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LOCKF(3)                   Linux Programmer's Manual                  LOCKF(3)
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():
           _XOPEN_SOURCE >= 500
               || /* Glibc since 2.19: */ _DEFAULT_SOURCE
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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
       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 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
              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 F_LOCK and this lock  operation  would  cause  a
              deadlock.
       EINVAL An invalid operation was specified in cmd.
       ENOLCK Too many segment locks open, lock table is full.
ATTRIBUTES
       For   an   explanation   of   the  terms  used  in  this  section,  see
       attributes(7).
       +----------+---------------+---------+
       |Interface | Attribute     | Value   |
       +----------+---------------+---------+
       |lockf()   | Thread safety | MT-Safe |
       +----------+---------------+---------+
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, SVr4.
SEE ALSO
       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)
COLOPHON
       This  page  is  part of release 4.15 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/.
GNU                               2017-09-15                          LOCKF(3)