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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/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 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 co- operating 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). The flock() function operates on the local system only. It does not make any attempt to coordinate a file's lock status with other systems. In a distributed environment, use the fcntl() or lockf() interfaces to place advisory locks on files, as they provide a superset of flock() features. The locking mechanism allows two types of locks: shared locks and ex- clusive 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, 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 ex- plicitly unlocks the file, the parent will lose its lock. Processes blocked awaiting a lock may be awakened by signals. The file locks set by the flock()function do not interact in any way with the file locks set by the fcntl() and lockf() functions. If a process sets an exclusive lock on a file using the flock() function, the lock will not affect any process that is setting or clearing locks on the same file using the fcntl() or lockf() functions. It is there- fore possible for an inconsistency to arise if a file is locked by dif- ferent processes using flock() and fcntl(). (The fcntl() and lockf() functions use the same mechanism for record locking.) RETURN VALUES Upon successful completion, 0 (zero) is returned. Otherwise, -1 is re- turned 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 in- teruppted 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: open(2), close(2), exec(2), fcntl(2), fork(2), lockf(3) de- lim off flock(2)