On Wed, Jul 03, 2019 at 12:50:59PM +0200, Hendrik Leppkes wrote: > On Wed, Jul 3, 2019 at 10:46 AM Michael Niedermayer > <mich...@niedermayer.cc> wrote: > > > > On Wed, Jul 03, 2019 at 09:41:41AM +0200, Reimar Döffinger wrote: > > > > > > > > > On 03.07.2019, at 08:29, Michael Niedermayer <mich...@niedermayer.cc> > > > wrote: > > > > > > > On Tue, Jul 02, 2019 at 08:42:43PM -0300, James Almer wrote: > > > >> On 7/2/2019 5:56 PM, Michael Niedermayer wrote: > > > >>> Signed-off-by: Michael Niedermayer <mich...@niedermayer.cc> > > > >>> --- > > > >>> doc/APIchanges | 3 +++ > > > >>> libavutil/mem.h | 13 +++++++++++++ > > > >>> libavutil/version.h | 2 +- > > > >>> 3 files changed, 17 insertions(+), 1 deletion(-) > > > >>> > > > >>> diff --git a/doc/APIchanges b/doc/APIchanges > > > >>> index b5fadc2a48..65b8ed74ad 100644 > > > >>> --- a/doc/APIchanges > > > >>> +++ b/doc/APIchanges > > > >>> @@ -15,6 +15,9 @@ libavutil: 2017-10-21 > > > >>> > > > >>> API changes, most recent first: > > > >>> > > > >>> +2019-07-XX - XXXXXXXXXX - lavu 56.31.100 - mem.h > > > >>> + Add av_memcpy() > > > >>> + > > > >>> 2019-06-21 - XXXXXXXXXX - lavu 56.30.100 - frame.h > > > >>> Add FF_DECODE_ERROR_DECODE_SLICES > > > >>> > > > >>> diff --git a/libavutil/mem.h b/libavutil/mem.h > > > >>> index 5fb1a02dd9..a35230360d 100644 > > > >>> --- a/libavutil/mem.h > > > >>> +++ b/libavutil/mem.h > > > >>> @@ -506,6 +506,19 @@ void *av_memdup(const void *p, size_t size); > > > >>> */ > > > >>> void av_memcpy_backptr(uint8_t *dst, int back, int cnt); > > > >>> > > > >>> +/** > > > >>> + * memcpy() implementation without a NULL pointer special case > > > >>> + * > > > >>> + * @param dst Destination buffer > > > >>> + * @param src Source buffer > > > >>> + * @param cnt Number of bytes to copy; must be >= 0 > > > >>> + */ > > > >>> +static inline void av_memcpy(void *dst, const void *src, size_t cnt) > > > >> > > > >> How many cases are there in the codebase where cnt can be 0, and dst or > > > >> src NULL, without it having been checked before calling memcpy? And > > > >> from > > > >> those, which would not be from situations where the code should have > > > >> instead aborted and returned ENOMEM, or EINVAL if either of them are > > > >> function arguments? > > > > > > > > There are around 2500 occurances of memcpy in the codebase > > > > To awnser your question it would be needed to review all of them and in > > > > many > > > > cases their surrounding code. > > > > So that is unlikely to be awnsered by anyone accuratly > > > > > > > > Also iam not sure i understand why you ask or why this would matter > > > > the suggested function allows to simplify cases where the NULL can > > > > occur, not where it cannot or should not. That is this is intended for > > > > the cases where we already have or are adding explicit checks to > > > > avoid the NULL case. > > > > > > > > i could rename this to av_memcpy_nullsafe which would make it clearer > > > > but > > > > also its more to write and read > > > > > > I admit I thought that a worthwhile idea originally, > > > but I have to think back to a long time ago that every function added to > > > our "API" has a cost of people having to know about it and how to use it. > > > And if it's currently only 2 places that would benefit I think James is > > > right to ask if it makes sense. > > > Of course another question might be if it might make sense to replace all > > > memcpy uses with it. > > > I mean, isn't it naturally expected behaviour that the pointers would be > > > ignored if the copy amount is 0? There might be a lot of code assuming > > > that we do not know about... > > > > in addition to the 2 there are these related commits found by very dumb git > > log greps > > In further addition there would be cases that deal with src == dst, > > something we > > could add a check for in av_memcpy() too > > > > Personally I really don't like hiding too much magic in a function > like this. It can easily lead to brittle code, someone may think the > pointer is always valid since its memcpy'ed to, and uses it > afterwards, and there you have a disaster.
Why would someone think that a pointer to an array of length 0 is valid ? malloc(0) for example does not gurantee that > Either the function name should make perfectly clear what it does, or > preferably it should just not exist and code should just validate its > stuff. what name would you suggest ? but we can just drop this and add the checks to memcpy() where needed i was just trying to make all the undefined behaviour fixes a bit less ugly as multiple people complained ... thanks [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB Asymptotically faster algorithms should always be preferred if you have asymptotical amounts of data
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