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LOCKF(3)		   Linux Programmer's Manual		      LOCKF(3)

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

       #include <unistd.h>

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

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

	   _XOPEN_SOURCE >= 500
	       || /* Glibc since 2.19: */ _DEFAULT_SOURCE
	       || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

       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
       leaves the relationship between lockf() and fcntl(2) locks unspecified.
       A  portable  application	 should	 probably  avoid mixing calls to these

       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 ear-
	      lier  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

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

	      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.

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

	      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.

	      The command was F_LOCK and this lock  operation  would  cause  a

       EINVAL An invalid operation was specified in cmd.

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

       For   an	  explanation	of   the  terms	 used  in  this	 section,  see

       |Interface | Attribute	  | Value   |
       |lockf()	  | Thread safety | MT-Safe |
       POSIX.1-2001, POSIX.1-2008, SVr4.

       fcntl(2), flock(2)

       locks.txt and mandatory-locking.txt in the Linux kernel	source	direc-
       tory  Documentation/filesystems	(on  older  kernels,  these  files are
       directly under the Documentation directory,  and	 mandatory-locking.txt
       is called mandatory.txt)

       This  page  is  part of release 4.10 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

GNU				  2016-03-15			      LOCKF(3)