#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

char buf[123];

int main() {
  int fd = open("/mnt/abc", O_WRONLY, 0123);
  for (int i = 0; i < sizeof(buf); ++i)
    buf[i] = i + 1;
  ssize_t res = write(fd, buf, sizeof(buf));
  fprintf(stderr, "res = %ld %lx\n", res, res);
  return 0;
}