first form binds the socket to the given port on all interfaces, while the second only binds to the matching interface. A typical example is below, causing the socket to listen only on port 8080 on the local machine's network.

        tcp_bind(Socket, localhost:8080)

  If Port is unbound, the system picks an arbitrary free port and unifies Port with the selected port number. Port is either an integer or the name of a registered service. See also tcp_connect/4.

[det]tcp_listen(+SocketId, +BackLog)
  Tells, after tcp_bind/2, the socket to listen for incoming requests for connections. Backlog indicates how many pending connection requests are allowed. Pending requests are requests that are not yet acknowledged using tcp_accept/3. If the indicated number is exceeded, the requesting client will be signalled that the service is currently not available. A commonly used default value for Backlog is 5.
[det]tcp_accept(+Socket, -Slave, -Peer)
  This predicate waits on a server socket for a connection request by a client. On success, it creates a new socket for the client and binds the identifier to Slave. Peer is bound to the IP-address of the client or the atom `af_unix` if Socket is an AF_UNIX socket (see unix_domain_socket/1).
[det]tcp_connect(+SocketId, +Address)
  Connect SocketId. After successful completion, tcp_open_socket/3 can be used to create I/O-Streams to the remote socket. This predicate is part of the low level client API. A connection to a particular host and port is realised using these steps:

          tcp_socket(Socket),
          tcp_connect(Socket, Host:Port),
          tcp_open_socket(Socket, StreamPair)

  Typical client applications should use the high level interface provided by tcp_connect/3 which avoids resource leaking if a step in the process fails, and can be hooked to support proxies. For example:

          setup_call_cleanup(
              tcp_connect(Host:Port, StreamPair, []),
              talk(StreamPair),
              close(StreamPair))

  If SocketId is an AF_UNIX socket (see unix_domain_socket/1), Address is an atom or string denoting a file name.

[det]tcp_connect(+Socket, +Address, -Read, -Write)
  Connect a (client) socket to Address and return a bi-directional connection through the stream-handles Read and Write. This predicate may be hooked by defining socket:tcp_connect_hook/4 with the same signature. Hooking can be used to deal with proxy connections. E.g.,

      :- multifile socket:tcp_connect_hook/4.

      socket:tcp_connect_hook(Socket, Address, Read, Write) :-
          proxy(ProxyAdress),
          tcp_connect(Socket, ProxyAdress),
          tcp_open_socket(Socket, Read, Write),
          proxy_connect(Address, Read, Write).

  deprecated
    New code should use tcp_connect/3 called as `tcp_connect(+Address, -StreamPair, +Options)`.
[det]tcp_connect(+Address, -StreamPair, +Options)
[det]tcp_connect(+Socket, +Address, -StreamPair)
  Establish a TCP communication as a client. The +,-,+ mode is the preferred way for a client to establish a connection. This predicate can be hooked to support network proxies. To use a proxy, the hook proxy_for_url/3 must be defined. Permitted options are:
  bypass_proxy(+Boolean)
    Defaults to `false`. If `true`, do not attempt to use any proxies to obtain the connection
  nodelay(+Boolean)
    Defaults to `false`. If `true`, set nodelay on the resulting socket using `tcp_setopt(Socket, nodelay)`

  The +,+,- mode is deprecated and does not support proxies. It behaves like tcp_connect/4, but creates a stream pair (see stream_pair/3).

  Address | is either a Host:Port term or a file name (atom or string). The latter connects to an AF_UNIX socket and requires unix_domain_socket/1.

  Errors
    `proxy_error(tried(ResultList))` is raised by mode (+,-,+) if proxies are defines by proxy_for_url/3 but no proxy can establsh the connection. ResultList contains one or more terms of the form `false(Proxy)` for a hook that simply failed or `error(Proxy, ErrorTerm)` for a hook that raised an exception.
  See also
    `library(http/http_proxy)` defines a hook that allows to connect through HTTP proxies that support the `CONNECT` method.
tcp_select(+ListOfStreams, -ReadyList, +TimeOut)
  Same as the built-in wait_for_input/3. Used to allow for interrupts and timeouts on Windows. A redesign of the Windows socket interface makes it impossible to do better than Windows `select()` call underlying wait_for_input/3. As input multiplexing typically happens in a background thread anyway we accept the loss of timeouts and interrupts.
  deprecated
    Use wait_for_input/3
[semidet,multifile]try_proxy(+Proxy, +TargetAddress, -Socket, -StreamPair)
  Attempt a socket-level connection via the given proxy to TargetAddress. The Proxy argument must match the output argument of proxy_for_url/3. The predicate tcp_connect/3 (and http_open/3 from the `library(http/http_open)`) collect the results of failed proxies and raise an exception no proxy is capable of realizing the connection.

  The default implementation recognises the values for Proxy described below. The `library(http/http_proxy)` adds `proxy(Host,Port)` which allows for HTTP proxies using the `CONNECT` method.

  direct
    Do not use any proxy
  socks(Host, Port)
    Use a SOCKS5 proxy
[nondet,multifile]proxy_for_url(+URL, +Hostname, -Proxy)
  This hook can be implemented to return a proxy to try when connecting to URL. Returned proxies are tried in the order in which they are returned by the multifile hook try_proxy/4. Pre-defined proxy methods are:
  direct
    connect directly to the resource
  proxy(Host, Port)
    Connect to the resource using an HTTP proxy. If the resource is not an HTTP URL, then try to connect using the CONNECT verb, otherwise, use the GET verb.
  socks(Host, Port)
    Connect to the resource via a SOCKS5 proxy

  These correspond to the proxy methods defined by PAC Proxy auto-config. Additional methods can be returned if suitable clauses for http:http_connection_over_proxy/6 or try_proxy/4 are defined.

[det]tcp_setopt(+SocketId, +Option)
  Set options on the socket. Defined options are:
  reuseaddr
    Allow servers to reuse a port without the system being completely sure the port is no longer in use.
  bindtodevice(+Device)
    Bind the socket to Device (an atom). For example, the code below binds the socket to the loopback device that is typically used to realise the localhost. See the manual pages for `setsockopt()` and the socket interface (e.g., `socket(7)` on Linux) for details.

        tcp_socket(Socket),
        tcp_setopt(Socket, bindtodevice(lo))

  nodelay
  nodelay(true)
    If `true`, disable the Nagle optimization on this socket, which is enabled by default on almost all modern TCP/IP stacks. The Nagle optimization joins small packages, which is generally desirable, but sometimes not. Please note that the underlying TCP_NODELAY setting to `setsockopt()` is not available on all platforms and systems may require additional privileges to change this option. If the option is not supported, tcp_setopt/2 raises a domain_error exception. See Wikipedia for details.
  broadcast
    UDP sockets only: broadcast the package to all addresses matching the address. The address is normally the address of the local subnet (i.e. 192.168.1.255). See udp_send/4.
  ip_add_membership(+MultiCastGroup)
  ip_add_membership(+MultiCastGroup, +LocalInterface)
  ip_add_membership(+MultiCastGroup, +LocalInterface, +InterfaceIndex)
  ip_drop_membership(+MultiCastGroup)
  ip_drop_membership(+MultiCastGroup, +LocalInterface)
  ip_drop_membership(+MultiCastGroup, +LocalInterface, +InterfaceIndex)
    Join/leave a multicast group. Calls `setsockopt()` with the corresponding arguments.
  dispatch(+Boolean)
    In GUI environments (using XPCE or the Windows `swipl-win.exe` executable) this flags defines whether or not any events are dispatched on behalf of the user interface. Default is `true`. Only very specific situations require setting this to `false`.
  sndbuf(+Integer)
    Sets the send buffer size to Integer (bytes). On Windows this defaults (now) to 64kb. Higher latency links may benefit from increasing this further since the maximum theoretical throughput on a link is given by buffer-size / latency. See https://support.microsoft.com/en-gb/help/823764/slow-performance-occurs-when-you-copy-data-to-a-tcp-server-by-using-a for Microsoft's discussion
[det]tcp_fcntl(+Stream, +Action, ?Argument)
  Interface to the `fcntl()` call. Currently only suitable to deal switch stream to non-blocking mode using:

        tcp_fcntl(Stream, setfl, nonblock),

  An attempt to read from a non-blocking stream while there is no data available returns -1 (or `end_of_file` for read/1), but at_end_of_stream/1 fails. On actual end-of-input, at_end_of_stream/1 succeeds.

