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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)):

           _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  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; 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 deadlock.

       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 attributes(7).

       │InterfaceAttributeValue   │
       │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    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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