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flock(2) [freebsd man page]

FLOCK(2)						      BSD System Calls Manual							  FLOCK(2)

NAME
flock -- apply or remove an advisory lock on an open file LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/file.h> #define LOCK_SH 0x01 /* shared file lock */ #define LOCK_EX 0x02 /* exclusive file lock */ #define LOCK_NB 0x04 /* do not block when locking */ #define LOCK_UN 0x08 /* unlock file */ int flock(int fd, int operation); DESCRIPTION
The flock() system call applies or removes an advisory lock on the file associated with the file descriptor fd. A lock is applied by speci- fying an operation argument 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. The flock(), fcntl(2), and lockf(3) locks are compatible. Processes using different locking interfaces can cooperate over the same file safely. However, only one of such interfaces should be used within the same process. If a file is locked by a process through flock(), any record within the file will be seen as locked from the viewpoint of another process using fcntl(2) or lockf(3), and vice versa. Processes blocked awaiting a lock may be awakened by signals. RETURN VALUES
The flock() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error. ERRORS
The flock() system 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. [EOPNOTSUPP] The argument fd refers to an object that does not support file locking. [ENOLCK] A lock was requested, but no locks are available. SEE ALSO
close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), flopen(3), lockf(3) HISTORY
The flock() system call appeared in 4.2BSD. BSD
November 9, 2011 BSD

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flock(3UCB)					     SunOS/BSD Compatibility Library Functions					       flock(3UCB)

NAME
flock - apply or remove an advisory lock on an open file SYNOPSIS
/usr/ucb/cc[ flag ... ] file ... #include <sys/file.h> int flock( fd, operation) int fd, operation; DESCRIPTION
flock() applies or removes an advisory lock on the file associated with the file descriptor fd. The compatibility version of flock() has been implemented on top of fcntl(2) locking. It does not provide complete binary compatibility. Advisory locks allow cooperating processes to perform consistent operations on files, but do not guarantee exclusive access (that is, pro- cesses 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. More than one process may hold a shared lock for a file at any given time, but multiple exclusive, or both shared and exclusive, locks may not exist simultaneously on a file. A lock is applied by specifying an operation parameter LOCK_SH for a shared lock or LOCK_EX for an exclusive lock. The operation paramerer may be ORed with LOCK_NB to make the operation non-blocking. To unlock an existing lock, the operation should be LOCK_UN. Read permission is required on a file to obtain a shared lock, and write permission is required to obtain an exclusive lock. Locking a seg- ment that is already locked by the calling process causes the old lock type to be removed and the new lock type to take effect. Requesting a lock on an object that is already locked normally causes the caller to block 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. RETURN VALUES
flock() returns: 0 on success. -1 on failure and sets errno to indicate the error. ERRORS
EBADF The argument fd is an invalid descriptor. EINVAL operation is not a valid argument. EOPNOTSUPP The argument fd refers to an object other than a file. EWOULDBLOCK The file is locked and the LOCK_NB option was specified. SEE ALSO
cc(1B), lockd(1M), chmod(2), close(2), dup(2), exec(2), fcntl(2), fork(2), open(2), lockf(3C) NOTES
Use of these interfaces should be restricted to only applications written on BSD platforms. Use of these interfaces with any of the system libraries or in multi-thread applications is unsupported. 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. Locks are not inherited by a child process. Processes blocked awaiting a lock may be awakened by signals. Mandatory locking may occur, depending on the mode bits of the file. See chmod(2). Locks obtained through the flock() mechanism under SunOS 4.1 were known only within the system on which they were placed. This is no longer true. SunOS 5.11 30 Oct 2007 flock(3UCB)

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