mem: Document the non-compatibility of av_realloc() and av_malloc()

libavutil/mem.h

@@ -106,8 +106,12 @@ av_alloc_size(1, 2) static inline void *av_malloc_array(size_t nmemb, size_t siz
  * reallocated.
  * @return Pointer to a newly-reallocated block or NULL if the block
  * cannot be reallocated or the function is used to free the memory block.
- * @note av_realloc() is not guaranteed to maintain the alignment of
- *       pointers originating from the av_malloc() family of functions.
+ * @warning Pointers originating from the av_malloc() family of functions must
+ *          not be passed to av_realloc(). The former can be implemented using
+ *          memalign() (or other functions), and there is no guarantee that
+ *          pointers from such functions can be passed to realloc() at all.
+ *          The situation is undefined according to POSIX and may crash with
+ *          some libc implementations.
  * @see av_fast_realloc()
  */
 void *av_realloc(void *ptr, size_t size) av_alloc_size(2);
@@ -122,8 +126,12 @@ void *av_realloc(void *ptr, size_t size) av_alloc_size(2);
  * @param size Size of the single element
  * @return Pointer to a newly-reallocated block or NULL if the block
  * cannot be reallocated or the function is used to free the memory block.
- * @note av_realloc_array() is not guaranteed to maintain the alignment of
- *       pointers originating from the av_malloc() family of functions.
+ * @warning Pointers originating from the av_malloc() family of functions must
+ *          not be passed to av_realloc(). The former can be implemented using
+ *          memalign() (or other functions), and there is no guarantee that
+ *          pointers from such functions can be passed to realloc() at all.
+ *          The situation is undefined according to POSIX and may crash with
+ *          some libc implementations.
  */
 av_alloc_size(2, 3) void *av_realloc_array(void *ptr, size_t nmemb, size_t size);
 
@@ -137,8 +145,12 @@ av_alloc_size(2, 3) void *av_realloc_array(void *ptr, size_t nmemb, size_t size)
  * @param nmemb Number of elements
  * @param size Size of the single element
  * @return Zero on success, an AVERROR error code on failure.
- * @note av_reallocp_array() is not guaranteed to maintain the alignment of
- *       pointers originating from the av_malloc() family of functions.
+ * @warning Pointers originating from the av_malloc() family of functions must
+ *          not be passed to av_realloc(). The former can be implemented using
+ *          memalign() (or other functions), and there is no guarantee that
+ *          pointers from such functions can be passed to realloc() at all.
+ *          The situation is undefined according to POSIX and may crash with
+ *          some libc implementations.
  */
 av_alloc_size(2, 3) int av_reallocp_array(void *ptr, size_t nmemb, size_t size);