Selecting advisory or mandatory locking

The use of mandatory locking is not recommended for reasons that will be made clear in a subsequent section. Whether or not locks are enforced by the I/O system calls is determined at the time the calls are made by the permissions on the file (see chmod(2)). For locks to be under mandatory enforcement, the file must be a regular file with the set-group-ID bit on and the group execute permission off. If either condition fails, all record locks are advisory. Mandatory enforcement can be assured by the following code:

   #include <sys/types.h>
   #include <sys/stat.h>
   

   int mode;
   struct stat buf;
   		.
   		.
   		.
   	if (stat(filename, &buf) < 0) {
   		perror("program");
   		exit (2);
   	}
   	/* get currently set mode */
   	mode = buf.st_mode;
   	/* remove group execute permission from mode */
   	mode &= ~(S_IEXEC>>3);
   	/* set 'set group id bit' in mode */
   	mode |= S_ISGID;
   	if (chmod(filename, mode) < 0) {
   		perror("program");
   		exit(2);
   	}
   		.
   		.
   		.

Files that are to be record locked should never have any type of execute permission set on them. This is because the operating system does not obey the record locking protocol when executing a file.

The chmod(1) command can also be easily used to set a file to have mandatory locking. This can be done with the command:

   chmod +l file

The ls(1) command shows this setting when you ask for the long listing format:

   ls -l file

causes the following to be printed:

-rw---l--- 1 user group size mod_time file

Advisory versus mandatory locking

Note the following points when choosing whether to implement advisory or mandatory locking:

• Mandatory locking only protects those portions of a file that are locked. Other portions of the file that are not locked may be accessed according to normal UNIX system file permissions.

• If multiple reads or writes are necessary for an atomic transaction, the process should explicitly lock all such pieces before any I/O begins. Thus advisory enforcement is sufficient for all programs that perform in this way.

• As stated earlier, arbitrary programs should not have unrestricted access permission to files that are important enough to record lock.

• Advisory locking is more efficient because a record lock check does not have to be performed for every I/O request.