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
          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 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
   directory  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 4.09 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/.

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