lockf(3p) — Linux manual page


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

PROLOG         top

       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         top

       lockf — record locking on files

SYNOPSIS         top

       #include <unistd.h>

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

DESCRIPTION         top

       The lockf() function shall lock sections of a file with advisory-
       mode locks. Calls to lockf() from threads in other processes
       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

       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-

       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 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

       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 accessing a locked
       section of another process. 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

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

RETURN VALUE         top

       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         top

       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.

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

              The function argument is F_LOCK and a deadlock is

       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.

              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().

              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.

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

       The following sections are informative.

EXAMPLES         top

   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);


       Record-locking should not be used in combination with the
       fopen(), fread(), fwrite(), and other stdio functions. 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         top




SEE ALSO         top

       alarm(3p), chmod(3p), close(3p), creat(3p), fcntl(3p), fopen(3p),
       mmap(3p), open(3p), read(3p), write(3p)

       The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 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 .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group               2017                         LOCKF(3P)

Pages that refer to this page: unistd.h(0p)mmap(3p)