[unix-history] / usr / src / lib / libc / sys / flock.2

.\" Copyright (c) 1983 The Regents of the University of California.
.\" All rights reserved.
.\"
.\" %sccs.include.redist.man%
.\"
.\"	@(#)flock.2	6.6 (Berkeley) %G%
.\"
.TH FLOCK 2 ""
.UC 5
.SH NAME
flock \- apply or remove an advisory lock on an open file
.SH SYNOPSIS
.nf
.ft B
#include <sys/file.h>
.PP
.ft B
.DT
#define	LOCK_SH	1	/* shared lock */
#define	LOCK_EX	2	/* exclusive lock */
#define	LOCK_NB	4	/* don't block when locking */
#define	LOCK_UN	8	/* unlock */
.PP
.ft B
flock(fd, operation)
int fd, operation;
.fi
.SH DESCRIPTION
.I Flock
applies or removes an
.I advisory
lock on the file associated with the file descriptor
.IR fd .
A lock is applied by specifying an
.I operation
parameter that is the inclusive or of
LOCK_SH or LOCK_EX and, possibly, LOCK_NB.  To unlock
an existing lock
.I operation
should be LOCK_UN.
.PP
Advisory locks allow cooperating processes to perform
consistent operations on files, but do not guarantee
consistency (i.e., processes may still access files
without using advisory locks possibly resulting in
inconsistencies).
.PP
The locking mechanism allows two types of locks:
.I shared
locks and
.I exclusive
locks.
At any time multiple shared locks may be applied to a file,
but at no time are multiple exclusive, or both shared and exclusive,
locks allowed simultaneously on a file.  
.PP
A shared lock may be
.I upgraded
to an exclusive lock, and vice versa, simply by specifying
the appropriate lock type; this results in the previous
lock being released and the new lock applied (possibly
after other processes have gained and released the lock).
.PP
Requesting a lock on an object that is already locked
normally causes the caller to be blocked until the lock may be
acquired.  If LOCK_NB is included in
.IR operation ,
then this will not happen; instead the call will fail and
the error EWOULDBLOCK will be returned.
.SH NOTES
Locks are on files, not file descriptors.  That is, file descriptors
duplicated through
.IR dup (2)
or
.IR fork (2)
do not result in multiple instances of a lock, but rather multiple
references to a single lock.  If a process holding a lock on a file
forks and the child explicitly unlocks the file, the parent will
lose its lock.
.PP
Processes blocked awaiting a lock may be awakened by signals.
.SH "RETURN VALUE
Zero is returned if the operation was successful;
on an error a \-1 is returned and an error code is left in
the global location \fIerrno\fP.
.SH "ERRORS
The \fIflock\fP call fails if:
.TP 20
[EWOULDBLOCK]
The file is locked and the LOCK_NB option was specified.
.TP 20
[EBADF]
The argument \fIfd\fP is an invalid descriptor.
.TP 20
[EINVAL]
The argument \fIfd\fP refers to an object other than a file.
.SH "SEE ALSO"
open(2), close(2), dup(2), execve(2), fork(2)
Commit	Line	Data
88b3ccf2 KB	1	.\" Copyright (c) 1983 The Regents of the University of California.
88b3ccf2 KB	2	.\" All rights reserved.
619ee695	3	.\"
91cff1e1	4	.\" %sccs.include.redist.man%
88b3ccf2	5	.\"
91cff1e1	6	.\" @(#)flock.2 6.6 (Berkeley) %G%
619ee695	7	.\"
9b1f9fbd	8	.TH FLOCK 2 ""
619ee695 KM	9	.UC 5
	10	.SH NAME
	11	flock \- apply or remove an advisory lock on an open file
	12	.SH SYNOPSIS
	13	.nf
	14	.ft B
	15	#include <sys/file.h>
	16	.PP
	17	.ft B
	18	.DT
	19	#define LOCK_SH 1 /* shared lock */
	20	#define LOCK_EX 2 /* exclusive lock */
	21	#define LOCK_NB 4 /* don't block when locking */
	22	#define LOCK_UN 8 /* unlock */
	23	.PP
	24	.ft B
	25	flock(fd, operation)
	26	int fd, operation;
	27	.fi
	28	.SH DESCRIPTION
	29	.I Flock
	30	applies or removes an
	31	.I advisory
	32	lock on the file associated with the file descriptor
	33	.IR fd .
	34	A lock is applied by specifying an
	35	.I operation
38edb05c	36	parameter that is the inclusive or of
619ee695 KM	37	LOCK_SH or LOCK_EX and, possibly, LOCK_NB. To unlock
	38	an existing lock
	39	.I operation
	40	should be LOCK_UN.
	41	.PP
	42	Advisory locks allow cooperating processes to perform
	43	consistent operations on files, but do not guarantee
4decc76b	44	consistency (i.e., processes may still access files
619ee695 KM	45	without using advisory locks possibly resulting in
	46	inconsistencies).
	47	.PP
	48	The locking mechanism allows two types of locks:
	49	.I shared
	50	locks and
	51	.I exclusive
	52	locks.
	53	At any time multiple shared locks may be applied to a file,
	54	but at no time are multiple exclusive, or both shared and exclusive,
	55	locks allowed simultaneously on a file.
	56	.PP
	57	A shared lock may be
	58	.I upgraded
	59	to an exclusive lock, and vice versa, simply by specifying
	60	the appropriate lock type; this results in the previous
	61	lock being released and the new lock applied (possibly
	62	after other processes have gained and released the lock).
	63	.PP
38edb05c	64	Requesting a lock on an object that is already locked
19ecbc2b	65	normally causes the caller to be blocked until the lock may be
619ee695 KM	66	acquired. If LOCK_NB is included in
	67	.IR operation ,
	68	then this will not happen; instead the call will fail and
	69	the error EWOULDBLOCK will be returned.
	70	.SH NOTES
	71	Locks are on files, not file descriptors. That is, file descriptors
	72	duplicated through
	73	.IR dup (2)
	74	or
	75	.IR fork (2)
	76	do not result in multiple instances of a lock, but rather multiple
	77	references to a single lock. If a process holding a lock on a file
	78	forks and the child explicitly unlocks the file, the parent will
	79	lose its lock.
	80	.PP
	81	Processes blocked awaiting a lock may be awakened by signals.
	82	.SH "RETURN VALUE
	83	Zero is returned if the operation was successful;
	84	on an error a \-1 is returned and an error code is left in
	85	the global location \fIerrno\fP.
	86	.SH "ERRORS
	87	The \fIflock\fP call fails if:
	88	.TP 20
	89	[EWOULDBLOCK]
	90	The file is locked and the LOCK_NB option was specified.
	91	.TP 20
	92	[EBADF]
	93	The argument \fIfd\fP is an invalid descriptor.
	94	.TP 20
	95	[EINVAL]
	96	The argument \fIfd\fP refers to an object other than a file.
	97	.SH "SEE ALSO"
	98	open(2), close(2), dup(2), execve(2), fork(2)