EXIF library (libexif) API: The libexif library

EXIF library (libexif) API 0.6.23

The libexif library

# General Notes

This documentation is work in progress, as is the code itself.

# Using libexif

#include <libexif/exif-data.h>

libexif provides a libexif.pc file for use with pkgconfig on the libexif installation. If you are using libtool to build your package, you can also make use of libexif-uninstalled.pc.

An application using libexif would typically first create an ExifLoader to load EXIF data into memory. From there, it would extract that data as an ExifData to start manipulating it. Each IFD is represented by its own ExifContent within that ExifData, which contains all the tag data in ExifEntry form. If the MakerNote data is required, an ExifMnoteData can be extracted from the ExifData and manipulated with the MakerNote functions.

libexif is written in C using an object-based style that defines sets of functions that operate on each data structure.

# Primary Data Structures

ExifLoader State maintained by the loader interface while importing EXIF data from an external file or memory

ExifData The entirety of EXIF data found in an image

ExifContent All EXIF tags in a single IFD

ExifEntry Data found in a single EXIF tag

ExifMnoteData All data found in the MakerNote tag

ExifLog State maintained by the logging interface

# String Conventions

Strings are 8 bit characters ("char*"). When libexif is compiled with NLS, character set and encoding are as set in the current locale, except for strings that come directly from the data in EXIF tags which are generally returned in raw form. Most EXIF strings are defined to be plain 7-bit ASCII so this raw form should be acceptable in any UNIX locale, but some software ignores the specification and writes 8-bit characters. It is up to the application to detect this and deal with it intelligently.

# Memory Management Patterns

For pointers to data objects, libexif uses reference counting. The pattern is to use the foo_new() function to create a data object, foo_ref() to increase the reference counter, and foo_unref() to decrease the reference counter and possibly free(3)ing the memory.

Libexif by default relies on the calloc(3), realloc(3), and free(3) functions, but the libexif user can tell libexif to use their special memory management functions at runtime.

# Thread Safety

libexif is thread safe when the underlying C library is also thread safe. Some C libraries may require defining a special macro (like _REENTRANT) to ensure this, or may require linking to a special thread-safe version of the library.

The programmer must ensure that each object allocated by libexif is only used in a single thread at once. For example, an ExifData* allocated in one thread can't be used in a second thread if there is any chance that the first thread could use it at the same time. Multiple threads can use libexif without issues if they never share handles.