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LOCKF(3P)                  POSIX Programmer's Manual                 LOCKF(3P)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may dif-
       fer (consult the corresponding Linux manual page for details of Linux behavior), or the interface  may  not  be
       implemented on Linux.

NAME
       lockf - record locking on files

SYNOPSIS
       #include <unistd.h>

       int lockf(int fildes, int function, off_t size);


DESCRIPTION
       The  lockf()  function  shall  lock  sections  of  a file with advisory-mode locks. Calls to lockf() from other
       threads which attempt to lock the locked file section shall either return an error value  or  block  until  the
       section  becomes  unlocked. All the locks for a process are removed when the process terminates. Record locking
       with lockf() shall be supported for regular files and may be supported for other files.

       The fildes argument is an open file descriptor. To establish a lock with this  function,  the  file  descriptor
       shall be opened with write-only permission (O_WRONLY) or with read/write permission (O_RDWR).

       The  function  argument  is  a control value which specifies the action to be taken. The permissible values for
       function are defined in <unistd.h> as follows:

                                   Function  Description
                                   F_ULOCK   Unlock locked sections.
                                   F_LOCK    Lock a section for exclusive use.
                                   F_TLOCK   Test and lock a section for exclusive use.
                                   F_TEST    Test a section for locks by other processes.

       F_TEST shall detect if a lock by another process is present on the specified section.

       F_LOCK and F_TLOCK shall both lock a section of a file if the section is available.

       F_ULOCK shall remove locks from a section of the file.

       The size argument is the number of contiguous bytes to be locked or unlocked.  The  section  to  be  locked  or
       unlocked  starts  at  the  current offset in the file and extends forward for a positive size or backward for a
       negative size (the preceding bytes up to but not including the current offset). If size is 0, the section  from
       the  current  offset through the largest possible file offset shall be locked (that is, from the current offset
       through the present or any future end-of-file). An area need not be allocated to the file to be locked  because
       locks may exist past the end-of-file.

       The  sections  locked  with F_LOCK or F_TLOCK may, in whole or in part, contain or be contained by a previously
       locked section for the same process. When this occurs, or if adjacent locked sections would occur, the sections
       shall be combined into a single locked section. If the request would cause the number of locks to exceed a sys-
       tem-imposed limit, the request shall fail.

       F_LOCK and F_TLOCK requests differ only by the action taken if the section is not available. F_LOCK shall block
       the  calling  thread until the section is available. F_TLOCK shall cause the function to fail if the section is
       already locked by another process.

       File locks shall be released on first close by the locking process of any file descriptor for the file.

       F_ULOCK requests may release (wholly or in part) one or more locked sections controlled by the process.  Locked
       sections shall be unlocked starting at the current file offset through size bytes or to the end-of-file if size
       is (off_t)0.  When all of a locked section is not released (that is, when the beginning or end of the  area  to
       be  unlocked  falls within a locked section), the remaining portions of that section shall remain locked by the
       process. Releasing the center portion of a locked section shall cause the remaining locked  beginning  and  end
       portions  to  become two separate locked sections. If the request would cause the number of locks in the system
       to exceed a system-imposed limit, the request shall fail.

       A potential for deadlock occurs if the threads of a process controlling a locked section are blocked by access-
       ing  another  process' locked section. If the system detects that deadlock would occur, lockf() shall fail with
       an [EDEADLK] error.

       The interaction between fcntl() and lockf() locks is unspecified.

       Blocking on a section shall be interrupted by any signal.

       An F_ULOCK request in which size is non-zero and the offset of the last byte of the requested  section  is  the
       maximum  value  for an object of type off_t, when the process has an existing lock in which size is 0 and which
       includes the last byte of the requested section, shall be treated as a request to unlock from the start of  the
       requested  section  with  a  size  equal  to  0. Otherwise, an F_ULOCK request shall attempt to unlock only the
       requested section.

       Attempting to lock a section of a file that is associated with a buffered stream produces unspecified  results.

RETURN VALUE
       Upon  successful  completion,  lockf()  shall return 0. Otherwise, it shall return -1, set errno to indicate an
       error, and existing locks shall not be changed.

ERRORS
       The lockf() function shall fail if:

       EBADF  The fildes argument is not a valid open file descriptor; or function is F_LOCK or F_TLOCK and fildes  is
              not a valid file descriptor open for writing.

       EACCES or EAGAIN

              The function argument is F_TLOCK or F_TEST and the section is already locked by another process.

       EDEADLK
              The function argument is F_LOCK and a deadlock is detected.

       EINTR  A signal was caught during execution of the function.

       EINVAL The  function  argument is not one of F_LOCK, F_TLOCK, F_TEST, or F_ULOCK; or size plus the current file
              offset is less than 0.

       EOVERFLOW
              The offset of the first, or if size is not 0 then the last, byte in the requested section cannot be rep-
              resented correctly in an object of type off_t.


       The lockf() function may fail if:

       EAGAIN The function argument is F_LOCK or F_TLOCK and the file is mapped with mmap().

       EDEADLK or ENOLCK

              The function argument is F_LOCK, F_TLOCK, or F_ULOCK, and the request would cause the number of locks to
              exceed a system-imposed limit.

       EOPNOTSUPP or EINVAL

              The implementation does not support the locking of files of the type indicated by the fildes argument.


       The following sections are informative.

EXAMPLES
   Locking a Portion of a File
       In the following example, a file named /home/cnd/mod1 is being modified. Other processes that use  locking  are
       prevented  from changing it during this process. Only the first 10000 bytes are locked, and the lock call fails
       if another process has any part of this area locked already.


              #include <fcntl.h>
              #include <unistd.h>


              int fildes;
              int status;
              ...
              fildes = open("/home/cnd/mod1", O_RDWR);
              status = lockf(fildes, F_TLOCK, (off_t)10000);

APPLICATION USAGE
       Record-locking should not be used in combination with the fopen(), fread(), fwrite(),  and  other  stdio  func-
       tions.  Instead, the more primitive, non-buffered functions (such as open()) should be used. Unexpected results
       may occur in processes that do buffering in the user address space. The process may later read/write data which
       is/was locked. The stdio functions are the most common source of unexpected buffering.

       The alarm() function may be used to provide a timeout facility in applications requiring it.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       alarm(),  chmod(),  close(),  creat(),  fcntl(), fopen(), mmap(), open(), read(), write(), the Base Definitions
       volume of IEEE Std 1003.1-2001, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard  for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifica-
       tions Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers,  Inc  and  The
       Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Stan-
       dard, the original IEEE and The Open Group Standard is the referee  document.  The  original  Standard  can  be
       obtained online at http://www.opengroup.org/unix/online.html .



IEEE/The Open Group                  2003                            LOCKF(3P)