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lockf - apply, test or remove a POSIX lock on an open file
#include <sys/file.h> int lockf(int fd, int cmd, off_t len);
Apply, test or remove a POSIX lock on an open file. The file is specified by fd. This call is just an interface for fcntl(2). Valid operations are given below: F_LOCK Set an exclusive lock to the file. Only one process may hold an exclusive lock for a given file at a given time. If the file is already locked it blocks until the previous lock is released. F_TLOCK Same as F_LOCK but never blocks and return error instead if the file is already locked. F_ULOCK Unlock the file. F_TEST Test the lock: return 0 if fd is unlocked or locked by this process; return -1, set errno to EACCES, if another process holds the lock.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
EAGAIN The file is locked and the LOCK_NB flag was selected, or operation is prohibited because the file has been memory-mapped by another process. EBADF fd is not an open file descriptor. EDEADLK Specified lock operation would cause a deadlock. EINVAL An invalid operation was specified in fd. ENOLCK Too many segment locks open, lock table is full.