Significance of last item in this array of structs

Question

How is the last item in myarray valid? Supposedly this is an "empty-terminated list". This is C code.

typedef struct sFoo
{
    char *a;
    char *b;
} SFOO;

static SFOO my_sfoo_array[] =
    {
        { 0x1000, 0x2000 },
        { 0x3000, 0x4000 },
        { }        /* what?! */
    };

Are the missing structure elements automatically supplied as 0, so that the last entry { } is really {0, 0}?

I think it is not a standard thing though, but a compiler extension. Standard C does not allow empty initializers. — Eugene Sh., Jan 21 '21 at 20:27

Vlad from Moscow · Accepted Answer · 2021-01-21T20:42:06.097

0

This is not a valid C initialization

static SFOO my_sfoo_array[] =
    {
        { 0x1000, 0x2000 },
        { 0x3000, 0x4000 },
        { }        /* what?! */
    };

C does not allow empty braces.

In C++ it means value initialization.

So in C it would be correctly to write

static SFOO my_sfoo_array[3] =
    {
        { 0x1000, 0x2000 },
        { 0x3000, 0x4000 },
    };

In this case the members of the third element will be initialized by null-pointers. The initializer in C is defined like

initializer:
    assignment-expression
    { initializer-list }
    { initializer-list , }

as you can see the initializer-list is not optional.

edited Jan 21 '21 at 20:42

answered Jan 21 '21 at 20:28

Vlad from Moscow

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Not valid in standard C, but is works in common compilers https://stackoverflow.com/questions/17589533/is-an-empty-initializer-list-valid-c-code/ – Eugene Sh. Jan 21 '21 at 20:30
@EugeneSh. It does not matter what compiler allows such an initialization. We are speaking about C. – Vlad from Moscow Jan 21 '21 at 20:36
Remove a comma in the second snippet – klutt Jan 21 '21 at 20:39
@klutt It is allowed.;) – Vlad from Moscow Jan 21 '21 at 20:39
@VladfromMoscow Really? Didn't know that. – klutt Jan 21 '21 at 20:40
1

@klutt Not only allowed, but considered a good practice (well, I guess depends). For example when you add another line to the arrays, it will be only that line shown in `diff`. While without the comma two lines will be different. – Eugene Sh. Jan 21 '21 at 20:44
@EugeneSh. I find it a very good practice. Just didn't know it was allowed. Those kind of nice syntactic sugar is not very typical to C. :) – klutt Jan 21 '21 at 20:49

Significance of last item in this array of structs

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