lockf - record locking on files
#include <unistd.h>
int
lockf(int filedes, 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 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
(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:
Function Description
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.
The F_ULOCK function 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 endof-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.
The 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) 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.
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.
lockf(), fcntl(2) and flock(2) locks may be safely used concurrently.
Blocking on a section is interrupted by any signal.
If successful, the lockf() function returns 0. Otherwise,
it returns -1,
sets the global variable errno to indicate an error, and existing locks
are not changed.
lockf() 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
descriptor.
The argument function is F_LOCK or F_TLOCK,
and filedes is
not a valid file descriptor open for writing.
[EDEADLK] The argument function is F_LOCK and a deadlock
is detected.
[EINTR] The argument function is F_LOCK and lockf()
was interrupted
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 support
locking.
[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 exceeding a systemimposed
limit.
fcntl(2), flock(2)
The lockf() function conforms to X/Open Portability Guide
Issue 4.2
(``XPG4.2'').
OpenBSD 3.6 December 19, 1997
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