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    Command:

    lockf

    
    
    

    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 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.
           EINVAL An invalid operation was specified in cmd.
    
           ENOLCK Too many segment locks open, lock table is full.
    
    
    

    CONFORMING TO

           SVr4, POSIX.1-2001.
    
    
    

    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)
    
    
    

    GNU 2014-02-11 LOCKF(3)

    
    
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