mmap.c

#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <string.h>

int main(int argc,char *argv[])
{
  struct stat st;
  char content[17];
  char *new_content = "***New Content***";
  void *map;
  
  // Opening the file in read and write mode (O_RDWR)
  // Need to match exacly the PROT argument in the mmap function!
  int f=open(argv[1], O_RDWR);
  fstat(f, &st);

  // Map the file to memory
  // NULL = the kernel wil choose the address at which he will create the mapping
  // PROT_READ | PROT_WRITE (PROT argument)!
  // MAP_SHARED = Share this mapping.  Updates to the mapping are visible to other processes that map this file
  map=mmap(NULL, st.st_size, PROT_READ | PROT_WRITE, MAP_SHARED, f, 0);

  // Read from the file via the mapped memory
  memcpy(content, map, 17);
  content[17] = '\0';
  printf("read: %s\n", content);
  
  // Write to the file via the mapped memory
  memcpy(map, new_content, strlen(new_content));

  // Clean up to save changes!
  munmap(map, st.st_size);
  close(f);
  return 0;

}