flock(2) [osx man page]
FLOCK(2) BSD System Calls Manual FLOCK(2) NAME
flock -- apply or remove an advisory lock on an open file SYNOPSIS
#include <sys/file.h> #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 */ int flock(int fd, int operation); DESCRIPTION
Flock() applies or removes an advisory lock on the file associated with the file descriptor fd. A lock is applied by specifying an operation parameter that is one of LOCK_SH or LOCK_EX with the optional addition of LOCK_NB. To unlock an existing lock operation should be LOCK_UN. 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). The locking mechanism allows two types of locks: shared locks and 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. A shared lock may be upgraded to an exclusive lock, and vice versa, simply by specifying the appropriate lock type; this results in the pre- vious 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, then this will not happen; instead the call will fail and the error EWOULDBLOCK will be returned. NOTES
Locks are on files, not file descriptors. That is, file descriptors duplicated through dup(2) or 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. Processes blocked awaiting a lock may be awakened by signals. RETURN VALUES
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 errno. ERRORS
The flock() call fails if: [EWOULDBLOCK] The file is locked and the LOCK_NB option was specified. [EBADF] The argument fd is an invalid descriptor. [EINVAL] The argument fd refers to an object other than a file. [ENOTSUP] The referenced descriptor is not of the correct type. SEE ALSO
close(2), dup(2), execve(2), fork(2), open(2) HISTORY
The flock() function call appeared in 4.2BSD. 4.2 Berkeley Distribution December 11, 1993 4.2 Berkeley Distribution
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flock(2) System Calls Manual flock(2) Name flock - apply or remove an advisory lock on an open file Syntax #include <sys/file.h> #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 */ flock(fd, operation) int fd, operation; Description The system call applies or removes an advisory lock on the file associated with the file descriptor, fd. A lock is applied by specifying an operation parameter that is the inclusive OR of LOCK_SH or LOCK_EX and, possibly, LOCK_NB. To unlock an existing lock, operation should be LOCK_UN. 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. The locking mechanism allows two types of locks: shared locks and 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. 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 opera- tion, because there is a possibility that other processes have requests for locks, or have gained or released a lock. 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 operation, the call is not blocked; instead, the call fails and the error EWOULDBLOCK is returned. Locks are on files, not file descriptors. That is, file descriptors duplicated through or 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. Processes blocked awaiting a lock may be awakened by signals. 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, errno. Diagnostics The call fails under the following conditions: [EWOULDBLOCK] The file is locked and the LOCK_NB option was specified. [EBADF] The argument fd is an invalid descriptor. [EINVAL] The argument fd refers to an object other than a file. [EOPNOTSUPP] Invalid operation is requested. The argument fd refers to a socket. Restrictions File region locking is not supported over NFS. See Also close(2), dup(2), execve(2), fork(2), open(2) flock(2)