lockf(3C) COMPATIBILITY FUNCTIONS lockf(3C)
NAME
lockf - record locking on files
SYNOPSIS
#include <unistd.h>
int lockf (int fildes, int function, long size);
DESCRIPTION
lockf allows sections of a file to be locked; advisory or
mandatory write locks depending on the mode bits of the file
[see chmod(2)]. Locking calls from other processes that
attempt to lock the locked file section will either return
an error value or be put to sleep until the resource becomes
unlocked. All the locks for a process are removed when the
process terminates. [See fcntl(2) for more information
about record locking.]
fildes is an open file descriptor. The file descriptor must
have OWRONLY or ORDWR permission in order to establish
locks with this function call.
function is a control value that specifies the action to be
taken. The permissible values for function are defined in
unistd.h as follows:
#define FULOCK 0 /* unlock previously locked section */
#define FLOCK 1 /* lock section for exclusive use */
#define FTLOCK 2 /* test & lock section for exclusive use */
#define FTEST 3 /* test section for other locks */
All other values of function are reserved for future exten-
sions and will result in an error return if not implemented.
FTEST is used to detect if a lock by another process is
present on the specified section. FLOCK and FTLOCK both
lock a section of a file if the section is available.
FULOCK removes locks from a section of the file.
size is the number of contiguous bytes to be locked or
unlocked. The resource to be locked or unlocked starts at
the current offset in the file and extends forward for a
positive size and backward for a negative size (the preced-
ing bytes up to but not including the current offset). If
size is zero, the section from the current offset through
the largest file offset is locked (i.e., from the current
offset through the present or any future end-of-file). An
area need not be allocated to the file in order to be locked
as such locks may exist past the end-of-file.
The sections locked with FLOCK or FTLOCK may, in whole or
in part, contain or be contained by a previously locked sec-
tion for the same process. Locked sections will be unlocked
Last change: C Programming Language Utilities 1
lockf(3C) COMPATIBILITY FUNCTIONS lockf(3C)
starting at the the point of the offset through size bytes
or to the end of file if size is (offt) 0. When this
situation occurs, or if this situation occurs in adjacent
sections, the sections are combined into a single section.
If the request requires that a new element be added to the
table of active locks and this table is already full, an
error is returned, and the new section is not locked.
FLOCK and FTLOCK requests differ only by the action taken
if the resource is not available. FLOCK will cause the
calling process to sleep until the resource is available.
FTLOCK will cause the function to return a -1 and set errno
to EACCES if the section is already locked by another pro-
cess.
FULOCK requests may, in whole or in part, release one or
more locked sections controlled by the process. When sec-
tions are not fully released, the remaining sections are
still locked by the process. Releasing the center section
of a locked section requires an additional element in the
table of active locks. If this table is full, an errno is
set to ENOLK and the requested section is not released.
A potential for deadlock occurs if a process controlling a
locked resource is put to sleep by requesting another
process's locked resource. Thus calls to lockf or fcntl
scan for a deadlock prior to sleeping on a locked resource.
An error return is made if sleeping on the locked resource
would cause a deadlock.
Sleeping on a resource is interrupted with any signal. The
alarm system call may be used to provide a timeout facility
in applications that require this facility.
lockf will fail if one or more of the following are true:
EBADF fildes is not a valid open descriptor.
EAGAIN cmd is FTLOCK or FTEST and the section is already
locked by another process.
EDEADLKcmd is FLOCK and a deadlock would occur.
ENOLK cmd is FLOCK, FTLOCK, or FULOCK and the number of
entries in the lock table would exceed the number
allocated on the system.
ECOMM fildes is on a remote machine and the link to that
machine is no longer active.
SEE ALSO
intro(2), alarm(2), chmod(2), close(2), creat(2), fcntl(2),
Last change: C Programming Language Utilities 2
lockf(3C) COMPATIBILITY FUNCTIONS lockf(3C)
open(2), read(2), write(2).
DIAGNOSTICS
Upon successful completion, a value of 0 is returned. Oth-
erwise, a value of -1 is returned and errno is set to indi-
cate the error.
NOTES
Unexpected results may occur in processes that do buffering
in the user address space. The process may later read/write
data that is/was locked. The standard I/O package is the
most common source of unexpected buffering.
Because in the future the variable errno will be set to
EAGAIN rather than EACCES when a section of a file is
already locked by another process, portable application pro-
grams should expect and test for either value.
Last change: C Programming Language Utilities 3