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System Interface Guide

File and Record Locking

You don't need to use traditional file I/O to do locking of file elements. The lighter weight synchronization mechanisms described in Multithreaded Programming Guide can be used effectively with mapped files and are much more efficient than the old style file locking described in this section.

You lock files, or portions of files, to prevent the errors that can occur when two or more users of a file try to update information at the same time.

File locking and record locking are really the same thing, except that file locking blocks access to the whole file, while record locking blocks access to only a specified segment of the file. (In the SunOS 5.0 through 5.8 system, all files are a sequence of bytes of data: a record is a concept of the programs that use the file.)

Supported File Systems

Both advisory and mandatory locking are supported on the following types of file systems:

Choosing a Lock Type

Mandatory locking suspends a process until the requested file segments are free. Advisory locking returns a result indicating whether the lock was obtained or not: processes can ignore the result and do the I/O anyway. You cannot use both mandatory and advisory file locking on the same file at the same time. The mode of a file at the time it is opened determines whether locks on a file are treated as mandatory or advisory.

Of the two basic locking calls, fcntl(2) is more portable, more powerful, and less easy to use than lockf(3C).fcntl(2) is specified in Posix 1003.1 standard. lockf(3C) is provided to be compatible with older applications.

Terminology

Some useful definitions for reading the rest of this section are shown below:

record 

An arbitrary sequence of bytes in a file. The UNIX operating system supports no record structure. Programs that use the files can impose any arbitrary record structure.  

cooperating processes

Two or more processes using some mechanism to regulate access to a shared resource. 

read lock

Lets other processes also apply a read lock and/or perform reads, and blocks other processes from writing or applying a write lock. 

write lock

Blocks all other process from reading, writing, or applying any lock.  

advisory lock

Returns an error without blocking to a process that does not hold the lock. Advisory locking is not enforced on creat(2), open(2), read(2), or write(2) operations.

mandatory lock

Blocks execution of processes that do not hold the lock. Access to locked records is enforced on creat(2), open(2), read(2), and write(2) operations.

Opening a File for Locking

A lock can only be requested on a file with a valid open descriptor. For read locks, the file must be opened with at least read access. For write locks, the file must also be opened with write access. In this example, a file is opened for both read and write access. 

...
 	filename = argv[1];
 	fd = open (filename, O_RDWR);
 	if (fd < 0) {
 		perror(filename);
 		exit(2);
 	}
 	...

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:

Setting and Removing Record Locks

Locking a record is done the same way as locking a file except that the starting point and length of the lock segment is not set to zero.

Plan a failure response for when you cannot obtain all the required locks. Contention for data is why you use record locking, so different programs might:

Getting Lock Information

You can determine which process, if any, is holding a lock. Use this as a simple test or to find locks on a file. A lock is set, as in the previous examples, and F_GETLK is used in fcntl(2). The next example finds and prints indentifying data on all the locked segments of a file.

Example 5-2 

struct flock lck;

 	lck.l_whence = 0;
 	lck.l_start = 0L;
 	lck.l_len = 0L;
 	do {
 		lck.l_type = F_WRLCK;
 		(void) fcntl(fd, F_GETLK, &lck);
 		if (lck.l_type != F_UNLCK) {
 			(void) printf("%d %d %c %8ld %8ld\n", lck.l_sysid, lck.l_pid, 
						(lck.l_type == F_WRLCK) ? 'W' : 'R', lck.l_start, lck.l_len);
 			/* If this lock goes to the end of the address space, no
 			 * need to look further, so break out. */
 			if (lck.l_len == 0) {
 			/* else, look for new lock after the one just found. */
 					lck.l_start += lck.l_len;
 			}
 		}
 	} while (lck.l_type != F_UNLCK);

fcntl(2) with the F_GETLK command can sleep while waiting for a server to respond, and it can fail (returning ENOLCK) if there is a resource shortage on either the client or server.

lockf(3C) with the F_TEST command can be used to test if a process is holding a lock. This function does not return information about where the lock is and which process owns it. 

(void) lseek(fd, 0, 0L);
 /* set the size of the test region to zero (0). to test until the
    end of the file address space. */
 if (lockf(fd, (off_t)0, SEEK_SET) < 0) {
 	switch (errno) {
 		case EACCES:
 		case EAGAIN:
 			(void) printf("file is locked by another process\n");
 			break;
 		case EBADF:
 			/* bad argument passed to lockf */
 			perror("lockf");
 			break;
 		default:
 			(void) printf("lockf: unexpected error <%d>\n", errno);
 			break;
 	}

Forking and Locks

When a process forks, the child receives a copy of the file descriptors that the parent opened. Locks are not inherited by the child because they are owned by a specific process. The parent and child share a common file pointer for each file. Both processes can try to set locks on the same location in the same file. This problem happens with both lockf(3C) and fcntl(2). If a program holding a record lock forks, the child process should close the file and reopen it to set a new, separate file pointer.

Deadlock Handling

The UNIX locking facilities provide deadlock detection/avoidance. Deadlocks can happen only when the system is about to put a record locking function to sleep. A search is made to determine whether process A will wait for a lock that B holds while B is waiting for a lock that A holds. If a potential deadlock is detected, the locking function fails and sets errno to indicate deadlock. Processes setting locks using F_SETLK do not cause a deadlock because they do not wait when the lock cannot be granted immediately.

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.

Cautions About Mandatory Locking