mm-test.c

/**
 * mm-test: A simple test of using mmap() to manipulate files as if they are
 * regions of memory. This example has full error checking, and allows the user
 * to specify a file, as well as replacement text and an optional offset value.
 *
 * Andrew Duffy, 2016
 */
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mman.h>
#include <sys/stat.h>

  int
main(int argc, char *argv[])
{
  int fd, offset = 0;
  char *ptr;
  struct stat st;

  if (argc < 3) {
    fprintf(stderr, "Usage: %s FILE \"TEXT\" [OFFSET]\n", argv[0]);
    return 1;
  }

  if (argc == 4) {
    offset = atoi(argv[3]);
  }


  if ((fd = open(argv[1], O_RDWR)) < 0) {
    fprintf(stderr, "Error, could not open %s\n", argv[1]);
    return 1;
  }

  if (fstat(fd, &st) < 0) {
    fprintf(stderr, "Could not fstat() file %s\n", argv[1]);
    return 1;
  }

  int fsize = st.st_size;

  if (strlen(argv[2]) + offset >= fsize) {
      fprintf(stderr, "Aborting, combined offset + inserted text exceeds size of the file\n");
      return 1;
  }

  fprintf(stderr, "File size = %d bytes\n", fsize);
  // Map the entire region into memory
  if ((ptr = mmap(NULL, fsize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)) == MAP_FAILED) {
    fprintf(stderr, "error, failed to map\n");
    return 1;
  }

  fprintf(stderr, "Mapped file %s to address 0x%x\n", argv[1], ptr);

  strcpy(ptr + offset, argv[2]); // copy into the file

  if (munmap(ptr, 256) < 0) {
    fprintf(stderr, "Error, could not unmap region\n");
    return 1;
  }
  return 0;

}