LOCKF(3)                  (2019-03-06)                   LOCKF(3)

          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

              _XOPEN_SOURCE >= 500
                  || /* Glibc since 2.19: */ _DEFAULT_SOURCE
                  || /* Glibc versions <= 2.19: */ _BSD_SOURCE ||

          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 pre-
          sent  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 applica-
          tion should probably avoid mixing calls to these interfaces.

          Valid operations are given below:

               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

               Same as F_LOCK but the call never blocks and returns an
               error instead if the file is already locked.

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               Unlock the indicated section of  the  file.   This  may
               cause a locked section to be split into two locked sec-

               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.

          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.

               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.

               While waiting to acquire a lock, the  call  was  inter-
               rupted by delivery of a signal caught by a handler; see

               An invalid operation was specified in cmd.

               Too many segment locks open, lock table is full.

          For an explanation of the terms used in  this  section,  see
          attributes(7).     allbox;    lb    lb    lb    l    l    l.
          Interface Attribute Value     T{     lockf()     T}   Thread
          safety  MT-Safe

          POSIX.1-2001, POSIX.1-2008, SVr4.

          fcntl(2), flock(2)

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          locks.txt and  mandatory-locking.txt  in  the  Linux  kernel
          source  directory  Documentation/filesystems  (on older ker-
          nels, these  files  are  directly  under  the  Documentation
          directory,     and     mandatory-locking.txt    is    called

          This page is part of release 5.10  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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