mmap.c

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <errno.h>

int main(int argc, char *argv[])
{
  int fd, offset;
  char * data;
  struct stat sbuf;

  if (argc != 2) {
    fprintf(stderr, "usage: %s offset\n", argv[0]);
    exit(1);
  }

  if ((fd = open("mmap.c", O_RDWR)) == -1) {
    perror("open");
    exit(1);
  }

  if (stat("mmap.c", &sbuf) == -1) {
    perror("stat");
    exit(1);
  }

  offset = atoi(argv[1]);
  if (offset < 0 || offset > sbuf.st_size - 1) {
    fprintf(stderr, "offset must be in the range 0-%lu\n", sbuf.st_size - 1);
    exit(1);
  }

  if ((data = mmap((caddr_t)0,
                   sbuf.st_size,
                   PROT_READ|PROT_WRITE,
                   MAP_SHARED, fd, 0)) == (caddr_t)(-1)) {
    perror("mmap");
    exit(1);
  }

  printf("byte at offset %d is '%c'\n", offset, data[offset]);
  printf("change it, and press enter\n");

  getchar();

  printf("byte at offset %d is now '%c'\n", offset, data[offset]);

  return 0;
}