[semidet]tcp_getopt(+Socket, ?Option)
  Get information about Socket. Defined properties are below. Requesting an unknown option results in a `domain_error` exception.
  file_no(-File)
    Get the OS file handle as an integer. This may be used for debugging and integration.
[det]tcp_host_to_address(?HostName, ?Address)
  Translate between a machines host-name and it's (IP-)address. If HostName is an atom, it is resolved using `getaddrinfo()` and the IP-number is unified to Address using a term of the format `ip(Byte1,Byte2,Byte3,Byte4)`. Otherwise, if Address is bound to an `ip(Byte1,Byte2,Byte3,Byte4)` term, it is resolved by `gethostbyaddr()` and the canonical hostname is unified with HostName.
  To be done
    This function should support more functionality provided by gethostbyaddr, probably by adding an option-list.
[det]gethostname(-Hostname)
  Return the canonical fully qualified name of this host. This is achieved by calling `gethostname()` and return the canonical name returned by `getaddrinfo()`.
[det]negotiate_socks_connection(+DesiredEndpoint, +StreamPair)
  Negotiate a connection to DesiredEndpoint over StreamPair. DesiredEndpoint should be in the form of either:

  - hostname : port
  - `ip(A,B,C,D)` : port
  Errors
    `socks_error(Details)` if the SOCKS negotiation failed.

### 6.5 Unix domain sockets

Unix domain sockets (sockets with address family AF_UNIX) are represented as a (socket) file in the file system. They can only be used to connect processes on the same host. The main advantage of AF_UNIX sockets is that name conflicts are much easier to manage than port conflicts and that access is determined by the file system permission rules.

unix_domain_socket(-Socket)
  Creates an AF_UNIX-domain stream-socket and unifies an identifier to it with Socket. This predicate does not exist if the OS does not support the AF_UNIX address family (e.g. MS-Windows).

  Unix domain socket affect tcp_connect/2 (for clients) and tcp_bind/2 and tcp_accept/3 (for servers). The address is an atom or string that is handled as a file name. On most systems the length of this file name is limited to 128 bytes (including null terminator), but according to the Linux documentation (unix(7)), portable applications must keep the address below 92 bytes. Note that these lengths are in bytes. Non-ascii characters may be represented as multiple bytes. If the length limit is exceeded a `representation_error(af_unix_name)` exception is raised.

### 6.6 UDP protocol support

The socket library provides support for UDP sockets. The UDP protocol is a connection-less and unreliable datagram based protocol. That means that messages sent may or may not arrive at the client side, may arrive in a different order then they were sent or even may arrive multiple times. UDP messages are often used for data replication, streaming media, or service discovery.

The library `library(udp_broadcast)` provides a high-level interface that builds on top of the `library(broadcast)` facilitating an unreliable publish-subscribe pattern based communication channel. The `library(udp_broadcast)` library supports multiple network architectures, e.g., classical LAN broadcasting, multicast and unicast.

udp_socket(-Socket)
  Similar to tcp_socket/1, but create a socket using the `SOCK_DGRAM` protocol, ready for UDP connections.
udp_receive(+Socket, -Data, -From, +Options)
  Wait for and return the next datagram. The data is returned as a Prolog string object (see string_to_list/2). From is a term of the format ip(A,B,C,D):Port indicating the sender of the message. Socket can be waited for using wait_for_input/3. Defined Options:
  as(+Type)
    Defines the returned term-type. Type is one of `atom`, `codes` or `string` (default).
  max_message_size(+Size)
    Specify the maximum number of bytes to read from a UDP datagram. Size must be within the range 0-65535. If unspecified, a maximum of 4096 bytes will be read.

  The typical sequence to receive UDP data is:

      receive(Port) :-
              udp_socket(S),
              tcp_bind(S, Port),
              repeat,
                  udp_receive(Socket, Data, From, [as(atom)]),
                  format('Got ~q from ~q~n', [Data, From]),
                  fail.

udp_send(+Socket, +Data, +To, +Options)
  Send a UDP message. Data is a string, atom or code-list providing the data. To is an address of the form Host:Port where Host is either the hostname or a term ip/4. Options is currently unused.

  A simple example to send UDP data is:

      send(Host, Port, Message) :-
              udp_socket(S),
              udp_send(S, Message, Host:Port, []),
              tcp_close_socket(S).

  A broadcast is achieved by using `tcp_setopt(Socket, broadcast)` prior to sending the datagram and using the local network broadcast address as a ip/4 term.

The normal mechanism to discover a service on the local network is for the client to send a broadcast message to an agreed port. The server receives this message and replies to the client with a message indicating further details to establish the communication.

## 7 The stream_pool library

The `library(streampool)` library dispatches input from multiple streams based on wait_for_input/3. It is part of the clib package as it is used most of the time together with the `library(socket)` library. On non-Unix systems it often can only be used with socket streams.

With SWI-Prolog 5.1.x, multi-threading often provides a good alternative to using this library. In this schema one thread watches the listening socket waiting for connections and either creates a thread per connection or processes the accepted connections with a pool of worker threads. The library `library(http/thread_httpd)` provides an example realising a mult-threaded HTTP server.

add_stream_to_pool(+Stream, :Goal)
  Add Stream, which must be an input stream and ---on non-unix systems--- connected to a socket to the pool. If input is available on Stream, Goal is called.
delete_stream_from_pool(+Stream)
  Delete the given stream from the pool. Succeeds, even if Stream is no member of the pool. If Stream is unbound the entire pool is emtied but unlike close_stream_pool/0 the streams are not closed.
close_stream_pool
  Empty the pool, closing all streams that are part of it.
dispatch_stream_pool(+TimeOut)
  Wait for maximum of TimeOut for input on any of the streams in the pool. If there is input, call the Goal associated with add_stream_to_pool/2. If Goal fails or raises an exception a message is printed. TimeOut is described with wait_for_input/3.

  If Goal is called, there is some input on the associated stream. Goal must be careful not to block as this will block the entire pool.1This is hard to achieve at the moment as none of the Prolog read-commands provide for a timeout.

stream_pool_main_loop
  Calls dispatch_stream_pool/1 in a loop until the pool is empty.

Below is a very simple example that reads the first line of input and echos it back.

    :- use_module(library(streampool)).

    server(Port) :-
            tcp_socket(Socket),
            tcp_bind(Socket, Port),
            tcp_listen(Socket, 5),
            tcp_open_socket(Socket, In, _Out),
            add_stream_to_pool(In, accept(Socket)),
            stream_pool_main_loop.

    accept(Socket) :-
            tcp_accept(Socket, Slave, Peer),
            tcp_open_socket(Slave, In, Out),
            add_stream_to_pool(In, client(In, Out, Peer)).

    client(In, Out, _Peer) :-
            read_line_to_codes(In, Command),
            close(In),
            format(Out, 'Please to meet you: ~s~n', [Command]),
            close(Out),
            delete_stream_from_pool(In).

## 8 library(uri): Process URIs

This library provides high-performance C-based primitives for manipulating URIs. We decided for a C-based implementation for the much better performance on raw character manipulation. Notably, URI handling primitives are used in time-critical parts of RDF processing. This implementation is based on RFC-3986:

    http://labs.apache.org/webarch/uri/rfc/rfc3986.html

The URI processing in this library is rather liberal. That is, we break URIs according to the rules, but we do not validate that the components are valid. Also, percent-decoding for IRIs is liberal. It first tries UTF-8; then ISO-Latin-1 and finally accepts %-characters verbatim.

Earlier experience has shown that strict enforcement of the URI syntax results in many errors that are accepted by many other web-document processing tools.

