21:09% cat g.c                                                           /tmp/test
#include <stdio.h>

int
main(void)
{
	const int foo = 3;
	char bar[foo];
	for (int i = 0; i < foo; i ++) {
		bar [i] = 0x30;
	}
	puts(bar);
	return 0;
}

21:09% clang g.c                                                         /tmp/test
21:09% ./a.out                                                           /tmp/test
000
21:09% gcc g.c                                                           /tmp/test
g.c: In function ‘main’:
g.c:8:2: error: ‘for’ loop initial declarations are only allowed in C99 or C11 mode
  for (int i = 0; i < foo; i ++) {
  ^
g.c:8:2: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
21:10% gcc g.c -std=c11                                                  /tmp/test
21:10% ./a.out                                                           /tmp/test
000