Are the av_free* functions in ffmpeg idempotent?

Question

Specifically I'm looking at avformat_free_context. In normal execution of my code, avformat_free_context is called appropriately. I would also like to call it in the destructor of a class, in case there was an exceptional case that caused the code using the AVFormatContext to return early. Do the ffmpeg free functions behave appropriately when called on already freed data structures?

Answer 1

No, not all of them are.

Links are to function definitions; see for yourself!

av_freep

avformat_free_context calls av_freep on some pointers held by the AVFormatContext which doesn't perform a NULL check before passing the address of the memory to be freed off to av_free (which is essentially just a call to free; see below). It does, however, set the pointer to NULL.

av_free

If ffmpeg was compiled with CONFIG_MEMALIGN_HACK defined, av_free will do a NULL check before freeing. This will effectively make av_freep idempotent for memory which is freed by av_freep(&pointer_to_data);. This might not be the case for your build.

others

Additionally, some of the other functions called by avformat_free_context are idempotent. They achieve this by clearing pointers and performing NULL checks, or decreasing array indexes to prevent a double free. Examples include ff_free_stream and av_opt_free.

avformat_free_context

avformat_free_context will perform a NULL check before attempting to free the context. So if you pass NULL to it, you don't have to worry. However, if you double-free an AVFormatContext you will end up doing a low-level double-free, because avformat_free_context calls av_free directly on the AVFormatContext.

Advice

The safest thing to do when freeing a context is:

avformat_free_context(pcontext);
pcontext = NULL;