Ubuntu Manpage: flock - apply or remove an advisory lock on an open file

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 the open file specified by fd. The argument operation
is one of the following:

LOCK_SH Place a shared lock. More than one process may hold a shared lock for a
given file at a given time.

LOCK_EX Place an exclusive lock. Only one process may hold an exclusive lock for a
given file at a given time.

LOCK_UN Remove an existing lock held by this process.

A call to flock() may block if an incompatible lock is held by another process. To make a
nonblocking request, include LOCK_NB (by ORing) with any of the above operations.

A single file may not simultaneously have both shared and exclusive locks.

Locks created by flock() are associated with an open file description (see open(2)). This
means that duplicate file descriptors (created by, for example, fork(2) or dup(2)) refer
to the same lock, and this lock may be modified or released using any of these file
descriptors. Furthermore, the lock is released either by an explicit LOCK_UN operation on
any of these duplicate file descriptors, or when all such file descriptors have been
closed.

If a process uses open(2) (or similar) to obtain more than one file descriptor for the
same file, these file descriptors are treated independently by flock(). An attempt to
lock the file using one of these file descriptors may be denied by a lock that the calling
process has already placed via another file descriptor.

A process may hold only one type of lock (shared or exclusive) on a file. Subsequent
flock() calls on an already locked file will convert an existing lock to the new lock
mode.

Locks created by flock() are preserved across an execve(2).

A shared or exclusive lock can be placed on a file regardless of the mode in which the
file was opened.

RETURN VALUE

       On success, zero is returned.  On error, -1 is returned, and errno is set to indicate  the
       error.

ERRORS

       EBADF  fd is not an open file descriptor.

       EINTR  While  waiting  to acquire a lock, the call was interrupted by delivery of a signal
              caught by a handler; see signal(7).

       EINVAL operation is invalid.

       ENOLCK The kernel ran out of memory for allocating lock records.

       EWOULDBLOCK
              The file is locked and the LOCK_NB flag was selected.

CONFORMING TO

       4.4BSD (the flock() call first appeared  in  4.2BSD).   A  version  of  flock(),  possibly
       implemented in terms of fcntl(2), appears on most UNIX systems.

NOTES

       Since  kernel  2.0,  flock()  is implemented as a system call in its own right rather than
       being emulated in the GNU C library as a call  to  fcntl(2).   With  this  implementation,
       there  is  no  interaction  between  the types of lock placed by flock() and fcntl(2), and
       flock() does not detect deadlock.  (Note, however, that  on  some  systems,  such  as  the
       modern BSDs, flock() and fcntl(2) locks do interact with one another.)

       flock()  places  advisory  locks  only; given suitable permissions on a file, a process is
       free to ignore the use of flock() and perform I/O on the file.

       flock() and fcntl(2) locks have different semantics with respect to forked  processes  and
       dup(2).   On  systems that implement flock() using fcntl(2), the semantics of flock() will
       be different from those described in this manual page.

       Converting a lock (shared to exclusive, or vice versa) is not guaranteed to be atomic: the
       existing  lock  is  first  removed, and then a new lock is established.  Between these two
       steps, a pending lock request by another process may be granted, with the result that  the
       conversion  either  blocks,  or fails if LOCK_NB was specified.  (This is the original BSD
       behavior, and occurs on many other implementations.)

   NFS details
       In Linux kernels up to 2.6.11, flock() does not lock files over NFS (i.e.,  the  scope  of
       locks  was  limited  to  the  local  system).   Instead, one could use fcntl(2) byte-range
       locking, which does work over NFS, given a sufficiently recent  version  of  Linux  and  a
       server which supports locking.

       Since  Linux 2.6.12, NFS clients support flock() locks by emulating them as fcntl(2) byte-
       range locks on the entire file.  This means that fcntl(2) and flock()  locks  do  interact
       with  one  another  over NFS.  It also means that in order to place an exclusive lock, the
       file must be opened for writing.

       Since Linux 2.6.37, the kernel supports a compatibility mode  that  allows  flock()  locks
       (and  also  fcntl(2)  byte region locks) to be treated as local; see the discussion of the
       local_lock option in nfs(5).

   CIFS details
       In Linux kernels up to 5.4, flock() is not propagated over SMB.  A file  with  such  locks
       will not appear locked for remote clients.

       Since  Linux 5.5, flock() locks are emulated with SMB byte-range locks on the entire file.
       Similarly to NFS, this means that fcntl(2) and flock() locks interact  with  one  another.
       Another  important  side-effect  is  that  the locks are not advisory anymore: any IO on a
       locked file will always fail with EACCES when done from a separate file descriptor.   This
       difference  originates  from  the  design  of  locks  in  the SMB protocol, which provides
       mandatory locking semantics.

       Remote and mandatory locking semantics may vary  with  SMB  protocol,  mount  options  and
       server type.  See mount.cifs(8) for additional information.

COLOPHON

       This page is part of release 5.13 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.