lockf() function allows sections of a file to be
locked with advisory-mode locks. Calls to
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 fd is an open file descriptor.
The file descriptor must have been opened either for write-only
O_WRONLY) or read/write
The function argument is a control value
which specifies the action to be taken. The permissible values for
function are as follows:
- unlock locked sections
- lock a section for exclusive use
- test and lock a section for exclusive use
- test a section for locks by other processes
F_ULOCK removes locks from a section of
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_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.
requests differ only by the action taken if the section is not available.
F_LOCK blocks the calling process until the section
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.
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
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
locks are compatible. Processes using different locking interfaces can
cooperate over the same file safely. However, only one of such interfaces
should be used within the same process. If a file is locked by a process
any record within the file will be seen as locked from the viewpoint of
another process using
lockf(), and vice versa.
Blocking on a section is interrupted by any signal.