[det]uri_components(+URI, -Components)
[det]uri_components(-URI, +Components)
  Break a URI into its 5 basic components according to the RFC-3986 regular expression:

      ^(([^:/?#]+):)?(//([^/?#]*))?([^?#]*)(\?([^#]*))?(#(.*))?
       12            3  4          5       6  7        8 9

  Components | is a term `uri_components(Scheme, Authority, Path, Search, Fragment)`. If a URI is parsed, i.e., using mode (+,-), components that are not found are left uninstantiated (variable). See uri_data/3 for accessing this structure.

[semidet]uri_data(?Field, +Components, ?Data)
  Provide access the uri_component structure. Defined field-names are: `scheme`, `authority`, `path`, `search` and `fragment`
[semidet]uri_data(+Field, +Components, +Data, -NewComponents)
  NewComponents is the same as Components with Field set to Data.
[det]uri_normalized(+URI, -NormalizedURI)
  NormalizedURI is the normalized form of URI. Normalization is syntactic and involves the following steps:

  - 6.2.2.1. Case Normalization
  - 6.2.2.2. Percent-Encoding Normalization
  - 6.2.2.3. Path Segment Normalization
[det]iri_normalized(+IRI, -NormalizedIRI)
  NormalizedIRI is the normalized form of IRI. Normalization is syntactic and involves the following steps:

  - 6.2.2.1. Case Normalization
  - 6.2.2.3. Path Segment Normalization
  See also
    This is similar to uri_normalized/2, but does not do normalization of %-escapes.
[det]uri_normalized_iri(+URI, -NormalizedIRI)
  As uri_normalized/2, but percent-encoding is translated into IRI Unicode characters. The translation is liberal: valid UTF-8 sequences of %-encoded bytes are mapped to the Unicode character. Other %XX-sequences are mapped to the corresponding ISO-Latin-1 character and sole % characters are left untouched.
  See also
    uri_iri/2.
[semidet]uri_is_global(+URI)
  True if URI has a scheme. The semantics is the same as the code below, but the implementation is more efficient as it does not need to parse the other components, nor needs to bind the scheme. The condition to demand a scheme of more than one character is added to avoid confusion with DOS path names.

      uri_is_global(URI) :-
              uri_components(URI, Components),
              uri_data(scheme, Components, Scheme),
              nonvar(Scheme),
              atom_length(Scheme, Len),
              Len > 1.

[det]uri_resolve(+URI, +Base, -GlobalURI)
  Resolve a possibly local URI relative to Base. This implements http://labs.apache.org/webarch/uri/rfc/rfc3986.html#relative-transform
[det]uri_normalized(+URI, +Base, -NormalizedGlobalURI)
  NormalizedGlobalURI is the normalized global version of URI. Behaves as if defined by:

      uri_normalized(URI, Base, NormalizedGlobalURI) :-
              uri_resolve(URI, Base, GlobalURI),
              uri_normalized(GlobalURI, NormalizedGlobalURI).

[det]iri_normalized(+IRI, +Base, -NormalizedGlobalIRI)
  NormalizedGlobalIRI is the normalized global version of IRI. This is similar to uri_normalized/3, but does not do %-escape normalization.
[det]uri_normalized_iri(+URI, +Base, -NormalizedGlobalIRI)
  NormalizedGlobalIRI is the normalized global IRI of URI. Behaves as if defined by:

      uri_normalized(URI, Base, NormalizedGlobalIRI) :-
              uri_resolve(URI, Base, GlobalURI),
              uri_normalized_iri(GlobalURI, NormalizedGlobalIRI).

[det]uri_query_components(+String, -Query)
[det]uri_query_components(-String, +Query)
  Perform encoding and decoding of an URI query string. Query is a list of fully decoded (Unicode) Name=Value pairs. In mode (-,+), query elements of the forms Name(Value) and Name-Value are also accepted to enhance interoperability with the option and pairs libraries. E.g.

      ?- uri_query_components(QS, [a=b, c('d+w'), n-'VU Amsterdam']).
      QS = 'a=b&c=d%2Bw&n=VU%20Amsterdam'.

      ?- uri_query_components('a=b&c=d%2Bw&n=VU%20Amsterdam', Q).
      Q = [a=b, c='d+w', n='VU Amsterdam'].

[det]uri_authority_components(+Authority, -Components)
[det]uri_authority_components(-Authority, +Components)
  Break-down the authority component of a URI. The fields of the structure Components can be accessed using uri_authority_data/3.
[semidet]uri_authority_data(+Field, ?Components, ?Data)
  Provide access the uri_authority structure. Defined field-names are: `user`, `password`, `host` and `port`
[det]uri_encoded(+Component, +Value, -Encoded)
[det]uri_encoded(+Component, -Value, +Encoded)
  Encoded is the URI encoding for Value. When encoding (Value`->`Encoded), Component specifies the URI component where the value is used. It is one of `query_value`, `fragment`, `path` or `segment`. Besides alphanumerical characters, the following characters are passed verbatim (the set is split in logical groups according to RFC3986).
  query_value, fragment
    "-._`~`" `|` "!$’()*,;" `|` "@" `|` "/?"
  path
    "-._`~`" `|` "!$&’()*,;=" `|` "@" `|` "/"
  segment
    "-._`~`" `|` "!$&’()*,;=" `|` "@"
[det]uri_iri(+URI, -IRI)
[det]uri_iri(-URI, +IRI)
  Convert between a URI, encoded in US-ASCII and an IRI. An IRI is a fully expanded Unicode string. Unicode strings are first encoded into UTF-8, after which %-encoding takes place.
  Errors
    `syntax_error(Culprit)` in mode (+,-) if URI is not a legally percent-encoded UTF-8 string.
[semidet]uri_file_name(+URI, -FileName)
[det]uri_file_name(-URI, +FileName)
  Convert between a URI and a local file_name. This protocol is covered by RFC 1738. Please note that file-URIs use absolute paths. The mode (-, +) translates a possible relative path into an absolute one.

## 9 CGI Support library

This is currently a very simple library, providing support for obtaining the form-data for a CGI script:

cgi_get_form(-Form)
  Decodes standard input and the environment variables to obtain a list of arguments passed to the CGI script. This predicate both deals with the CGI GET method as well as the POST method. If the data cannot be obtained, an `existence_error` exception is raised.

Below is a very simple CGI script that prints the passed parameters. To test it, compile this program using the command below, copy it to your cgi-bin directory (or make it otherwise known as a CGI-script) and make the query `http://myhost.mydomain/cgi-bin/cgidemo?hello=world`

    % pl -o cgidemo --goal=main --toplevel=halt -c cgidemo.pl

    :- use_module(library(cgi)).

    main :-
            set_stream(current_output, encoding(utf8)),
            cgi_get_form(Arguments),
            format('Content-type: text/html; charset=UTF-8~n~n', []),
            format('<html>~n', []),
            format('<head>~n', []),
            format('<title>Simple SWI-Prolog CGI script</title>~n', []),
            format('</head>~n~n', []),
            format('<body>~n', []),
            format('<p>', []),
            print_args(Arguments),
            format('</body>~n</html>~n', []).

    print_args([]).
    print_args([A0|T]) :-
            A0 =.. [Name, Value],
            format('<b>~w</b>=<em>~w</em><br>~n', [Name, Value]),
            print_args(T).

### 9.1 Some considerations

Printing an HTML document using format/2 is not a neat way of producing HTML because it is vulnerable to required escape sequences. A high-level alternative is provided by `library(http/html_write)` from the HTTP library.

The startup-time of Prolog is relatively long, in particular if the program is large. In many cases it is much better to use the SWI-Prolog HTTP server library and make the main web-server relay requests to the SWI-Prolog webserver. See the SWI-Prolog HTTP package for details.

The CGI standard is unclear about handling Unicode data. The above two declarations ensure the CGI script will send all data in UTF-8 and thus provide full support of Unicode. It is assumed that browsers generally send form-data using the same encoding as the page in which the form appears, UTF-8 or ISO Latin-1. The current version of cgi_get_form/1 assumes the CGI data is in UTF-8.

