Le septidi 7 fructidor, an CCXXIV, Ronald S. Bultje a écrit : > On Tue, Apr 21, 2015 at 9:43 AM, Nicolas George <geo...@nsup.org> wrote: > > Le duodi 2 floréal, an CCXXIII, wm4 a écrit : > > > Isn't the problem that realloc() doesn't keep the alignment? > > No. That problem exists too > > > That seems counter-intuitive. The problem does not exist, but it does exist > too? > > If you're saying that "the problem exists, but a user of av_realloc should > be aware that the alignment will be lost", that's fine.
I do not remember the exact terms of the discussion. There are two issues to be aware of: 1. Memory obtained from av_malloc() should not be used with av_realloc(). 2. Memory obtained from av_realloc() is not aligned enough. I do not remember which issue was "the" "problem" in the discussion. Issue #1 is treacherous because it will almost always work. It comes from POSIX, that says about realloc "If ptr does not match a pointer returned earlier by calloc(), malloc(), or realloc() [...] the behavior is undefined.": av_malloc() uses posix_memalign() when available, and posix_memalign() is neither calloc(), malloc() nor realloc(), and POSIX neglected to include a posix_realloc_memalign(). Still, with most implementations, it will work, because "it works" is an acceptable undefined behavior; valgrind does not even report it. I do not remember if someone found an actual libc where it did not work, or if we had trouble with MEMALIGN_HACK between av_malloc() and av_realloc(), or if it is purely theoretical. Issue #2 can be relevant when ignoring issue #1; it was the cause of this: https://trac.ffmpeg.org/ticket/1692 HtH. Regards, -- Nicolas George
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