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

int main(int argc, char* argv[]) {

  if (argc != 4) {
    printf("proc-2  pid  addr  length\n");
    exit(1);
  }

  int           pid  = strtol (argv[1], NULL, 10);
  unsigned long addr = strtoul(argv[2], NULL, 16);
  int           len  = strtol (argv[3], NULL, 10);

  char* proc_mem = malloc(50);
  sprintf(proc_mem, "/proc/%d/mem", pid);

  printf("opening %s, address is %ld\n", proc_mem, addr);
  int fd_proc_mem = open(proc_mem, O_RDWR);
  if (fd_proc_mem == -1) {
    printf("Could not open %s\n", proc_mem);
    exit(1);
  }

  char* buf = malloc(len);

  lseek(fd_proc_mem, addr, SEEK_SET);
  read (fd_proc_mem, buf , len     );

  printf("String at %ld in process %d is:\n", addr, pid);
  printf("  %s\n", buf);

  printf("\nNow, this string is modified\n");
  strncpy(buf, "Hello from proc-2", len);

  lseek(fd_proc_mem, addr, SEEK_SET);
  if (write (fd_proc_mem, buf , len     ) == -1) {
     printf("Error while writing\n");
     exit(1);
  }

  free(buf);
  free(proc_mem);
}