FLOCK Section: Linux Programmer's Manual (2) Updated: 11 December 2025 Index   NAME flock - apply or remove an advisory lock on an open file   SYNOPSIS #include <sys/file.h> int flock(int fd, int operation)   DESCRIPTION Apply or remove an advisory lock on an open file. The file is specified by fd. Valid operations are given below: LOCK_SH Shared lock. More than one process may hold a shared lock for a given file at a given time. LOCK_EX Exclusive lock. Only one process may hold an exclusive lock for a given file at a given time. LOCK_UN Unlock. LOCK_NB Don't block when locking. May be specified (by or'ing) along with one of the other operations. A single file may not simultaneously have both shared and exclusive locks. A file is locked (i.e., the inode), not the file descriptor. So, and do not create multiple instances of a lock.   RETURN VALUE On success, zero is returned. On error, -1 is returned, and errno is set appropriately.   ERRORS EWOULDBLOCK The file is locked and the LOCK_NB flag was selected.   CONFORMING TO 4.4BSD (the call first appeared in 4.2BSD).   NOTES does not lock files over NFS. Use instead: that does work over NFS, given a sufficiently recent version of Linux and a server which supports locking. and locks have different semantics with respect to forked processes and   SEE ALSO There are also locks.txt and mandatory.txt in /usr/src/linux/Documentation.   Index NAME SYNOPSIS DESCRIPTION RETURN VALUE ERRORS CONFORMING TO NOTES SEE ALSO This document was created by using the manual pages. Time: 11:56:05 GMT, March 24, 2025