Selecting Advisory or Mandatory Locking

For mandatory locks, 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. Set a mandatory lock as follows.

#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 to be record locked should never have any execute permission set. This is because the operating system ignores record locks when executing a file.

The chmod(1) command can also be used to set a file to permit mandatory locking. For example:

$ chmod +l file

This command sets the O20n0 permission bit in the file mode, which indicates mandatory locking on the file. If n is even, the bit is interpreted as enabling mandatory locking. If n is odd, the bit is interpreted as "set group ID on execution."

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

$ ls -l file

displays following information:

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

The letter "l" in the permissions indicates that the set-group-ID bit is on, so mandatory locking is enabled, along with the normal semantics of set group ID.