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



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

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 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 (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
       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 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 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 aa Portion of aa 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 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
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       alarm(), chmod(), close(), creat(), fcntl(), fopen(), mmap(), open(),
       read(), write()

       The Base Definitions volume of POSIX.1-2017, <unistd.h>

COPYRIGHT
       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(3)
