XOR two files together

xor.c

/*
 * gcc -std=gnu99 -O2 -s -o otp otp.c
 */
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

#include <sys/stat.h>
#include <sys/mman.h>

#include <inttypes.h>
#include <string.h>

#define error_file(action, file) { fprintf(stderr, "Could not " #action " %s : %s\n", file, strerror(errno)); exit(errno); }

int main (int argc, char **argv)
{
	if (argc != 3 && argc != 4)
		return fprintf(stderr, "Usage: %s <source> <source> [output]\n", argv[0]), EXIT_FAILURE;

	int64_t in1 = open(argv[1], O_RDONLY);
	if (in1 < 0)
		error_file(open, argv[1]);

	int64_t in2 = open(argv[2], O_RDONLY);
	if (in2 < 0)
		error_file(open, argv[2]);

	int64_t out = STDOUT_FILENO;
	if (argc == 4)
		out = open(argv[3], O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
	if (out < 0)
		error_file(open, argv[3]);

	struct stat s1;
	fstat(in1, &s1);
	uint8_t *m1 = mmap(NULL, s1.st_size, PROT_READ, MAP_SHARED, in1, 0);
	if (m1 == MAP_FAILED)
		error_file(mmap, argv[1]);

	struct stat s2;
	fstat(in2, &s2);
	uint8_t *m2 = mmap(NULL, s2.st_size, PROT_READ, MAP_SHARED, in2, 0);
	if (m2 == MAP_FAILED)
		error_file(mmap, argv[2]);

	size_t oz = s1.st_size < s2.st_size ? s1.st_size :s2.st_size;
	ftruncate(out, oz);
	uint8_t *mo = NULL;
	if (out != STDOUT_FILENO)
		if ((mo = mmap(NULL, oz, PROT_READ | PROT_WRITE, MAP_SHARED, out, 0)) == MAP_FAILED)
			error_file(mmap, argv[3]);

	for (size_t i = 0; i < oz; i++)
		if (mo)
			mo[i] = m1[i] ^ m2[i];
		else
			putchar(m1[i] ^ m2[i]);

	munmap(m1, s1.st_size);
	munmap(m2, s2.st_size);
	if (mo)
		munmap(mo, oz);

	if (out != STDOUT_FILENO)
		close(out);
	close(in2);
	close(in1);

	return EXIT_SUCCESS;
}