FGETLK

FGETLK is a command used with the fcntl system call to examine advisory file-region locks on a file descriptor without actually acquiring them. It is used to determine whether a requested lock could be granted and to obtain information about any existing conflicting locks.

To use FGETLK, a program fills a struct flock with the region and lock type it would like to obtain (l_type, l_whence, l_start, l_len). The call fcntl(fd, F_GETLK, &lock) is then issued. On return, if the region could be locked, the kernel sets lock.l_type to F_UNLCK and lock.l_pid to 0, indicating no conflict. If another process holds a conflicting lock, lock.l_type reflects the type of the existing lock (F_RDLCK or F_WRLCK) and lock.l_pid contains the process ID of the locking process. The region described by l_start, l_len, and l_whence is the region being tested, and the file’s current locks are not changed by FGETLK.

The operation is non-blocking: it reports immediately whether the requested lock would be possible and provides details about any blocking lock. This makes FGETLK useful as a pre-check before attempting to acquire a lock with F_SETLK or F_SETLKW, or for diagnosing lock conflicts in multi-process environments.

Notes and variations: Locking with fcntl is advisory, so behavior depends on all cooperating processes using locking. Some platforms offer equivalent 64-bit variants, such as F_GETLK64, to accommodate large-file structures. FGETLK is part of a family that also includes F_SETLK and F_SETLKW for setting locks, with F_GETLK serving as the non-blocking inquiry.