I don't know C

2014 年 03 月 05 日
C
CPP

C is purer than C++. It does not have so many obscure features and ambiguous grammar. I understand these facts and thought there would be nothing more to learn about the C language itself. This was almost true in my mind until I met some open-source projects coded in C, i.e., x264 and ffmpeg. In this article, I will not talk about the x264 techniques but only the C language.

A colleague poked me yesterday and asked how to read the array structure below, which was originally found in x264 implementation. I edited it for explanation:

int16_t (*mv[2][2])[2];

For me this kind of presentation of array structure declaration was seen rarely. I paused for a few seconds and recalled a spirial rule I had learnt in college (probably 5 years ago). Back to that time I did not pay much attention to that because I could not understand it due to lack of coding experience. I did not manage to decipher it in a way both of us could understand at first and thus I went through the spirial rule.

So to speak in spirial rule, we may draw it in such way:

                     +-----------+
                     | +---+     |
                     | ^   |     |
            int16_t (*mv[2][2])[2];
             ^       ^     |     |
             |       +-----+     |
             +-------------------+

In speaking, it could be explained in the following English statement:

mv is a 2x2 2D array of pointers to int16_t[2].

It may still unclear to understand. I extend it in this way:

mv is a 2x2 2D array. Each of the array element is a pointer. Each pointer is pointing to one int16_t[2] element.

For now I think it will not be that wired to see why x264 accesses mv in, for example mv[0][1][6376][1], patterns.

In debugging we found that x264 sometimes use negative indexes in an array. e.g.:

int t = some_random_int_array[-1]; 

it is like why the hell can indexes are negative? However it turns out to be totally legal and not like python, negative indexes indicate elements before the first element of the array. This is because the pattern array[idx] is equivalent to *(array + idx). This SO thread explains and quotes the following from C99 §6.5.2.1/2:

The deﬁnition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))).

The story of learning new facts wen on and then I met designated initializers but I do not want to repeat every details of the specification here. As an short example in ffmpeg, I saw this:

AVCodec ff_libx264_encoder = {
    .name             = "libx264",
    .long_name        = NULL_IF_CONFIG_SMALL("libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10"),
    .type             = AVMEDIA_TYPE_VIDEO,
    .id               = AV_CODEC_ID_H264,
    .priv_data_size   = sizeof(X264Context),
    .init             = X264_init,
    .encode2          = X264_frame,
    .close            = X264_close,
    .capabilities     = CODEC_CAP_DELAY | CODEC_CAP_AUTO_THREADS,
    .priv_class       = &x264_class,
    .defaults         = x264_defaults,
    .init_static_data = X264_init_static,
};

I can guess what is the dot variable name is about, but did not ever imagine C can do something like this!

These all kinds of both new/old facts refreshed my attitude towards C. I knew C++ is a language hard to master all the details, but I have always underestimated C as well. Language is evolving itself all the time even for C.

References

The ``Clockwise/Spiral Rule’’

Negative array indexes in c

Designated Initializers

Archive 5