C program that checks if the given STDIN stream is standard ASCII

is_ascii.c

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
	int found_non_ascii = 0;

	while (!ferror(stdin) && !feof(stdin)) {
		size_t s;
		char buf[1024];
		s = fread(buf, sizeof(buf), 1, stdin);
		for (; !found_non_ascii && s > 0; s--) {
			if (buf[s - 1] < 0)
				found_non_ascii = 1;
		}
	}

	return ferror(stdin) || found_non_ascii;
}