#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/mman.h> /* mmap() is defined in this header */
#include <fcntl.h>
#include <stdarg.h>

#define FILE_MODE       (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)

void err_sys(char *fmt, ...)
{
   char buf[1024];

   va_list ap;

   va_start(ap, fmt);
   vsnprintf(buf, sizeof(buf), fmt, ap);

   fprintf(stderr, "%s", buf);

   va_end(ap);

   exit(EXIT_FAILURE);
}

int main (int argc, char *argv[])
{
 int fdout;
 char *dst;
 char buf[1024];

 if (argc != 2)
   err_sys ("usage: %s <file>\n", argv[0]);

 /* open/create the output file */
 if ((fdout = open (argv[1], O_RDWR | O_CREAT | O_TRUNC, FILE_MODE)) < 0)
   err_sys ("can't create %s for writing", argv[1]);

 /* go to the location corresponding to the last byte */
 if (lseek (fdout, sizeof(buf) - 1, SEEK_SET) == -1)
   err_sys ("lseek error");
 
 /* write a dummy byte at the last location */
 if (write (fdout, "", 1) != 1)
   err_sys ("write error");

 if ((dst = mmap (0, sizeof(buf), PROT_READ | PROT_WRITE, MAP_SHARED, fdout, 0)) == (caddr_t) -1)
   err_sys ("mmap error for output");

 // we do a strcpy from one buffer to another, and file's contents will be updated
 while ( fgets(buf, sizeof(buf), stdin) ) {
    strcpy(dst, buf);
 }

 return 0;

}