Setting a File Lock

To lock an entire file, set the offset to zero and set the size to zero.

There are several ways to set a lock on a file. Choice of method depends on how the lock interacts with the rest of the program, performance, and portability. This example uses the POSIX standard-compatible fcntl(2) function. It tries to lock a file until one of the following happens:

• The file is successfully locked

• There is an error

• MAX_TRY is exceeded, and the program gives up trying to lock the file

  ---

  #include <fcntl.h>
  
   ...
   	struct flock lck;
  
   ...
   	lck.l_type = F_WRLCK;	/* setting a write lock */
   	lck.l_whence = 0;	/* offset l_start from beginning of file */
   	lck.l_start = (off_t)0;	
   	lck.l_len = (off_t)0;	/* until the end of the file */
   	if (fcntl(fd, F_SETLK, &lck) <0) {
   		if (errno == EAGAIN || errno == EACCES) {
   			(void) fprintf(stderr, "File busy try again later!\n");
   			return;
   		}
   		perror("fcntl");
   		exit (2);
   	}
   	...

  ---

  Using fcntl(2), you can set the type and start of the lock request by setting a few structure variables.

  ---

  Note -

  Mapped files cannot be locked with flock(3UCB). See mmap(2). However, the multithread-oriented synchronization mechanisms (in either POSIX or Solaris styles) can be used with mapped files. See mutex(3THR), condition(3THR), semaphore(3THR), and rwlock(3THR).

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