flock.2

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)flock.2	8.1	9/11/90
.TH flock 2
.SH Name
flock \- apply or remove an advisory lock on an open file
.SH Syntax
.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
.NXR "flock system call"
.NXR "file" "applying advisory lock"
The
.PN flock
system call 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; that is, processes might still access files
without using advisory locks, possibly resulting in
inconsistencies.
.NXR "advisory lock" "defined"
.PP
The locking mechanism allows two types of locks:
.I shared
locks and
.I exclusive
locks.
At any time, multiple shared locks can be applied to a file.
However, multiple exclusive locks, or shared and exclusive
locks cannot be applied simultaneously on a file.  
.PP
A shared lock can be
upgraded to be 
an exclusive lock, and an exclusive lock can become shared, simply by specifying
the appropriate lock type. This change results in the previous
lock being released and the new lock applied. When upgrading,
do not include LOCK_NB in
.IR operation,
because there is a possibility that other processes have
requests for locks, or have gained or released a lock.
.PP
Requesting a lock on an object that is already locked
normally causes the caller to blocked until the lock can be
acquired.  If LOCK_NB is included in
.IR operation ,
the call is not blocked; instead, the call fails and
the error EWOULDBLOCK is returned.
.PP
Locks are on files, not file descriptors.  That is, file descriptors
duplicated through 
.PN dup 
or
.PN fork 
call 
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 
loses its lock.
.PP
Processes blocked awaiting a lock may be awakened by signals.
.SH Return Values
Zero is returned if the operation was successful;
on an error, a \-1 is returned and an error code is stored in
the global variable, \fIerrno\fP.
.SH Diagnostics
The 
.PN flock
call fails under the following conditions: 
.TP 15
[EWOULDBLOCK]
The file is locked and the LOCK_NB option was specified.
.TP 15
[EBADF]
The argument \fIfd\fP is an invalid descriptor.
.TP 15
[EINVAL]
The argument \fIfd\fP refers to an object other than a file.
.TP 15
[EOPNOTSUPP]
Invalid operation is requested.  The argument \fIfd\fR
refers to a socket.
.SH Restrictions
File region locking is not supported over NFS.
.SH See Also
close(2), dup(2), execve(2), fork(2), open(2)