## 10 Password encryption library

The `library(crypt)` library defines crypt/2 for encrypting and testing passwords. The clib package also provides crytographic hashes as described in section 11

crypt(+Plain, ?Encrypted)
  This predicate can be used in three modes. To test whether a password matches an encrypted version thereof, simply run with both arguments fully instantiated. To generate a default encrypted version of Plain, run with unbound Encrypted and this argument is unified to a list of character codes holding an encrypted version.

  The library supports two encryption formats: traditional Unix DES-hashes2On non-Unix systems, crypt() is provided by the NetBSD library. The license header is added at the end of this document. and FreeBSD compatible MD5 hashes (all platforms). MD5 hashes start with the magic sequence `$1$`, followed by an up to 8 character salt. DES hashes start with a 2 character salt. Note that a DES hash considers only the first 8 characters. The MD5 considers the whole string.

  Salt and algorithm can be forced by instantiating the start of Encrypted with it. This is typically used to force MD5 hashes:

      ?- phrase("$1$", E, _),
         crypt("My password", E),
         format('~s~n', [E]).

      $1$qdaDeDZn$ZUxSQEESEHIDCHPNc3fxZ1

  Encrypted is always a list of ASCII character codes. Plain only supports ISO-Latin-1 passwords in the current implementation.

  Plain is either an atom, SWI-Prolog string, list of characters or list of character-codes. It is not advised to use atoms, as this implies the password will be available from the Prolog heap as a defined atom.

  NOTE: crypt/2 provides an interface to the Unix password hashing API. Above we already introduced support for classical DES and MD5 hashes, both hashes that are considered insecure by today's standards.3Insecure means that the password can realistically be derived from the password hash using a brute-force attack. This implies that leaking the password database is an immediate security risk. The crypt() API of modern Unix systems typically support more secure hashes. Using crypt/2 is suitable if compatibility with OS passwords is required. If strong hashes and platform independence are important to you, use crypto_password_hash/2 provided by library `library(crypto)` from the ssl package.

## 11 SHA* Secure Hash Algorithms

The library `library(sha)` provides Secure Hash Algorihms approved by FIPS (Federal Information Processing Standard). Quoting Wikipedia: “The SHA (Secure Hash Algorithm) hash functions refer to five FIPS-approved algorithms for computing a condensed digital representation (known as a message digest) that is, to a high degree of probability, unique for a given input data sequence (the message). These algorithms are called‘secure' because (in the words of the standard), “for a given algorithm, it is computationally infeasible 1) to find a message that corresponds to a given message digest, or 2) to find two different messages that produce the same message digest. Any change to a message will, with a very high probability, result in a different message digest.''

The current library supports all 5 approved algorithms, both computing the hash-key from data and the hash Message Authentication Code (HMAC).

A general secure hash interface is provided by `library(crypto)`, part of the ssl package.

Input is text, represented as an atom, packed string object or code-list. Note that these functions operate on byte-sequences and therefore are not meaningful on Unicode text. The result is returned as a list of byte-values. This is the most general format that is comfortable supported by standard Prolog and can easily be transformed in other formats. Commonly used text formats are ASCII created by encoding each byte as two hexadecimal digits and ASCII created using base64 encoding. Representation as a large integer can be desirable for computational processing.

sha_hash(+Data, -Hash, +Options)
  Hash is the SHA hash of Data. Data is either an atom, packed string or list of character codes. Hash is unified with a list of bytes (integers in the range 0..255) representing the hash. See hash_atom/2 to convert this into the more commonly seen hexadecimal representation. The conversion is controlled by Options:
  algorithm(+Algorithm)
    One of `sha1` (default), `sha224`, `sha256`, `sha384` or `sha512`
  encoding(+Encoding)
    This option defines the mapping from Prolog (Unicode) text to bytes on which the SHA algorithm is performed. It has two values. The defualt is `utf8`, which implies that Unicode text is encoded as UTF-8 bytes. This option can deal with any atom. The alternative is `octet`, which implies that the text is considered as a sequence of bytes. This is suitable for e.g., atoms that represent binary data. An error is raised if the text contains code-points outside the range 0..255.
hmac_sha(+Key, +Data, -HMAC, +Options)
  Quoting Wikipedia: “A keyed-hash message authentication code, or HMAC, is a type of message authentication code (MAC) calculated using a cryptographic hash function in combination with a secret key. As with any MAC, it may be used to simultaneously verify both the data integrity and the authenticity of a message. Any iterative cryptographic hash function, such as MD5 or SHA-1, may be used in the calculation of an HMAC; the resulting MAC algorithm is termed HMAC-MD5 or HMAC-SHA-1 accordingly. The cryptographic strength of the HMAC depends upon the cryptographic strength of the underlying hash function, on the size and quality of the key and the size of the hash output length in bits.''

  Key and Data are either an atom, packed string or list of character codes. HMAC is unified with a list of integers representing the authentication code. Options is the same as for sha_hash/3, but currently only `sha1` and `sha256` are supported.

hash_atom(+Hash, -HexAtom)
  True when HexAtom is the commonly used hexadecimal encoding of the hash code. E.g.,

      ?- sha_hash('SWI-Prolog', Hash, []),
         hash_atom(Hash, Hex).
      Hash = [61, 128, 252, 38, 121, 69, 229, 85, 199|...],
      Hex = '3d80fc267945e555c730403bd0ab0716e2a68c68'.

### 11.1 License terms

The underlying SHA-2 library is an unmodified copy created by Dr Brian Gladman, Worcester, UK. It is distributed under the license conditions below.

The free distribution and use of this software in both source and binary form is allowed (with or without changes) provided that:

1. distributions of this source code include the above copyright notice, this list of conditions and the following disclaimer;

2. distributions in binary form include the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other associated materials;

3. the copyright holder's name is not used to endorse products built using this software without specific written permission.

ALTERNATIVELY, provided that this notice is retained in full, this product may be distributed under the terms of the GNU General Public License (GPL), in which case the provisions of the GPL apply INSTEAD OF those given above.

## 12 library(md5): MD5 hashes

See also
  `library(sha)`, `library(hash_stream)` and `library(crypto)`.

Compute MD5 hashes from a Prolog string. This library provides a lightweight alternative to the general secure hash interface provided by `library(crypto)` from the `ssl` package.

[det]md5_hash(+Data, -Hash, +Options)
  Hash is the MD5 hash of Data, The conversion is controlled by Options:
  encoding(+Encoding)
    If Data is a sequence of character codes, this must be translated into a sequence of bytes, because that is what the hashing requires. The default encoding is `utf8`. The other meaningful value is `octet`, claiming that Data contains raw bytes.

  Data | is either an atom, string, code-list or char-list.
  Hash | is an atom holding 32 characters, representing the hash in hexadecimal notation

## 13 library(hash_stream): Maintain a hash on a stream

See also
  In addition to this hash library, SWI-Prolog provides `library(md5)`, `library(sha)` and hash functions through `library(crypto)`, part of the `ssl` package.

This library defines a filter stream that maintains a hash of the data that passes through the stream. It can be used to compute the hash of input data while it is being processed. This is notably interesting if data is processed from a socket as it avoids the need for collecting the data first in a temporary file.

A typical processing sequence is illustrated below, where process/2 somehow processed the data and save_result/3 records the result as obtained from URL with content digest SHA256 its Result.

        ...,
        http_open(URL, In0, []),
        open_hash_stream(In0, In, [algorithm(sha256)]),
        process(In, Result),
        stream_hash(In, SHA256),
        close(In),
        save_result(URL, SHA256, Result)

This library can also be used to compute the hash for the content of a file. The advantage is that this code doesn't rely on external tools. It is considerably faster for short files, but considerably slower on large files because Prolog I/O is based on character streams rather than blocks.

    file_hash(Algorithm, File, Hash) :-
        setup_call_cleanup(
            open(File, read, In0, [type(binary)]),
            setup_call_cleanup(
                open_hash_stream(In0, In,
                                 [ algorithm(Algorithm),
                                   close_parent(false)
                                 ]),
                ( setup_call_cleanup(
                      open_null_stream(Null),
                      copy_stream_data(In, Null),
                      close(Null)),
                  stream_hash(In, Hash)
                ),
                close(In)),
            close(In0)).

