LOCKF(3) BSD Library Functions Manual LOCKF(3)
NAME
lockf -- record locking on files
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
##include <>
int
lockf(int filedes, int function, offt size);
DESCRIPTION
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 (OWRONLY) or read/write
(ORDWR) operation.
The function argument is a control value which specifies the action to be
taken. The permissible values for function are as follows:
Function Description
FULOCK unlock locked sections
FLOCK lock a section for exclusive use
FTLOCK test and lock a section for exclusive use
FTEST test a section for locks by other processes
FULOCK removes locks from a section of the file; FLOCK and FTLOCK both
lock a section of a file if the section is available; FTEST 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
future end-of-file).
The sections locked with FLOCK or FTLOCK 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
will fail.
FLOCK and FTLOCK requests differ only by the action taken if the sec-
tion is not available. FLOCK blocks the calling process until the sec-
tion is available. FTLOCK 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.
FULOCK 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 FULOCK 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
offt, 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 FULOCK request will attempt to unlock only the
requested section.
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.
RETURN VALUES
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.
ERORS
The lockf() function will fail if:
[EAGAIN] The argument function is FTLOCK or FTEST and the
section is already locked by another process.
[EBADF] The argument filedes is not a valid open file descrip-
tor.
The argument function is FLOCK or FTLOCK, and
filedes is not a valid file descriptor open for writ-
ing.
[EDEADLK] The argument function is FLOCK and a deadlock is
detected.
[EINTR] The argument function is FLOCK and lockf() was inter-
rupted by the delivery of a signal.
[EINVAL] The argument function is not one of FULOCK, FLOCK,
FTLOCK or FTEST.
The argument filedes refers to a file that does not
support locking.
[ENOLCK] The argument function is FULOCK, FLOCK or FTLOCK,
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 ALSO
fcntl(2), flock(2)
STANDARDS
The lockf() function conforms to X/Open Portability Guide Issue 4,
Version 2 (``XPG4.2'').
BSD December 19, 1997 BSD
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