LOCKF(3) BSD Library Functions Manual LOCKF(3)NAMElockf — record locking on files
Standard C Library (libc, -lc)
lockf(int filedes, int function, off_t size);
The lockf() function allows sections of a file to be locked with advi‐
sory-mode locks. Calls to lockf() from other processes which attempt to
lock the locked file section will either return an error value or block
until the section becomes unlocked. All the locks for a process are
removed when the process terminates.
The argument filedes is an open file descriptor. The file descriptor
must have been opened either for write-only (O_WRONLY) or read/write
The function argument is a control value which specifies the action to be
taken. The permissible values for function are as follows:
F_ULOCK unlock locked sections
F_LOCK lock a section for exclusive use
F_TLOCK test and lock a section for exclusive use
F_TEST test a section for locks by other processes
F_ULOCK removes locks from a section of the file; F_LOCK and F_TLOCK both
lock a section of a file if the section is available; F_TEST detects if a
lock by another process is present on the specified section.
The size argument is the number of contiguous bytes to be locked or
unlocked. The section to be locked or unlocked starts at the current
offset in the file and extends forward for a positive size or backward
for a negative size (the preceding bytes up to but not including the cur‐
rent offset). However, it is not permitted to lock a section that starts
or extends before the beginning of the file. If size is 0, the section
from the current offset through the largest possible file offset is
locked (that is, from the current offset through the present or any
The sections locked with F_LOCK or F_TLOCK may, in whole or in part, con‐
tain or be contained by a previously locked section for the same process.
When this occurs, or if adjacent locked sections would occur, the sec‐
tions are combined into a single locked section. If the request would
cause the number of locks to exceed a system-imposed limit, the request
F_LOCK and F_TLOCK requests differ only by the action taken if the sec‐
tion is not available. F_LOCK blocks the calling process until the sec‐
tion is available. F_TLOCK makes the function fail if the section is
already locked by another process.
File locks are released on first close by the locking process of any file
descriptor for the file.
F_ULOCK requests release (wholly or in part) one or more locked sections
controlled by the process. Locked sections will be unlocked starting at
the current file offset through size bytes or to the end of file if size
is 0. When all of a locked section is not released (that is, when the
beginning or end of the area to be unlocked falls within a locked sec‐
tion), the remaining portions of that section are still locked by the
process. Releasing the center portion of a locked section will cause the
remaining locked beginning and end portions to become two separate locked
sections. If the request would cause the number of locks in the system
to exceed a system-imposed limit, the request will fail.
An F_ULOCK request in which size is non-zero and the offset of the last
byte of the requested section is the maximum value for an object of type
off_t, when the process has an existing lock in which size is 0 and which
includes the last byte of the requested section, will be treated as a
request to unlock from the start of the requested section with a size
equal to 0. Otherwise an F_ULOCK request will attempt to unlock only the
A potential for deadlock occurs if a process controlling a locked region
is put to sleep by attempting to lock the locked region of another
process. This implementation detects that sleeping until a locked region
is unlocked would cause a deadlock and fails with an EDEADLK error.
The lockf(), fcntl(2), and flock(2) locks are compatible. Processes
using different locking interfaces can cooperate over the same file
safely. However, only one of such interfaces should be used within the
same process. If a file is locked by a process through flock(2), any
record within the file will be seen as locked from the viewpoint of
another process using fcntl(2) or lockf(), and vice versa.
Blocking on a section is interrupted by any signal.
The lockf() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error. In the case of a failure, existing locks are not changed.
The lockf() function will fail if:
[EAGAIN] The argument function is F_TLOCK or F_TEST and the
section is already locked by another process.
[EBADF] The argument filedes is not a valid open file descrip‐
The argument function is F_LOCK or F_TLOCK, and
filedes is not a valid file descriptor open for writ‐
[EDEADLK] The argument function is F_LOCK and a deadlock is
[EINTR] The argument function is F_LOCK and lockf() was inter‐
rupted by the delivery of a signal.
[EINVAL] The argument function is not one of F_ULOCK, F_LOCK,
F_TLOCK or F_TEST.
The argument filedes refers to a file that does not
[ENOLCK] The argument function is F_ULOCK, F_LOCK or F_TLOCK,
and satisfying the lock or unlock request would result
in the number of locked regions in the system exceed‐
ing a system-imposed limit.
SEE ALSOfcntl(2), flock(2)STANDARDS
The lockf() function conforms to X/Open Portability Guide Issue 4,
Version 2 (“XPG4.2”).
BSD December 19, 1997 BSD