lockf(3) - Man pages

lockf(3) Library Functions Manual lockf(3)

NAME
lockf - apply, test or remove a POSIX lock on an open file

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#include <unistd.h>

int lockf(int fd, int cmd, off_t len);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

lockf():
_XOPEN_SOURCE >= 500
|| /* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION
Apply, test, or remove a POSIX lock on a section of an open file. The
file is specified by fd, a file descriptor open for writing, the action
by cmd, and the section consists of byte positions pos..pos+len-1 if
len is positive, and pos-len..pos-1 if len is negative, where pos is
the current file position, and if len is zero, the section extends from
the current file position to infinity, encompassing the present and fu-
ture end-of-file positions. In all cases, the section may extend past
current end-of-file.

On Linux, lockf() is just an interface on top of fcntl(2) locking.
Many other systems implement lockf() in this way, but note that POSIX.1
leaves the relationship between lockf() and fcntl(2) locks unspecified.
A portable application should probably avoid mixing calls to these in-
terfaces.

Valid operations are given below:

F_LOCK Set an exclusive lock on the specified section of the file. If
(part of) this section is already locked, the call blocks until
the previous lock is released. If this section overlaps an ear-
lier locked section, both are merged. File locks are released
as soon as the process holding the locks closes some file de-
scriptor for the file. A child process does not inherit these
locks.

F_TLOCK
Same as F_LOCK but the call never blocks and returns an error
instead if the file is already locked.

F_ULOCK
Unlock the indicated section of the file. This may cause a
locked section to be split into two locked sections.

F_TEST Test the lock: return 0 if the specified section is unlocked or
locked by this process; return -1, set errno to EAGAIN (EACCES
on some other systems), if another process holds a lock.

RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set to indicate the error.

ERRORS
EACCES or EAGAIN
The file is locked and F_TLOCK or F_TEST was specified, or the
operation is prohibited because the file has been memory-mapped
by another process.

EBADF fd is not an open file descriptor; or cmd is F_LOCK or F_TLOCK
and fd is not a writable file descriptor.

EDEADLK
The command was F_LOCK and this lock operation would cause a
deadlock.

EINTR While waiting to acquire a lock, the call was interrupted by de-
livery of a signal caught by a handler; see signal(7).

EINVAL An invalid operation was specified in cmd.

ENOLCK Too many segment locks open, lock table is full.

ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).

┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│lockf() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘

STANDARDS
POSIX.1-2001, POSIX.1-2008, SVr4.