Selecting Advisory or Mandatory Locking

Selecting Advisory or Mandatory Locking

As discussed under "Advisory Locking", file locking is usually an in-memory service of the IRIX kernel. The kernel keeps a table of locks that have been placed. Processes anywhere in the system update the table by calling fcntl() or lockf() to request locks. When all processes that use a file do this, and respect the results, file integrity can be maintained.

It is possible to extend file locking by making it mandatory on all processes, whether or not they were designed to be part of the cooperating group. Mandatory locking, as defined under "Mandatory Locking", is enforced by the file I/O function calls. As a result, an independent process that calls write() to update a locked record will be blocked. The write() function tests for a contending lock internally. Of course, the same test is made by write() when called by a cooperating process, in which case it represents unnecessary overhead.

Mandatory locking is enforced on a file-by-file basis, triggered by a bit in the file inode that is set by chmod (see the chmod(1) and chmod(2) reference pages). In order to enforce mandatory locking on a particular file, turn on the set-group-ID bit along with a nonexecutable group permission ("2644" permissions, for example). This must be done before the file is opened; a change has no effect on a file that is already open.

Example 4-8 shows a fragment of code that sets mandatory lock mode on a given filename.

Example 4-8 : Setting Mandatory Locking Permission Bits

#include <sys/types.h>
#include <sys/stat.h>
int setMandatoryLocking(char *filename)
{
   int mode;
   struct stat buf;
   if (stat(filename, &buf) < 0)
   {
      perror("stat(2)");
      return error;
   }
   mode = buf.st_mode;
   /* ensure group execute permission 0010 bit is off */
   mode &= ~(S_IEXEC>>3);
   /* turn on 'set group id bit' in mode */
   mode |= S_ISGID;
   if (chmod(filename, mode) < 0)
   {
      perror("chmod(2)");
      return error;
   }
   return 0;
}

When IRIX opens a file, it checks to see whether both of two conditions are true:

Set group-ID bit is 1
Group execute permission is 0

When both are true, the file is marked for mandatory locking, and each use of creat(), open(), read(), and write() tests for contending locks.

Some points to remember about mandatory locking:

Mandatory locking does not protect against file truncation with the truncate() function (see the truncate(2) reference page), which does not look for locks on the truncated portion of the file.
Mandatory locking protects only 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.
Advisory locking is more efficient because a record lock check does not have to be performed for every I/O request.