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NAME

       lockf - apply, test or remove a POSIX lock on an open file

SYNOPSIS

       #include <unistd.h>

       int lockf(int fd, int cmd, off_t len);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       lockf():
           _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 ||
           _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED

DESCRIPTION

       Apply, test or remove a POSIX lock on a section of an open file.  The file is specified by
       fd,  a  file  descriptor  open for writing, the action by cmd, and the section consists of
       byte positions pos..pos+len-1 if len is positive, and pos-len..pos-1 if len  is  negative,
       where  pos  is the current file position, and if len is zero, the section extends from the
       current file position  to  infinity,  encompassing  the  present  and  future  end-of-file
       positions.  In all cases, the section may extend past current end-of-file.

       On  Linux,  lockf()  is  just an interface on top of fcntl(2) locking.  Many other systems
       implement lockf() in this way, but note that POSIX.1-2001 leaves the relationship  between
       lockf()  and  fcntl(2)  locks  unspecified.   A portable application should probably avoid
       mixing calls to these interfaces.

       Valid operations are given below:

       F_LOCK Set an exclusive lock on the specified section of the  file.   If  (part  of)  this
              section is already locked, the call blocks until the previous lock is released.  If
              this section overlaps an earlier locked section, both are merged.  File  locks  are
              released  as  soon as the process holding the locks closes some file descriptor for
              the file.  A child process does not inherit these locks.

       F_TLOCK
              Same as F_LOCK but the call never blocks and returns an error instead if  the  file
              is already locked.

       F_ULOCK
              Unlock  the  indicated  section of the file.  This may cause a locked section to be
              split into two locked sections.

       F_TEST Test the lock: return 0 if the specified section is  unlocked  or  locked  by  this
              process;  return -1, set errno to EAGAIN (EACCES on some other systems), if another
              process holds a lock.

RETURN VALUE

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS

       EACCES or EAGAIN
              The file is locked and F_TLOCK  or  F_TEST  was  specified,  or  the  operation  is
              prohibited because the file has been memory-mapped by another process.

       EBADF  fd  is  not  an  open  file descriptor; or cmd is F_LOCK or F_TLOCK and fd is not a
              writable file descriptor.

       EDEADLK
              The command was T_LOCK and this lock operation would cause a deadlock.

       EINVAL An invalid operation was specified in fd.

       ENOLCK Too many segment locks open, lock table is full.

CONFORMING TO

       SVr4, POSIX.1-2001.

SEE ALSO

       fcntl(2), flock(2)
       There are  also  locks.txt  and  mandatory-locking.txt  in  the  kernel  source  directory
       Documentation/filesystems.   (On  older  kernels,  these  files  are  directly  under  the
       Documentation/ directory, and mandatory-locking.txt is called mandatory.txt.)

COLOPHON

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       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.