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 - 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)