lockf(S)

lockf -- record locking on files

Syntax

cc . . . -lc

#include <unistd.h>

int lockf (fildes, function, size)
int fildes, function;
long size;

Description

The lockf command allows sections of a file to be locked; advisory or mandatory write locks are used depending on the mode bits of the file [see chmod(S)]. Locking calls from other processes which attempt to lock the locked file section either return an error value or are put to sleep until the resource becomes unlocked. All the locks initiated by a process are removed when the process terminates or when the process closes the file. See fcntl(S) for more information about record locking.

fildes is an open file descriptor. The file descriptor must have O_WRONLY or O_RDWR permission in order to establish a lock with this function call.

function is a control value which specifies the action to be taken. The permissible values for function are defined in <unistd.h> as follows:

   #define F_ULOCK  0    /* Unlock a previously locked section */
   #define F_LOCK   1    /* Lock a section for exclusive use */
   #define F_TLOCK  2    /* Test and lock a section for exclusive use */
   #define F_TEST   3    /* Test section for other processes locks */

All other values of function are reserved for future extensions and will result in an error return if not implemented.

F_TEST is used to detect if a lock by another process is present on the specified section. F_LOCK and F_TLOCK both lock a section of a file if the section is available. F_ULOCK removes locks from a section of the file.

size is the number of contiguous bytes to be locked or unlocked. The section to be locked starts at the current offset in the file and extends forward for a positive size and backward for a negative size (the preceding bytes up to but not including the current offset). If size is zero, the section from the current offset through the largest file offset is locked (that is, from the current offset through the present or any future end-of-file). An area need not be allocated to the file in order to be locked as such locks may exist past the end-of-file.

The sections locked with F_LOCK or F_TLOCK may, in whole or in part, contain or be contained by a previously locked section for the same process. When this occurs, or if adjacent sections occur, the sections are combined into a single section. If the request requires that a new element be added to the table of active locks and this table is already full, an error is returned, and the new section is not locked.

F_LOCK and F_TLOCK requests differ only by the action taken if the resource is not available. F_LOCK causes the calling process to sleep until the resource is available. F_TLOCK causes the function to return a -1 and set errno to [EACCES] if the section is already locked by another process.

F_ULOCK requests may, in whole or in part, release one or more locked sections controlled by the process. When sections are not fully released, the remaining sections are still locked by the process. Releasing the center section of a locked section requires an additional element in the table of active locks. If this table is full, an [EDEADLK] error is returned, and the requested section is not released.

A potential for deadlock occurs if a process controlling a locked resource is put to sleep by accessing another process's locked resource. Thus calls to lockf or fcntl scan for a deadlock prior to sleeping on a locked resource. An error return is made if sleeping on the locked resource would cause a deadlock.

Sleeping on a resource is interrupted with any signal. The alarm(S) function may be used to provide a timeout facility in applications which require this facility.

The lockf utility fails if one or more of the following is true:

[EBADF]: fildes is not a valid open descriptor.
[EACCES]: cmd is F_TLOCK or F_TEST and the section is already locked by another process.
[EDEADLK]: cmd is F_LOCK and a deadlock would occur. Also the cmd is either F_LOCK, F_TLOCK, or F_ULOCK and the number of entries in the lock table would exceed the number allocated on the system.
[ECOMM]: fildes is on a remote machine and the link to that machine is no longer active.

Diagnostics

Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

Warnings

Unexpected results may occur in processes that do buffering in the user address space. The process may later read/write data which is/was locked. The standard I/O package is the most common source of unexpected buffering.

Because in the future the variable errno will be set to EAGAIN rather than EACCES when a section of a file is already locked by another process, portable application programs should expect and test for either value.

Standards conformance