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lockf - 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)):
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
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
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.1-2001 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
Same as F_LOCK but 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.
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.
On success, zero is returned. On error, -1 is returned, and errno is
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.
The command was T_LOCK and this lock operation would cause a
EINVAL An invalid operation was specified in fd.
ENOLCK Too many segment locks open, lock table is full.
There are also locks.txt and mandatory-locking.txt in the kernel source
directory Documentation/filesystems. (On older kernels, these files
are directly under the Documentation/ directory, and mandatory-
locking.txt is called mandatory.txt.)
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