[det]open_hash_stream(+OrgStream, -HashStream, +Options)
  Open a filter stream on OrgStream that maintains a hash. The hash can be retrieved at any time using stream_hash/2. Provided options:
  algorithm(+Algorithm)
    One of `md5`, `sha1`, `sha224`, `sha256`, `sha384` or `sha512`. Default is `sha1`.
  close_parent(+Bool)
    If `true` (default), closing the filter stream also closes the original (parent) stream.
[det]stream_hash(+HashStream, -Digest:atom)
  Unify Digest with a hash for the bytes send to or read from HashStream. Note that the hash is computed on the stream buffers. If the stream is an output stream, it is first flushed and the Digest represents the hash at the current location. If the stream is an input stream the Digest represents the hash of the processed input including the already buffered data.

## 14 Memory files

The `library(memfile)` provides an alternative to temporary files, intended for temporary buffering of data. Memory files in general are faster than temporary files and do not suffer from security risks or naming conflicts associated with temporary-file management.

There is no limit to the number of memory streams, nor the size of them. However, a single memory file cannot have multiple streams at the same time, i.e., a memory file cannot be opened multiple times, not even for reading. Memory files are thread-safe and subject to (atom) garbage collection.

These predicates are first of all intended for building higher-level primitives such as open_codes_stream/3. See also format/3, atom_to_term/3, term_to_atom/2, term_string/2, etc.

new_memory_file(-Handle)
  Create a new memory file and return a unique opaque handle to it.
free_memory_file(+Handle)
  Discard the memory file and its contents. If the file is open it is first closed.
open_memory_file(+Handle, +Mode, -Stream)
  Open the memory-file. Mode is one of `read`, `write`, `append`, `update` or `insert`. The resulting Stream must be closed using close/1. When opened for `update` or `insert`, the current location is initialized at the start of the data and can be modified using seek/2 or set_stream_position/2. In `update` mode, existing content is replaced, while the size is enlarged after hitting the end of the data. In `insert` mode, the new data is inserted at the current point.
open_memory_file(+Handle, +Mode, -Stream, +Options)
  Open a memory-file as open_memory_file/3. Options:
  encoding(+Encoding)
    Set the encoding for a memory file and the created stream. Encoding names are the same as used with open/4. By default, memoryfiles represent UTF-8 streams, making them capable of storing arbitrary Unicode text. In practice the only alternative is `octet`, turning the memoryfile into binary mode. Please study SWI-Prolog Unicode and encoding issues before using this option.
  free_on_close(+Bool)
    If `true` (default `false` and the memory file is opened for reading, discard the file (see free_memory_file/1) if the input is closed. This is used to realise open_chars_stream/2 in library(charsio).
size_memory_file(+Handle, -Size)
  Return the content-length of the memory-file in characters in the current encoding of the memory file. The file should be closed and contain data.
size_memory_file(+Handle, -Size, +Encoding)
  Return the content-length of the memory-file in characters in the given Encoding. The file should be closed and contain data.
atom_to_memory_file(+Atom, -Handle)
  Turn an atom into a read-only memory-file containing the (shared) characters of the atom. Opening this memory-file in mode `write` yields a permission error.
insert_memory_file(+Handle, +Offset, +Data)
  Insert Data into the memory file at location Offset. The offset is specified in characters. Data can be an atom, string, code or character list. Other terms are first serialized using writeq/1. This predicate raises a domain_error exception if Offset is out of range and a permission_error if the memory file is read-only or opened.
delete_memory_file(+Handle, +Offset, +Length)
  Delete a Length characters from the memory file, starting at Offset. This predicate raises a domain_error exception if Offset or Offset+Length is out of range and a permission_error if the memory file is read-only or opened.
memory_file_to_atom(+Handle, -Atom)
  Return the content of the memory-file in Atom.
memory_file_to_atom(+Handle, -Atom, +Encoding)
  Return the content of the memory-file in Atom, pretending the data is in the given Encoding. This can be used to convert from one encoding into another, typically from/to bytes. For example, if we must convert a set of bytes that contain text in UTF-8, open the memory file as octet stream, fill it, and get the result using Encoding is `utf8`.
memory_file_to_codes(+Handle, -Codes)
  Return the content of the memory-file as a list of character-codes in Codes.
memory_file_to_codes(+Handle, -Codes, +Encoding)
  Return the content of the memory-file as a list of character-codes in Codes, pretending the data is in the given Encoding.
memory_file_to_string(+Handle, -String)
  Return the content of the memory-file as a string in -String.
memory_file_to_string(+Handle, -String, +Encoding)
  Return the content of the memory-file as a string in String, pretending the data is in the given Encoding.
memory_file_substring(+Handle, ?Before, ?Length, ?After, -SubString)
  SubString is a substring of the memory file. There are Before characters in the memory file before SubString, SubString contains Length character and is followed by After characters in the memory file. The signature is the same as sub_string/5 and sub_atom/5, but currently at least two of the 3 position arguments must be specified. Future versions might implement the full functionality of sub_string/5.
memory_file_line_position(+MF, ?Line, ?LinePos, ?Offset)
  True if the character offset Offset corresponds with the LinePos character on line Line. Lines are counted from one (1). Note that LinePos is not the column as each character counts for one, including backspace and tab.

## 15 Time and alarm library

This library is not available if SWI-Prolog is compiled without support for multiple threads.

The `library(time)` provides timing and alarm functions. Alarms are thread-specific, i.e., creating an alarm causes the alarm goal to be called in the thread that created it. The predicate current_alarm/4 only reports alarms that are related to the calling thread. If a thread terminates, all remaining alarms are silently removed. Most applications use call_with_time_limit/2.

alarm(+Time, :Callable, -Id, +Options)
  Schedule Callable to be called Time seconds from now. Time is a number (integer or float). Callable is called on the next pass through a call- or redo-port of the Prolog engine, or a call to the PL_handle_signals() routine from SWI-Prolog. Id is unified with a reference to the timer.

  The resolution of the alarm depends on the underlying implementation, which is based on pthread_cond_timedwait() (on Windows on the pthread emulation thereof). Long-running foreign predicates that do not call PL_handle_signals() may further delay the alarm. The relation to blocking system calls (sleep, reading from slow devices, etc.) is undefined and varies between implementations.

  Options is a list of `Name(Value)` terms. Defined options are:

  remove(Bool)
    If `true` (default `false`), the timer is removed automatically after fireing. Otherwise it must be destroyed explicitly using remove_alarm/1.
  install(Bool)
    If `false` (default `true`), the timer is allocated but not scheduled for execution. It must be started later using install_alarm/1.
alarm(+Time, :Callable, -Id)
  Same as `alarm(Time, Callable, Id,[])`.
alarm_at(+Time, :Callable, -Id, +Options)
  as alarm/3, but Time is the specification of an absolute point in time. Absolute times are specified in seconds after the Jan 1, 1970 epoch. See also date_time_stamp/2.
install_alarm(+Id)
  Activate an alarm allocated using alarm/4 with the option `install(false)` or stopped using uninstall_alarm/1.
install_alarm(+Id, +Time)
  As install_alarm/1, but specifies a new (relative) timeout value.
uninstall_alarm(+Id)
  Deactivate a running alarm, but do not invalidate the alarm identifier. Later, the alarm can be reactivated using either install_alarm/1 or install_alarm/2. Reinstalled using install_alarm/1, it will fire at the originally scheduled time. Reinstalled using install_alarm/2 causes the alarm to fire at the specified time from now.
remove_alarm(+Id)
  Remove an alarm. If it is not yet fired, it will not be fired any more.
current_alarm(?At, ?:Callable, ?Id, ?Status)
  Enumerate the not-yet-removed alarms. Status is one of `done` if the alarm has been called, `next` if it is the next to be fired and scheduled otherwise.
call_with_time_limit(+Time, :Goal)
  True if Goal completes within Time seconds. Goal is executed as in once/1. If Goal doesn't complete within Time seconds (wall time), exit using the exception `time_limit_exceeded`. See catch/3.

