Thread safety

Exiv2 heavily relies on standard C++ containers. Static or global variables are used read-only, with the exception of the XMP namespace registration function (see below). Thus Exiv2 is thread safe in the same sense as C++ containers:
  • Different instances of the same class can safely be used concurrently in multiple threads.
  • In order to use the same instance of a class concurrently in multiple threads the application must serialize all write access to the object.

The level of thread safety within Exiv2 varies depending on the type of metadata: The Exif and IPTC code is reentrant. The XMP code uses the Adobe XMP toolkit (XMP SDK), which according to its documentation is thread-safe. It actually uses mutexes to serialize critical sections. However, the XMP SDK initialization function is not mutex protected, thus Exiv2::XmpParser::initialize is not thread-safe. In addition, Exiv2::XmpProperties::registerNs writes to a static class variable, and is also not thread-safe.

Therefore, multi-threaded applications need to ensure that these two XMP functions are serialized, e.g., by calling them from an initialization section which is run before any threads are started.

See also: Issue #439

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