| LOCKF(3) | Library Functions Manual | LOCKF(3) | 
lockf —
#include <unistd.h>
int
  
  lockf(int
    filedes, int
    function, off_t
    size);
lockf() function allows sections of a file to be
  locked with advisory-mode locks. Calls to lockf() from
  other processes which attempt to lock the locked file section will either
  return an error value or block until the section becomes unlocked. All the
  locks for a process are removed when the process terminates.
The argument filedes is an open file
    descriptor. The file descriptor must have been opened either for write-only
    (O_WRONLY) or read/write
    (O_RDWR) operation.
The function argument is a control value which specifies the action to be taken. The permissible values for function are as follows:
F_ULOCK removes locks from a section of
    the file; F_LOCK and F_TLOCK
    both lock a section of a file if the section is available;
    F_TEST detects if a lock by another process is
    present on the specified section.
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). However, it is not permitted to lock a section that starts or extends before the beginning of the file. If size is 0, the section from the current offset through the largest possible file offset is locked (that is, from the current offset through the present or any future 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 are
    combined into a single locked section. If the request would cause the number
    of locks to exceed a system-imposed limit, the request will fail.
F_LOCK and F_TLOCK
    requests differ only by the action taken if the section is not available.
    F_LOCK blocks the calling process until the section
    is available. F_TLOCK makes the function fail if the
    section is already locked by another process.
File locks are released on first close by the locking process of any file descriptor for the file.
F_ULOCK requests release (wholly or in
    part) one or more locked sections controlled by the process. Locked sections
    will be unlocked starting at the current file offset through
    size bytes or to the end of file if size is 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 are still locked by the process. Releasing the
    center portion of a locked section will 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 will fail.
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, will 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 will attempt to unlock only the
    requested section.
A potential for deadlock occurs if a process controlling a locked
    region is put to sleep by attempting to lock the locked region of another
    process. This implementation detects that sleeping until a locked region is
    unlocked would cause a deadlock and fails with an
    EDEADLK error.
lockf(),
    fcntl(2) and
    flock(2) locks may be safely
    used concurrently.
Blocking on a section is interrupted by any signal.
lockf() function returns 0.
  Otherwise, it returns -1, sets errno to indicate an
  error, and existing locks are not changed.
lockf() will fail if:
EAGAIN]F_TLOCK or F_TEST and the
      section is already locked by another process.EBADF]The argument function is
        F_LOCK or F_TLOCK, and
        filedes is not a valid file descriptor open for
        writing.
EDEADLK]F_LOCK and a deadlock is detected.EINTR]lockf() was interrupted by the delivery of a
      signal.EINVAL]F_ULOCK, F_LOCK,
      F_TLOCK or F_TEST.
    The argument filedes refers to a file that does not support locking.
ENOLCK]F_ULOCK, F_LOCK or
      F_TLOCK, and satisfying the lock or unlock request
      would result in the number of locked regions in the system exceeding a
      system-imposed limit.lockf() function conforms to X/Open
  Portability Guide Issue 4, Version 2
  (“XPG4.2”) and IEEE Std 1003.1-2008
  (“POSIX.1”).
lockf() function first appeared in
  FreeBSD 1.4.
| October 15, 2011 | NetBSD 10.0 |