mmap2.c

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

int main(int argc, char *argv[])
{
  char *content="**New content**";
  char buffer[30];
  struct stat st;
  void *map;
  
  // open the file in read only mode.
  int f=open(argv[1], O_RDONLY);
  fstat(f, &st);
 
  // mapping the file into memory with read only flag because we use O_RDONLY!
  // MAP_PRIVATE = Create a private copy-on-write mapping.
  map=mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, f, 0);

  // Since we using the read only flag let's bypass it and open the current process memory file in Read and Write
  int fm=open("/proc/self/mem", O_RDWR);

  // Write some new content to the memory,
  // Starting at the 5th byte from the beginning.
  lseek(fm, map + 5, SEEK_SET);
  int result = write(fm, content, strlen(content));

  if (result == -1)
  	printf("write: failed.\n");
  else
  	printf("write: succeeded.\n");
  
  //Read the first 29 bytes from the memory
  memcpy(buffer, map, 29);
  buffer[29] = 0;
  printf("read: %s\n", buffer);

}