SYS_FLOCK(9) The Linux VFS SYS_FLOCK(9)NAME
sys_flock - flock system call.
SYNOPSIS
long sys_flock(unsigned int fd, unsigned int cmd);
ARGUMENTS
fd
the file descriptor to lock.
cmd
the type of lock to apply.
DESCRIPTION
Apply a FL_FLOCK style lock to an open file descriptor. The cmd can be one of
LOCK_SH -- a shared lock.
LOCK_EX -- an exclusive lock.
LOCK_UN -- remove an existing lock.
LOCK_MAND -- a `mandatory' flock. This exists to emulate Windows Share Modes.
LOCK_MAND can be combined with LOCK_READ or LOCK_WRITE to allow other processes read and write access respectively.
COPYRIGHT Kernel Hackers Manual 2.6. July 2010 SYS_FLOCK(9)
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FLOCK(2) Linux Programmer's Manual FLOCK(2)NAME
flock - apply or remove an advisory lock on an open file
SYNOPSIS
#include <sys/file.h>
int flock(int fd, int operation);
DESCRIPTION
Apply or remove an advisory lock on the open file specified by fd. The parameter operation is one of the following:
LOCK_SH Place a shared lock. More than one process may hold a shared lock for a given file at a given time.
LOCK_EX Place an exclusive lock. Only one process may hold an exclusive lock for a given file at a given time.
LOCK_UN Remove an existing lock held by this process.
A call to flock() may block if an incompatible lock is held by another process. To make a non-blocking request, include LOCK_NB (by ORing)
with any of the above operations.
A single file may not simultaneously have both shared and exclusive locks.
Locks created by flock() are associated with a file, or, more precisely, an open file table entry. This means that duplicate file descrip-
tors (created by, for example, fork(2) or dup(2)) refer to the same lock, and this lock may be modified or released using any of these
descriptors. Furthermore, the lock is released either by an explicit LOCK_UN operation on any of these duplicate descriptors, or when all
such descriptors have been closed.
A process may only hold one type of lock (shared or exclusive) on a file. Subsequent flock() calls on an already locked file will convert
an existing lock to the new lock mode.
Locks created by flock() are preserved across an execve(2).
A shared or exclusive lock can be placed on a file regardless of the mode in which the file was opened.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
EWOULDBLOCK
The file is locked and the LOCK_NB flag was selected.
EBADF fd is not a not an open file descriptor.
EINTR While waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler.
EINVAL operation is invalid.
ENOLCK The kernel ran out of memory for allocating lock records.
CONFORMING TO
4.4BSD (the flock(2) call first appeared in 4.2BSD). A version of flock(2), possibly implemented in terms of fcntl(2), appears on most
Unices.
NOTES flock(2) does not lock files over NFS. Use fcntl(2) instead: that does work over NFS, given a sufficiently recent version of Linux and a
server which supports locking.
Since kernel 2.0, flock(2) is implemented as a system call in its own right rather than being emulated in the GNU C library as a call to
fcntl(2). This yields true BSD semantics: there is no interaction between the types of lock placed by flock(2) and fcntl(2), and flock(2)
does not detect deadlock.
flock(2) places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of flock(2) and perform I/O
on the file.
flock(2) and fcntl(2) locks have different semantics with respect to forked processes and dup(2).
SEE ALSO open(2), close(2), dup(2), execve(2), fcntl(2), fork(2), lockf(3)
There are also locks.txt and mandatory.txt in /usr/src/linux/Documentation.
Linux 2002-04-24 FLOCK(2)