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       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)):

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


       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.

              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.

              The  command  was  T_LOCK  and this lock operation would cause a

       EINVAL An invalid operation was specified in fd.

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


       SVr4, POSIX.1-2001.


       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.)


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