  Please note that this predicate uses alarm/4 and therefore its effect on long-running foreign code and system calls is undefined. Blocking I/O can be handled using the timeout option of set_stream/2 or waiting for input using wait_for_input/3.

## 16 library(unix): Unix specific operations

See also
  `library(process)` provides a portable high level interface to create and manage processes.

The `library(unix)` library provides the commonly used Unix primitives to deal with process management. These primitives are useful for many tasks, including server management, parallel computation, exploiting and controlling other processes, etc.

The predicates in this library are modelled closely after their native Unix counterparts.

[det]fork(-Pid)
  Clone the current process into two branches. In the child, Pid is unified to child. In the original process, Pid is unified to the process identifier of the created child. Both parent and child are fully functional Prolog processes running the same program. The processes share open I/O streams that refer to Unix native streams, such as files, sockets and pipes. Data is not shared, though on most Unix systems data is initially shared and duplicated only if one of the programs attempts to modify the data.

  Unix `fork()` is the only way to create new processes and fork/1 is a simple direct interface to it.

  Errors
    `permission_error(fork, process, main)` is raised if the calling thread is not the only thread in the process. Forking a Prolog process with threads will typically deadlock because only the calling thread is cloned in the fork, while all thread synchronization are cloned.
[det]fork_exec(+Command)
  Fork (as fork/1) and exec (using exec/1) the child immediately. This behaves as the code below, but bypasses the check for the existence of other threads because this is a safe scenario.

      fork_exec(Command) :-
            (   fork(child)
            ->  exec(Command)
            ;   true
            ).

exec(+Command)
  Replace the running program by starting Command. Command is a callable term. The functor is the command and the arguments provide the command-line arguments for the command. Each command-line argument must be atomic and is converted to a string before passed to the Unix call `execvp()`. Here are some examples:

  - `exec(ls('-l'))`
  - `exec('/bin/ls'('-l', '/home/jan'))`

  Unix `exec()` is the only way to start an executable file executing. It is commonly used together with fork/1. For example to start netscape on an URL in the background, do:

      run_netscape(URL) :-
              (    fork(child),
                   exec(netscape(URL))
              ;    true
              ).

  Using this code, netscape remains part of the process-group of the invoking Prolog process and Prolog does not wait for netscape to terminate. The predicate wait/2 allows waiting for a child, while detach_IO/0 disconnects the child as a deamon process.

[det]wait(?Pid, -Status)
  Wait for a child to change status. Then report the child that changed status as well as the reason. If Pid is bound on entry then the status of the specified child is reported. If not, then the status of any child is reported. Status is unified with `exited(ExitCode)` if the child with pid Pid was terminated by calling `exit()` (Prolog halt/1). ExitCode is the return status. Status is unified with `signaled(Signal)` if the child died due to a software interrupt (see kill/2). Signal contains the signal number. Finally, if the process suspended execution due to a signal, Status is unified with `stopped(Signal)`.
[det]kill(+Pid, +Signal)
  Deliver a software interrupt to the process with identifier Pid using software-interrupt number Signal. See also on_signal/2. Signals can be specified as an integer or signal name, where signal names are derived from the C constant by dropping the `SIG` prefix and mapping to lowercase. E.g. `int` is the same as `SIGINT` in C. The meaning of the signal numbers can be found in the Unix manual.
[det]pipe(-InSream, -OutStream)
  Create a communication-pipe. This is normally used to make a child communicate to its parent. After pipe/2, the process is cloned and, depending on the desired direction, both processes close the end of the pipe they do not use. Then they use the remaining stream to communicate. Here is a simple example:

      :- use_module(library(unix)).

      fork_demo(Result) :-
              pipe(Read, Write),
              fork(Pid),
              (   Pid == child
              ->  close(Read),
                  format(Write, '~q.~n',
                         [hello(world)]),
                  flush_output(Write),
                  halt
              ;   close(Write),
                  read(Read, Result),
                  close(Read)
              ).

[det]dup(+FromStream, +ToStream)
  Interface to Unix `dup2()`, copying the underlying filedescriptor and thus making both streams point to the same underlying object. This is normally used together with fork/1 and pipe/2 to talk to an external program that is designed to communicate using standard I/O.

  Both FromStream and ToStream either refer to a Prolog stream or an integer descriptor number to refer directly to OS descriptors. See also `demo/pipe.pl` in the source-distribution of this package.

[det]detach_IO(+Stream)
  This predicate is intended to create Unix deamon processes. It performs two actions.

  1. The I/O streams `user_input`, `user_output` and `user_error` are closed if they are connected to a terminal (see `tty` property in stream_property/2). Input streams are rebound to a dummy stream that returns EOF. Output streams are reboud to forward their output to Stream.
  2. The process is detached from the current process-group and its controlling terminal. This is achieved using `setsid()` if provided or using `ioctl()` `TIOCNOTTY` on `/dev/tty`.

  To ignore all output, it may be rebound to a null stream. For example:

            ...,
            open_null_stream(Out),
            detach_IO(Out).

  The detach_IO/1 should be called only once per process. Subsequent calls silently succeed without any side effects.

  See also
    detach_IO/0 and `library(syslog)`.
[det]detach_IO
  Detach I/O similar to detach_IO/1. The output streams are bound to a file `/tmp/pl-out.<pid>`. Output is line buffered (see set_stream/2).
  See also
    `library(syslog)` allows for sending output to the Unix logging service.
  Compatibility
    Older versions of this predicate only created this file if there was output.
[semidet]sysconf(+Conf)
  Access system configuration. See `sysconf(1)` for details. Conf is a term Config(Value), where Value is always an integer. Config is the `sysconf()` name after removing =_SC_= and conversion to lowercase. Currently support the following configuration info: `arg_max`, `child_max`, `clk_tck`, `open_max`, `pagesize`, `phys_pages`, `avphys_pages`, `nprocessors_conf` and `nprocessors_onln`. Note that not all values may be supported on all operating systems.

## 17 Limiting process resources

The `library(rlimit)` library provides an interface to the POSIX getrlimit()/setrlimit() API that control the maximum resource-usage of a process or group of processes. This call is especially useful for servers such as CGI scripts and inetd-controlled servers to avoid an uncontrolled script claiming too much resources.

rlimit(+Resource, -Old, +New)
  Query and/or set the limit for Resource. Time-values are in seconds and size-values are counted in bytes. The following values are supported by this library. Please note that not all resources may be available and accessible on all platforms. This predicate can throw a variety of exceptions. In portable code this should be guarded with catch/3. The defined resources are:

  > `as` | Max address space
  > `cpu` | CPU time in seconds
  > `fsize` | Maximum filesize
  > `data` | max data size
  > `stack` | max stack size
  > `core` | max core file size
  > `rss` | max resident set size
  > `nproc` | max number of processes
  > `nofile` | max number of open files
  > `memlock` | max locked-in-memory address

  When the process hits a limit POSIX systems normally send the process a signal that terminates it. These signals may be caught using SWI-Prolog's on_signal/3 primitive. The code below illustrates this behaviour. Please note that asynchronous signal handling is dangerous, especially when using threads. 100% fail-safe operation cannot be guaranteed, but this procedure will inform the user properly‘most of the time'.

      rlimit_demo :-
              rlimit(cpu, _, 2),
              on_signal(xcpu, _, cpu_exceeded),
              ( repeat, fail ).

      cpu_exceeded(_Sig) :-
              format(user_error, 'CPU time exceeded~n', []),
              halt(1).

