C Programming: Strange looking array declaration

Question

I'm looking at the source code in one of the FFMPEG files and found a construct that looks really strange to me. Can sb please explain what is happening here?

init and query_formats are actually functions that have been declared before in the file.

AVFilter avfilter_vf_fade = {
 .name = "fade",
 .description = NULL_IF_CONFIG_SMALL("Fade in/out input video"),
 .init = init,
 .priv_size = sizeof(FadeContext),
 .query_formats = query_formats,
 .inputs = (AVFilterPad[]) {{ .name = "default",
 .type = AVMEDIA_TYPE_VIDEO,
 .config_props = config_props,
 .get_video_buffer = avfilter_null_get_video_buffer,
 .start_frame = avfilter_null_start_frame,
 .draw_slice = draw_slice,
 .end_frame = end_frame,
 .min_perms = AV_PERM_READ | AV_PERM_WRITE,
 .rej_perms = AV_PERM_PRESERVE, },
 { .name = NULL}},
 .outputs = (AVFilterPad[]) {{ .name = "default",
 .type = AVMEDIA_TYPE_VIDEO, },
 { .name = NULL}},
};

What are the "." doing in there. How would you access all these points. What would be saved in the compartments of the array (pointer addresses?!)?

I'm a bit confused..

Also, how do you learn about how the code of a third party programmer works, if there are almost no comments around? Documentation doesn't exist either..

PS: This is what the init function looks like:

static av_cold int init(AVFilterContext *ctx, const char *args, void *opaque)
{
...
}

• cnicutar: They are called "designated initializers".

Answer 1

This is C99.

It allows to initialize structures by name.

For example the structure:

struct foo {
 int x,y;
 char *name;
};

Can be initialized as:

struct foo f = { 
 .name = "Point",
 .x=10,
 .y=20
};

This requires up-to-date compiler that supports the latest standards: C99.

• Frank Vilea: Ok, I see. What does it mean if you use a function like has been done in the example .init = init where init is a function that was declared earlier? Does init now hold the result of the function or is it being assigned the address?

Answer 2

It's called a designated initializer, and is part of the C99 standard. Have a look here to learn more.