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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 differ (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  sup-
       ported 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 system-imposed
       limit, the request shall fail.
       F_LOCK and F_TLOCK requests differ only by the action taken if the sec-
       tion  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 por-
       tions 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 accessing another process' locked sec-
       tion. 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 represented 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  modi-
       fied.  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 functions. Instead, the more primi-
       tive, non-buffered functions (such as open())  should  be  used.  Unex-
       pected  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, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications 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 Standard, 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)