flock(2) [osf1 man page]
flock(2) System Calls Manual flock(2) NAME
flock - Applies or removes an advisory lock on an open file SYNOPSIS
#include <sys/fcntl.h> int flock( int filedes, int operation ); PARAMETERS
Specifies a file descriptor returned by a successful open() or fcntl() function, identifying the file to which the lock is to be applied or removed. Specifies one of the following constants for flock(), defined in the fcntl.h file: Apply a shared lock. Apply an exclusive lock. Do not block when locking. This value can be logically ORed with either LOCK_SH or LOCK_EX. Remove a lock. DESCRIPTION
The flock() function applies or removes an advisory lock on the 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). You can use the flock() function to coordinate a file's lock status on local, CFS, and NFS file systems. 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 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, then this will not happen; instead, the call will fail and errno will be set to [EWOULDBLOCK]. NOTES
Locks are on files, not file descriptors. That is, file descriptors duplicated using the dup() or fork() functions do not result in multi- ple 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. 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(). RETURN VALUES
Upon successful completion, 0 (zero) is returned. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
If the flock() function fails, errno may be set to one of the following values: The file is locked and the LOCK_NB option was specified. The filedes argument is not a valid open file descriptor. A signal interrupted the flock call. The operator is not valid. The lock table is full. Too many regions are already locked. The lock is blocked by some lock from another process. Putting the calling process to sleep while waiting for that lock to become free would cause a deadlock. RELATED INFORMATION
Functions: close(2), exec(2), fcntl(2), fork(2), open(2), lockf(3) delim off flock(2)
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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)