## 18 library(udp_broadcast): A UDP broadcast proxy

author
  Jeffrey Rosenwald (JeffRose@acm.org), Jan Wielemaker
See also
  `tipc.pl`
license
  BSD-2

SWI-Prolog's broadcast library provides a means that may be used to facilitate publish and subscribe communication regimes between anonymous members of a community of interest. The members of the community are however, necessarily limited to a single instance of Prolog. The UDP broadcast library removes that restriction. With this library loaded, any member on your local IP subnetwork that also has this library loaded may hear and respond to your broadcasts.

This library support three styles of networking as described below. Each of these networks have their own advantages and disadvantages. Please study the literature to understand the consequences.

broadcast
  Broadcast messages are sent to the LAN subnet. The broadcast implementation uses two UDP ports: a public to address the whole group and a private one to address a specific node. Broadcasting is generally a good choice if the subnet is small and traffic is low.
unicast
  Unicast sends copies of packages to known peers. Unicast networks can easily be routed. The unicast version uses a single UDP port per node. Unicast is generally a good choice for a small party, in particular if the peers are in different networks.
multicast
  Multicast is like broadcast, but it can be configured to work accross networks and may work more efficiently on VLAN networks. Like the broadcast setup, two UDP ports are used. Multicasting can in general deliver the most efficient LAN and WAN networks, but requires properly configured routing between the peers.

After initialization and, in the case of a unicast network managing the set of peers, communication happens through broadcast/1, broadcast_request/1 and listen/1,2,3.

A broadcast/1 or broadcast_request/1 of the shape `udp(Scope, Term)` or `udp(Scope, Term, TimeOut)` is forwarded over the UDP network to all peers that joined the same Scope. To prevent the potential for feedback loops, only the plain Term is broadcasted locally. The timeout is optional. It specifies the amount to time to wait for replies to arrive in response to a broadcast_request/1. The default period is 0.250 seconds. The timeout is ignored for broadcasts.

An example of three separate processes cooperating in the same scope called `peers`:

    Process A:

       ?- listen(number(X), between(1, 5, X)).
       true.

       ?-

    Process B:

       ?- listen(number(X), between(7, 9, X)).
       true.

       ?-

    Process C:

       ?- findall(X, broadcast_request(udp(peers, number(X))), Xs).
       Xs = [1, 2, 3, 4, 5, 7, 8, 9].

       ?-

It is also possible to carry on a private dialog with a single responder. To do this, you supply a compound of the form, Term:PortId, to a UDP scoped broadcast/1 or broadcast_request/1, where PortId is the ip-address and port-id of the intended listener. If you supply an unbound variable, PortId, to broadcast_request, it will be unified with the address of the listener that responds to Term. You may send a directed broadcast to a specific member by simply providing this address in a similarly structured compound to a UDP scoped broadcast/1. The message is sent via unicast to that member only by way of the member's broadcast listener. It is received by the listener just as any other broadcast would be. The listener does not know the difference.

For example, in order to discover who responded with a particular value:

    Host B Process 1:

       ?- listen(number(X), between(1, 5, X)).
       true.

       ?-

    Host A Process 1:

       ?- listen(number(X), between(7, 9, X)).
       true.

       ?-

    Host A Process 2:

       ?- listen(number(X), between(1, 5, X)).
       true.

       ?- bagof(X, broadcast_request(udp(peers,number(X):From,1)), Xs).
       From = ip(192, 168, 1, 103):34855,
       Xs = [7, 8, 9] ;
       From = ip(192, 168, 1, 103):56331,
       Xs = [1, 2, 3, 4, 5] ;
       From = ip(192, 168, 1, 104):3217,
       Xs = [1, 2, 3, 4, 5].

All incomming trafic is handled by a single thread with the alias `udp_inbound_proxy`. This thread also performs the internal dispatching using broadcast/1 and broadcast_request/1. Future versions may provide for handling these requests in separate threads.

### 18.1 Caveats

While the implementation is mostly transparent, there are some important and subtle differences that must be taken into consideration:

- UDP broadcast requires an initialization step in order to launch the broadcast listener proxy. See udp_broadcast_initialize/2.
- Prolog's broadcast_request/1 is nondet. It sends the request, then evaluates the replies synchronously, backtracking as needed until a satisfactory reply is received. The remaining potential replies are not evaluated. With UDP, all peers will send all answers to the query. The receiver may however stop listening.
- A UDP broadcast/1 is completely asynchronous.
- A UDP broadcast_request/1 is partially synchronous. A broadcast_request/1 is sent, then the sender balks for a period of time (default: 250 ms) while the replies are collected. Any reply that is received after this period is silently discarded. A optional second argument is provided so that a sender may specify more (or less) time for replies.
- Replies are presented to the user as a choice point on arrival, until the broadcast request timer finally expires. This allows traffic to propagate through the system faster and provides the requestor with the opportunity to terminate a broadcast request early if desired, by simply cutting choice points.
- Please beware that broadcast request transactions remain active and resources consumed until broadcast_request finally fails on backtracking, an uncaught exception occurs, or until choice points are cut. Failure to properly manage this will likely result in chronic exhaustion of UDP sockets.
- If a listener is connected to a generator that always succeeds (e.g. a random number generator), then the broadcast request will never terminate and trouble is bound to ensue.
- broadcast_request/1 with `udp_subnet` scope is not reentrant. If a listener performs a broadcast_request/1 with UDP scope recursively, then disaster looms certain. This caveat does not apply to a UDP scoped broadcast/1, which can safely be performed from a listener context.
- UDP broadcast's capacity is not infinite. While it can tolerate substantial bursts of activity, it is designed for short bursts of small messages. Unlike TIPC, UDP is unreliable and has no QOS protections. Congestion is likely to cause trouble in the form of non-Byzantine failure. That is, late, lost (e.g. infinitely late), or duplicate datagrams. Caveat emptor.
- A UDP broadcast_request/1 term that is grounded is considered to be a broadcast only. No replies are collected unless the there is at least one unbound variable to unify.
- A UDP broadcast/1 always succeeds, even if there are no listeners.
- A UDP broadcast_request/1 that receives no replies will fail.
- Replies may be coming from many different places in the network (or none at all). No ordering of replies is implied.
- Prolog terms are sent to others after first converting them to atoms using term_string/3. Serialization does not deal with cycles, attributes or sharing. The hook udp_term_string_hook/3 may be defined to change the message serialization and support different message formats and/or encryption.
- The broadcast model is based on anonymity and a presumption of trust--a perfect recipe for compromise. UDP is an Internet protocol. A UDP broadcast listener exposes a public port, which is static and shared by all listeners, and a private port, which is semi-static and unique to the listener instance. Both can be seen from off-cluster nodes and networks. Usage of this module exposes the node and consequently, the cluster to significant security risks. So have a care when designing your application. You must talk only to those who share and contribute to your concerns using a carefully prescribed protocol.
- UDP broadcast categorically and silently ignores all message traffic originating from or terminating on nodes that are not members of the local subnet. This security measure only keeps honest people honest!

udp_broadcast_close(+Scope)
  Close a UDP broadcast scope.
[semidet]udp_broadcast_initialize(+IPAddress, +Options)
  Initialized UDP broadcast bridge. IPAddress is the IP address on the network we want to broadcast on. IP addresses are terms `ip(A,B,C,D)` or an atom or string of the format `A.B.C.D`. Options processed:
  scope(+ScopeName)
    Name of the scope. Default is `subnet`.
  subnet_mask(+SubNet)
    Subnet to broadcast on. This uses the same syntax as IPAddress. Default classifies the network as class A, B or C depending on the the first octet and applies the default mask.
  port(+Port)
    Public port to use. Default is 20005.
  method(+Method)
    Method to send a message to multiple peers. One of
    broadcast
      Use UDP broadcast messages to the LAN. This is the default
    multicast
      Use UDP multicast messages. This can be used on WAN networks, provided the intermediate routers understand multicast.
    unicast
      Send the messages individually to all registered peers.

  For compatibility reasons Options may be the subnet mask.

