Anton Khirnov: > Quoting Guo, Yejun (2021-02-21 06:31:37) >> >> >>>> >>> FYI: The C standard does not mandate the sizes for most types (the uintx_t >>> types are an exception). It also gives the compiler absolute freedom on how >>> much padding to add to a structure. A compiler could very well add megabytes >>> of padding to StyleBox. The numbers you provide only pertain to one (or >>> actually none, see below) implementation, not to all of them. It is like >>> someone claiming to prove a general theorem by only checking it for one >>> example. >>> >>> Several of your statements are btw not only false for a hypothetical system >>> compliant with the C standard. They are false on common systems: >>> "SIZE_MAX / sizeof(*s->style_attributes) is always larger than uint_max" >>> is wrong on ordinary 32bit systems (where SIZE_MAX and UINT_MAX typically >>> coincide). And "sizeof(*s->style_attributes) is 12". Due to padding it is >>> normally >>> 16; it could be reduced to 12 (for ordinary >>> systems) by rearranging its elements. >>> >> >> thanks for the lucid explanation, yes, you're right. >> (and first to know that megabytes of padding is possible) >> >> btw, just as a learning, is there possible a real system where uint_max < >> uint32_max? >> If no, we might say that: size_max >= uint_max >= uint32_max > uint16_max. > > We assume int (and hence also unsigned int) is at least 32bit. > But AFAIK the only limit on size_t is that it's at least 16bit, > otherwise it can be smaller than int. > While it is true that the smallest permissible value of SIZE_MAX is 65535, we de facto require it to be much greater: We never check for truncation when using any allocation function with an unsigned parameter, ergo we rely on SIZE_MAX >= UINT_MAX. And some of our structures don't fit into 65535B (e.g. the context of the DVB subtitles decoder contains an "int clut_count2[257][256]") as do other objects (we seem to have at least 40 objects with a size >= 65536; the biggest is the VLC table for atrac3plus); H.2645 packets are even padded with 256KiB (see MAX_MBPAIR_SIZE). The latter would lead to nonsense behaviour on a system with SIZE_MAX == 65535, while the other two would lead to compilation errors.
- 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".
