|LOCKF(3)||Library Functions Manual||LOCKF(3)|
lockf — record
locking on files
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 the locks for a process are removed when the
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 function removes locks from a
section of the file;
F_TLOCK both lock a section of a file if the section
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_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_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) of 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 the 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.
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
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
Blocking on a section is interrupted by any signal.
If successful, the
returns 0. Otherwise, it returns -1, sets the global variable
errno to indicate an error, and existing locks are not
lockf() will fail if:
F_TESTand the section is already locked by another process.
The argument function is
filedes is not a valid file descriptor open for
F_LOCKand a deadlock is detected.
lockf() was interrupted by the delivery of a signal.
The argument filedes refers to a file that does not support locking.
F_TLOCK, and satisfying the lock or unlock request would result in the number of locked regions in the system exceeding a system-imposed limit.
lockf() function conforms to
X/Open Portability Guide Issue 4, Version 2
|June 5, 2013||OpenBSD-current|