Lock and Write Example

Open for writing or create ./testlocks.txt. While locking 64 bytes from offset is unsuccessful: get and print information about the locking process and move offset by another 64 bytes. Move offset bytes from the beginning of the file and write information about the current process. Wait for the Enter key to be pressed. Unlock the 64 locked bytes, output an error on failure. Close the file.

lockfile.c

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
int main(int argc, char * argv[])
{
  char str[64];
  memset(str, 32, 64);
  struct flock fi;
  sprintf(str, "Stored by process %i", getpid());
  int fd = open("testlocks.txt", O_RDWR|O_CREAT);
  fi.l_type = F_WRLCK;
  fi.l_whence = SEEK_SET;
  fi.l_start = 0;
  fi.l_len = 64;
  int off = 0; 
  while (fcntl(fd, F_SETLK, &fi) == -1)
  {
     fcntl(fd, F_GETLK, &fi);
     printf("bytes %i - %i blocked by process %i\n", off, off+64, fi.l_pid);
     off += 64;
     fi.l_start = off;
  }
  lseek(fd, off, SEEK_SET);
  write(fd, str, strlen(str));
  getchar();
  fi.l_type = F_UNLCK;
  if (fcntl(fd, F_SETLK, &fi) == -1) printf("ERROR while blocking!\n");
  close(fd);
}