Three dots operator "..." for initializing an array

Question

Consider the following example which initialize an array with a default value:

static unsigned int array[10] = { [ 0 ... 9 ] = 5 };

What exactly this operator does?

It is related to variadic macro __VA_ARGS__?

Possible duplicate of [Initialize an array with a single value in C (GCC)](https://stackoverflow.com/questions/7780405/initialize-an-array-with-a-single-value-in-c-gcc) — kiran Biradar, Jun 14 '19 at 13:13
No, it is not related to variadic macros or variadic functions. — John Bollinger, Jun 14 '19 at 13:22
This is not C. The token `three dots` is not valid in the context of the indices of an array in C. — alinsoar, Jun 14 '19 at 13:27

joH1 · Accepted Answer · 2021-02-03T14:55:46.820

5

In standard C, since C99, designated initializers allow to initialize individual elements of an array in the form:

int array[4] = {[1] = 42};

The syntax you stumbled upon is a range initializer, it is a GNU extension to initialize all elements between 0and 9 to the given value, thus strictly equivalent to:

static unsigned int array[10] = { [0] = 5, [1] = 5, [2] = 5, [3] = 5, [4] = 5, [5] = 5, [6] = 5, [7] = 5, [8] = 5, [9] = 5};

only less a burden to type and read.

Reference

edited Feb 03 '21 at 14:55

answered Jun 14 '19 at 13:17

joH1

555
1
9
27

I'm thinking this example should instead end with: , [9] = 5 }; – jaygeek Feb 02 '21 at 16:14
1

@jaygeek You're right! Fixed now. Thanks. – joH1 Feb 03 '21 at 14:56

Petr Skocik · Answer 2 · 2019-06-14T13:27:14.673

Nothing in ISO C. It's a nonstandard construct.

In GNU C (gcc/clang) it appears to initialize each of elements 0 through 9 to 5, i.e., it's shorthand for (C99)

static unsigned int array[10] = { [0]=5, [1]=5, [2]=5, /*...*/ [9]=5 };

or (C89)

static unsigned int array[10] = { 5, 5, 5, 5, /*...*/ };

The ... extension also works for cases:

_Bool lowercase_eh(char c)
{
    switch(c) case 'a' ... 'z': return 1;
    return 0; 
}

Apart from using the same ... token, it is unrelated to variadic macros or fucntions.

Three dots operator "..." for initializing an array

2 Answers2