The libexif library

0.6.17.1

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 integrate make use of libexif-uninstalled.pc.

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 of 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.

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 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.

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