Initialize all members of a C array to the same value.

For the GCC compiler, you can initialize an element to a particular value by specifying the array index and associated value.

For example:

...
int a[5] = { [1] = 42, [4] = 42 };

// Equivalent to:
int a[5] = { 0, 42, 0, 0, 42 };

You can also intialise a range of elements by using the format [first ... last] = value

This is a GNU extension.

Initialise All Elements to the Same Value

The following complete example shows how this technique can be used to initialize all elements in an array to the same value:

#include <stdio.h>
#define N 5

void printArray(int *arr, int n) {
	for (int i = 0; i < n; i++) {
		printf(i == 0 ? "[ %d" : ", %d", arr[i]);
	}
	puts(" ]\n");
}

int main()
{
	// Incorrect method
	int arr[N] = { 42 };
	printArray(arr, N);
	// Output: [ 42, 0, 0, 0, 0 ]
	
	int arr2[N] = { [0 ... N - 1] = 42 };
	printArray(arr2, N);
	// Output: [ 42, 42, 42, 42, 42 ]

	return 0;
}

References

• GCC Documentation