[det]udp_peer_add(+Scope, +Address)
[det]udp_peer_del(+Scope, ?Address)
[nondet]udp_peer(?Scope, ?Address)
  Manage and query the set of known peers for a unicast network. Address is either a term IP:Port or a plain IP address. In the latter case the default port registered with the scope is used.

  Address | has canonical form `ip(A,B,C,D)`:Port.

[det,multifile]udp_term_string_hook(+Scope, +Term, -String)
[semidet,multifile]udp_term_string_hook(+Scope, -Term, +String)
  Hook for serializing the message Term. The default writes `%prolog\n`, followed by the Prolog term in quoted notation while ignoring operators. This hook may use alternative serialization such as fast_term_serialized/2, use `library(ssl)` to realise encrypted messages, etc.

  Scope | is the scope for which the message is broadcasted. This can be used to use different serialization for different scopes.
  Term | encapsulates the term broadcasted by the application as follows: send(ApplTerm) Is sent by `broadcast(udp(Scope, ApplTerm))` request(Id, ApplTerm) Is sent by broadcast_request/1, where Id is a unique large (64 bit) integer. reply(Id, ApplTerm) Is sent to reply on a broadcast_request/1 request that has been received. Arguments are the same as above.

  throws
    The hook may throw `udp(invalid_message)` to stop processing the message.
[semidet,multifile]udp_unicast_join_hook(+Scope, +From, +Data)
  This multifile hook is called if an UDP package is received on the port of the unicast network identified by Scope. From is the origin IP and port and Data is the message data that is deserialized as defined for the scope (see udp_term_string/3).

  This hook is intended to initiate a new node joining the network of peers. We could in theory also omit the in-scope test and use a normal broadcast to join. Using a different channal however provides a basic level of security. A possibe implementation is below. The first fragment is a hook added to the server, the second is a predicate added to a client and the last initiates the request in the client. The excanged term (`join(X)`) can be used to exchange a welcome handshake.

      :- multifile udp_broadcast:udp_unicast_join_hook/3.
      udp_broadcast:udp_unicast_join_hook(Scope, From, join(welcome)) :-
          udp_peer_add(Scope, From),

      join_request(Scope, Address, Reply) :-
          udp_peer_add(Scope, Address),
          broadcast_request(udp(Scope, join(X))).

      ?- join_request(myscope, "1.2.3.4":10001, Reply).
      Reply = welcome.

## 19 library(prolog_stream): A stream with Prolog callbacks

This library defines a Prolog stream that realises its low-level I/O with callbacks to Prolog. The library was developed to bind normal Prolog I/O to Pengines I/O. This type of I/O redirection is probably the primary use case.

open_prolog_stream(+Module, +Mode, -Stream, +Options)
  Create a new stream that implements its I/O by calling predicates in Module. The called predicates are:
  Module`:``stream_write(+Stream, +String)`
    Called for a `Mode = write` stream if data is available. String contains the (textual) data that is written to Stream. The callback is called if the buffer of Stream overflows, the user calls `flush_output(Stream)` or Stream is closed and there is buffered data.
  Module`:``stream_read(+Stream, -Term)`
    Called for a `Mode == read` stream to get new data. On success the stream extracts text from the provided Term. Term is typically a string, atom, code or character list. If term is not one of the above, it is handed to writeq/1. To signal end-of-file, unify stream with an empty text, e.g., `stream_read(Stream, "")`.
  Module`:``stream_close(+Stream)`
    Called when the stream is closed. This predicate must succeed. The callback can be used to cleanup associated resources.

  The current implementation only deals with text streams. The stream uses the `wchar_t` encoding. The buffer size must be a multiple of `wchar_t`, i.e., a multiple of four for portability. The newline mode of the stream is `posix` on all platforms, disabling the translation `"\n" --> "\r\n"`.

  Options | is currently ignored.

  bug
    Futher versions might require additional callbacks. As we demand all callbacks to be defined, existing code needs to implement the new callbacks.

## NetBSD Crypt license

     * Copyright (c) 1989, 1993
     *      The Regents of the University of California.  All rights reserved.
     *
     * This code is derived from software contributed to Berkeley by
     * Tom Truscott.
     *
     * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     * 3. Neither the name of the University nor the names of its contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.

# Index

?
add_stream_to_pool/2
  7
alarm/3
  15
alarm/4
  15 15
alarm_at/4
atom_to_memory_file/2
atom_to_term/3
  14
call_with_time_limit/2
  15
catch/3
  15 17
cgi_get_form/1
  9.1
chmod/2
close/1
  14
close_stream_pool/0
  7
closelog/0
copy_directory/2
copy_file/2
crypt/2
  10 10 10
crypto_password_hash/2
  10
current_alarm/4
  15
date_time_stamp/2
  15
delete_directory_and_contents/1
delete_directory_contents/1
delete_memory_file/3
delete_stream_from_pool/1
directory_file_path/3
directory_member/3
dispatch_stream_pool/1
  7
dup/2
exec/1
fork/1
fork_exec/1
format/2
  9.1
format/3
  14
free_memory_file/1
  14
getegid/1
geteuid/1
getgid/1
getgroups/1
gethostname/1
getuid/1
group_data/3
group_info/2
hash_atom/2
  11
hmac_sha/4
initgroups/2
insert_memory_file/3
install_alarm/1
  15 15 15 15
install_alarm/2
  15 15
ip/4
  6.6 6.6
iri_normalized/2
iri_normalized/3
is_process/1
kill/2
link_file/3
make_directory_path/1
md5_hash/3
memory_file_line_position/4
memory_file_substring/5
memory_file_to_atom/2
memory_file_to_atom/3
memory_file_to_codes/2
memory_file_to_codes/3
memory_file_to_string/2
memory_file_to_string/3
negotiate_socks_connection/2
new_memory_file/1
on_signal/3
  17
once/1
  15
open/4
  14
open_chars_stream/2
  14
open_codes_stream/3
  14
open_hash_stream/3
open_memory_file/3
  14
open_memory_file/4
open_prolog_stream/4
openlog/3
pipe/2
process_create/3
process_group_kill/1
process_group_kill/2
process_id/1
process_id/2
process_kill/1
process_kill/2
process_release/1
process_set_method/1
process_wait/2
process_wait/3
prolog:debug_print_hook/3
proxy_for_url/3
relative_file_name/3
remove_alarm/1
  15
rlimit/3
seek/2
  14
set_stream/2
  15
set_stream_position/2
  14
set_time_file/3
set_user_and_group/1
set_user_and_group/2
setegid/1
seteuid/1
setgid/1
setgroups/1
setuid/1
sha_hash/3
  11
size_memory_file/2
size_memory_file/3
stream_hash/2
stream_pool_main_loop/0
string_to_list/2
  6.6
sub_atom/5
  14
sub_string/5
  14 14
sysconf/1
syslog/2
syslog/3
tcp_accept/3
  6.5
tcp_bind/2
  6.5
tcp_close_socket/1
tcp_connect/2
  6.5
tcp_connect/3
tcp_connect/4
tcp_fcntl/3
tcp_getopt/2
tcp_host_to_address/2
tcp_listen/2
tcp_open_socket/2
tcp_open_socket/3
tcp_select/3
tcp_setopt/2
tcp_socket/1
  6.6
term_string/2
  14
term_to_atom/2
  14
try_proxy/4
udp_broadcast_close/1
udp_broadcast_initialize/2
udp_peer/2
udp_peer_add/2
udp_peer_del/2
udp_receive/4
udp_send/4
udp_socket/1
udp_term_string_hook/3
udp_unicast_join_hook/3
uninstall_alarm/1
  15
unix_domain_socket/1
uri_authority_components/2
uri_authority_data/3
uri_components/2
uri_data/3
uri_data/4
uri_encoded/3
uri_file_name/2
uri_iri/2
uri_is_global/1
uri_normalized/2
uri_normalized/3
uri_normalized_iri/2
uri_normalized_iri/3
uri_query_components/2
uri_resolve/3
user_data/3
user_info/2
wait/2
wait_for_input/3
  6.6 7 7 15
writeq/1
  14
detach_IO/0
detach_IO/1