Provided by: manpages-dev_6.7-2_all 
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 op, 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 op, 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 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:
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 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.
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 op is F_LOCK or F_TLOCK and fd is not a
writable file descriptor.
EDEADLK
op was F_LOCK and this lock operation would cause a deadlock.
EINTR While waiting to acquire a lock, the call was interrupted by delivery of a signal
caught by a handler; see signal(7).
EINVAL An invalid operation was specified in op.
ENOLCK Too many segment locks open, lock table is full.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌───────────────────────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├───────────────────────────────────────────────────────────────┼───────────────┼─────────┤
│lockf() │ Thread safety │ MT-Safe │
└───────────────────────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, SVr4.
SEE ALSO
fcntl(2), flock(2)
locks.txt and mandatory-locking.txt in the Linux kernel source directory
Documentation/filesystems (on older kernels, these files are directly under the
Documentation directory, and mandatory-locking.txt is called mandatory.txt)