fhello-5.c

#include <stdio.h>
#include <errno.h>
#include <error.h>

int main (void)
{
  FILE *f = fopen ("fhello.txt", "w");
  if (!f)
     error (1, errno, "cannot open file");
  fprintf (f, "Hello, world!\n");
  fclose (f);
  return 0;
}