Tomas Härdin: > ons 2019-08-21 klockan 11:04 +0200 skrev Andreas Rheinhardt: >> Using avio_enum_protocols works as follows: One initializes a pointer to >> void and gives avio_enum_protocols the address of said pointer as >> argument; the pointer will be updated to point to a member of the >> url_protocols array. Now the address of the pointer can be reused for >> another call to avio_enum_protocols. >> Said array consists of constant pointers (to constant URLProtocols), >> but the user now has a pointer to non-const to it; of course it was always >> intended that the user is not allowed to modify what the pointer points >> to and this has been enforced by hiding the real type of the underlying >> object. But it is better to use a const void ** as parameter to enforce >> this. This way avio_enum_protocols can be implemented without resorting >> to casting a const away or ignoring constness as is done currently. >> >> Given that this amounts to an ABI and API break, this can only be done >> at the next major version bump; as usual, the break is currently hidden >> behind an appropriate #if. > > I'm fairly sure this is only an API break. C ABI doesn't care about > constness. But also: > >> @@ -805,7 +805,11 @@ int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer); >> * >> * @return A static string containing the name of current protocol or NULL >> */ >> +#if FF_API_NONCONST_ENUM_PROTOCOLS >> const char *avio_enum_protocols(void **opaque, int output); >> +#else >> +const char *avio_enum_protocols(const void **opaque, int output); >> +#endif > > This should still be perfectly compatible with all user code since > adding const is fine.. > No. void* can be safely and automatically converted to const void*; and the conversion from void** to void * const * is fine, too, but the conversion from void ** to const void ** is not safe for the reason already mentioned. I'll explain it once more. Imagine avio_enum_protocols already used a const void ** parameter and were const-correct and we called the function in the following way:
void *opaque = NULL; avio_enum_protocols(&opaque, 0); opaque now points to something const (namely the first element of the url_protocols array, an array of const pointers (to constant URLProtocols)), but opaque is not a pointer to something const, i.e. a violation of the const system has happened. Therefore one needs an explicit cast for this unsafe conversion; or the compiler complains. It is of course easy to fix this: Simply declare opaque as const void *. As mentioned already, the caller has no business modifying what this pointer points to anyway. - Andreas _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
