| LOCKF(3) | Library Functions Manual | LOCKF(3) |
lockf — record
locking on files
Standard C Library (libc, -lc)
#include
<unistd.h>
int
lockf(int
fildes, int
function, off_t
size);
The
lockf()
function allows sections of a file to be locked with advisory-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 of the locks for a process are
removed when the process terminates.
The argument fildes is an open file
descriptor. The file descriptor must have been opened either for write-only
(O_WRONLY) or read/write
(O_RDWR) operation.
The function argument is a control value which specifies the action to be taken. The permissible values for function are as follows:
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 current 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 F_LOCK or
F_TLOCK may, in whole or in part, contain or be
contained by a previously locked section for the same process. When this
occurs, or if adjacent locked sections would occur, the sections 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.
F_LOCK and F_TLOCK
requests differ only by the action taken if the section is not available.
F_LOCK blocks the calling process until the section
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 section), 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
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.
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]F_TLOCK or F_TEST and the
section is already locked by another process.EBADF]The argument function is
F_LOCK or F_TLOCK, and
fildes is not a valid file descriptor open for
writing.
EDEADLK]F_LOCK and a deadlock is detected.EINTR]lockf() was interrupted by the delivery of a
signal.EINVAL]F_LOCK, F_TEST,
F_TLOCK, or F_ULOCK.
The argument fildes refers to a file that does not support advisory locking.
ENOLCK]F_LOCK, F_TLOCK, or
F_ULOCK and satisfying the lock or unlock request
would result in the number of locked regions in the system exceeding a
system-imposed limit.EOPNOTSUPP]The lockf() function conforms to
X/Open Portability Guide Issue 4, Version 2
(“XPG4.2”).
| December 19, 1997 | macOS 15.2 |