Setting a File Lock

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

You can set a lock on a file in several ways. The 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() interface. The interface tries to lock a file until one of the following events happen:

• The file lock is set successfully

• An error occurs

• MAX_TRY is exceeded, and the program stops 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(), you can set the type and start of the lock request by setting structure variables. For more information, see the fcntl(2) man page.

  Note:

  You cannot lock mapped files with flock. However, you can use the multithread-oriented synchronization mechanisms with mapped files. These synchronization mechanisms can be used in POSIX styles and in Oracle Solaris styles.