Manpage of LOCKF
LOCKFSection: Linux Programmer's Manual (3)
NAMElockf - apply, test or remove a POSIX lock on an open file
int lockf(int fd, int cmd, off_t len);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
- _XOPEN_SOURCE >= 500
|| /* Glibc since 2.19: */ _DEFAULT_SOURCE
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
DESCRIPTIONApply, 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 lenis positive, and pos-len..pos-1 if lenis negative, where posis the current file position, and if lenis zero, the section extends from the current file position to infinity, encompassing the present and future 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 interfaces.
Valid operations are given below:
- Set an exclusive lock on the specified section of the file. If (part of) this section is already locked, the call blocks until the lock is released. If this section overlaps an earlier locked section, both are merged. File locks are released as soon as the process holding the locks closes some file descriptor for the file. A child process does not inherit these locks.
- Same as F_LOCKbut the call never blocks and returns an error instead if the file is already locked.
- Unlock the indicated section of the file. This may cause a locked section to be split into two locked sections.
- Test the lock: return 0 if the specified section is unlocked or locked by this process; return -1, set errnoto EAGAIN(EACCESon some other systems), if another process holds a lock.
RETURN VALUEOn success, zero is returned. On error, -1 is returned, and errnois set appropriately.
- EACCES or EAGAIN
- The file is locked and F_TLOCKor F_TESTwas specified, or the operation is prohibited because the file has been memory-mapped by another process.
- fdis not an open file descriptor; or cmdis F_LOCKor F_TLOCKand fdis not a writable file descriptor.
- The command was F_LOCKand this lock operation would cause a deadlock.
- An invalid operation was specified in cmd.
- Too many segment locks open, lock table is full.
ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7).
CONFORMING TOPOSIX.1-2001, POSIX.1-2008, SVr4.
SEE ALSOfcntl(2), flock(2)
locks.txtand mandatory-locking.txtin the Linux kernel source directory Documentation/filesystems(on older kernels, these files are directly under the Documentationdirectory, and mandatory-locking.txtis called mandatory.txt)
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