flock.2 (2010 09)

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ﬂock(2) ﬂock(2)

NAME

ﬂock() - apply or remove an advisory or enforced lock on an open ﬁle

SYNOPSIS

#include <sys/fcntl.h>

int flock(int filedes, int operation );

Parameters

filedes Speciﬁes a ﬁle descriptor returned by a successful

open(), creat(), dup(), dup2,or

fcntl() function, identifying the ﬁle on which the locking operation is to be performed.

operation Speciﬁes one of the following constants for

flock(), deﬁned in the <sys/fcntl.h>

ﬁle:

LOCK_SH Apply a shared lock.

LOCK_EX Apply an exclusive lock.

LOCK_UN Remove a lock.

In addition to the preceding constants, the following constant can be bitwise ORed with

either

LOCK_SH or LOCK_EX:

LOCK_NB Do not block when locking.

DESCRIPTION

The

flock() function applies or removes an advisory or enforced lock on the whole ﬁle associated with

the ﬁledes ﬁle descriptor. Advisory locks allow cooperating processes to perform consistent operations on

ﬁles, but do not guarantee consistency (that is, processes may still access ﬁles without using advisory

locks, possibly resulting in inconsistencies).Enforced locks are in effect if the enforcement bit in the ﬁle

mode is set (see chmod(2)). This might affect read(), write(), and other system calls performing ﬁle

operations.

You can use the

flock() function to coordinate a ﬁle’s lock status on local and NFS ﬁle systems.

The locking mechanism allows two types of locks: shared locks and exclusive locks. At any time mul-

tiple shared locks may be applied to a ﬁle, but at no time are multiple exclusive locks or both shared and

exclusive locks allowed simultaneously on a ﬁle.

A shared lock may be 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).

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 operation , this will not happen; instead, the call will

fail and errno will be set to [EWOULDBLOCK].

APPLICATION USAGE

Locks are on ﬁles, not ﬁle descriptors. This means that:

• Locks are not inherited by a child process resulting from a fork() call.

• All locks associated with a ﬁle for a given process are removed when any thread in the process closes

any ﬁle descriptor for that ﬁle.

Processes that are blocked awaiting a lock may be awakened by signals. An application needs to check

the return value to make sure that the lock is really acquired.

The flock() interface is not part of any UNIX standard. Therefore, if you are designing and writing

applications to be portable across platforms, you should use the fcntl() ﬁle locking interface instead of

flock().

NETWORKING FEATURES

NFS

The advisory ﬁle locking capabilities of

flock() are implemented throughout the network by the net-

work lock daemon (lockd). If the ﬁle server crashes and is rebooted, the lock daemon attempts to

recover all locks associated with the crashed server. If a lock cannot be reclaimed, the process that held

the lock is issued a SIGLOST signal. See lockd(1M).

File locking as implemented for NFS ﬁles is only advisory.

HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

Summary of content (2 pages)

PAGE 1
flock(2) flock(2) NAME flock() - apply or remove an advisory or enforced lock on an open file SYNOPSIS #include int flock(int filedes, int operation ); Parameters filedes Specifies a file descriptor returned by a successful open(), creat(), dup(), dup2, or fcntl() function, identifying the file on which the locking operation is to be performed. operation Specifies one of the following constants for flock(), defined in the file: LOCK_SH Apply a shared lock.
PAGE 2
flock(2) flock(2) RETURN VALUES flock() returns: 0 -1 for successful completion for unsuccessful completion; errno is set to indicate the error ERRORS If the flock() function fails, errno may be set to one of the following values: [EBADF] The filedes parameter is not a valid open file descriptor. A shared lock was requested and filedes was not opened for reading, or an exclusive lock was requested and filedes was not opened for writing.