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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 specifying 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 opera- tions on files, but do not guarantee consistency (i.e., processes may still access files without using advisory locks possibly resulting in in- consistencies). 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 exclu- sive, locks allowed simultaneously on a file. A shared lock may be upgraded to an exclusive lock, and vice versa, sim- ply by specifying the appropriate lock type; this results in the previous lock being released and the new lock applied (possibly after other pro- cesses 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 in- cluded 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 du- plicated 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 us- ing 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 speci- fied. [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 sup- port 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
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | NOTES | RETURN VALUES | ERRORS | SEE ALSO | HISTORY
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