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 speci­
                        fied (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  descrip­
       tor.   So,  dup(2)  and  fork(2)  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 flock(2) call first appeared in 4.2BSD).


NOTES

       flock(2) does not  lock  files  over  NFS.   Use  fcntl(2)
       instead:  that  does  work  over NFS, given a sufficiently
       recent version of Linux and a server which supports  lock­
       ing.
       respect to forked processes and dup(2).


SEE ALSO

       open(2), close(2), dup(2), execve(2),  fcntl(2),  fork(2),
       lockf(3).   There  are also locks.txt and mandatory.txt in
       /usr/src/linux/Documentation.