[unix-history] / usr / man / man2 / flock.2

.TH FLOCK 2 "27 July 1983"
.UC 4
.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 which 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 which is already locked
normally causes the caller to 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
48672018 C	1	.TH FLOCK 2 "27 July 1983"
	2	.UC 4
	3	.SH NAME
	4	flock \- apply or remove an advisory lock on an open file
	5	.SH SYNOPSIS
	6	.nf
	7	.ft B
	8	#include <sys/file.h>
	9	.PP
	10	.ft B
	11	.DT
	12	#define LOCK_SH 1 /* shared lock */
	13	#define LOCK_EX 2 /* exclusive lock */
	14	#define LOCK_NB 4 /* don't block when locking */
	15	#define LOCK_UN 8 /* unlock */
	16	.PP
	17	.ft B
	18	flock(fd, operation)
	19	int fd, operation;
	20	.fi
	21	.SH DESCRIPTION
	22	.I Flock
	23	applies or removes an
	24	.I advisory
	25	lock on the file associated with the file descriptor
	26	.IR fd .
	27	A lock is applied by specifying an
	28	.I operation
	29	parameter which is the inclusive or of
	30	LOCK_SH or LOCK_EX and, possibly, LOCK_NB. To unlock
	31	an existing lock
	32	.I operation
	33	should be LOCK_UN.
	34	.PP
	35	Advisory locks allow cooperating processes to perform
	36	consistent operations on files, but do not guarantee
	37	consistency (i.e. processes may still access files
	38	without using advisory locks possibly resulting in
	39	inconsistencies).
	40	.PP
	41	The locking mechanism allows two types of locks:
	42	.I shared
	43	locks and
	44	.I exclusive
	45	locks.
	46	At any time multiple shared locks may be applied to a file,
	47	but at no time are multiple exclusive, or both shared and exclusive,
	48	locks allowed simultaneously on a file.
	49	.PP
	50	A shared lock may be
	51	.I upgraded
	52	to an exclusive lock, and vice versa, simply by specifying
	53	the appropriate lock type; this results in the previous
	54	lock being released and the new lock applied (possibly
	55	after other processes have gained and released the lock).
	56	.PP
	57	Requesting a lock on an object which is already locked
	58	normally causes the caller to blocked until the lock may be
	59	acquired. If LOCK_NB is included in
	60	.IR operation ,
	61	then this will not happen; instead the call will fail and
	62	the error EWOULDBLOCK will be returned.
	63	.SH NOTES
	64	Locks are on files, not file descriptors. That is, file descriptors
65	duplicated through
66	.IR dup (2)
67	or
68	.IR fork (2)
69	do not result in multiple instances of a lock, but rather multiple
70	references to a single lock. If a process holding a lock on a file
71	forks and the child explicitly unlocks the file, the parent will
72	lose its lock.
73	.PP
74	Processes blocked awaiting a lock may be awakened by signals.
75	.SH "RETURN VALUE
76	Zero is returned if the operation was successful;
77	on an error a \-1 is returned and an error code is left in
78	the global location \fIerrno\fP.
79	.SH "ERRORS
80	The \fIflock\fP call fails if:
81	.TP 20
82	[EWOULDBLOCK]
83	The file is locked and the LOCK_NB option was specified.
84	.TP 20
85	[EBADF]
86	The argument \fIfd\fP is an invalid descriptor.
87	.TP 20
88	[EINVAL]
89	The argument \fIfd\fP refers to an object other than a file.
90	.SH "SEE ALSO"
91	open(2), close(2), dup(2), execve(2), fork(2)