.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "LOCKF" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" lockf 
.SH PROLOG
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.
.SH NAME
lockf \- record locking on files
.SH SYNOPSIS
.LP
\fB#include <unistd.h>
.br
.sp
int lockf(int\fP \fIfildes\fP\fB, int\fP \fIfunction\fP\fB, off_t\fP
\fIsize\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIlockf\fP() function shall lock sections of a file with advisory-mode
locks. Calls to \fIlockf\fP() from other threads
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 \fIlockf\fP() shall be supported for regular
files and may be supported for other files.
.LP
The \fIfildes\fP 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).
.LP
The \fIfunction\fP argument is a control value which specifies the
action to be taken. The permissible values for
\fIfunction\fP are defined in \fI<unistd.h>\fP as follows:
.TS C
center; l2 l.
\fBFunction\fP	\fBDescription\fP
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.
.TE
.LP
F_TEST shall detect if a lock by another process is present on the
specified section.
.LP
F_LOCK and F_TLOCK shall both lock a section of a file if the section
is available.
.LP
F_ULOCK shall remove locks from a section of the file.
.LP
The \fIsize\fP 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 \fIsize\fP 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.
.LP
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.
.LP
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.
.LP
File locks shall be released on first close by the locking process
of any file descriptor for the file.
.LP
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 \fIsize\fP bytes
or to the end-of-file if \fIsize\fP is (\fBoff_t\fP)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.
.LP
A potential for deadlock occurs if the threads of a process controlling
a locked section are blocked by accessing another
process' locked section. If the system detects that deadlock would
occur, \fIlockf\fP() shall fail with an [EDEADLK] error.
.LP
The interaction between \fIfcntl\fP() and \fIlockf\fP() locks is unspecified.
.LP
Blocking on a section shall be interrupted by any signal.
.LP
An F_ULOCK request in which \fIsize\fP is non-zero and the offset
of the last byte of the requested section is the maximum
value for an object of type \fBoff_t\fP, when the process has an existing
lock in which \fIsize\fP 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.
.LP
Attempting to lock a section of a file that is associated with a buffered
stream produces unspecified results.
.SH RETURN VALUE
.LP
Upon successful completion, \fIlockf\fP() shall return 0. Otherwise,
it shall return -1, set \fIerrno\fP to indicate an error,
and existing locks shall not be changed.
.SH ERRORS
.LP
The \fIlockf\fP() function shall fail if:
.TP 7
.B EBADF
The \fIfildes\fP argument is not a valid open file descriptor; or
\fIfunction\fP is F_LOCK or F_TLOCK and \fIfildes\fP is
not a valid file descriptor open for writing.
.TP 7
.B EACCES \fRor\fP EAGAIN
.sp
The \fIfunction\fP argument is F_TLOCK or F_TEST and the section is
already locked by another process.
.TP 7
.B EDEADLK
The \fIfunction\fP argument is F_LOCK and a deadlock is detected.
.TP 7
.B EINTR
A signal was caught during execution of the function.
.TP 7
.B EINVAL
The \fIfunction\fP argument is not one of F_LOCK, F_TLOCK, F_TEST,
or F_ULOCK; or \fIsize\fP plus the current file offset is
less than 0.
.TP 7
.B EOVERFLOW
The offset of the first, or if \fIsize\fP is not 0 then the last,
byte in the requested section cannot be represented
correctly in an object of type \fBoff_t\fP.
.sp
.LP
The \fIlockf\fP() function may fail if:
.TP 7
.B EAGAIN
The \fIfunction\fP argument is F_LOCK or F_TLOCK and the file is mapped
with \fImmap\fP().
.TP 7
.B EDEADLK \fRor\fP ENOLCK
.sp
The \fIfunction\fP argument is F_LOCK, F_TLOCK, or F_ULOCK, and the
request would cause the number of locks to exceed a
system-imposed limit.
.TP 7
.B EOPNOTSUPP \fRor\fP EINVAL
.sp
The implementation does not support the locking of files of the type
indicated by the \fIfildes\fP argument.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Locking a Portion of a File
.LP
In the following example, a file named \fB/home/cnd/mod1\fP 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.
.sp
.RS
.nf

\fB#include <fcntl.h>
#include <unistd.h>
.sp

int fildes;
int status;
\&...
fildes = open("/home/cnd/mod1", O_RDWR);
status = lockf(fildes, F_TLOCK, (off_t)10000);
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
Record-locking should not be used in combination with the \fIfopen\fP(),
\fIfread\fP(), \fIfwrite\fP(), and other \fIstdio\fP
functions. Instead, the more primitive, non-buffered functions (such
as \fIopen\fP()) 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 \fIstdio\fP functions are the most common
source of unexpected buffering.
.LP
The \fIalarm\fP() function may be used to provide a timeout facility
in applications
requiring it.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIalarm\fP(), \fIchmod\fP(), \fIclose\fP(), \fIcreat\fP(), \fIfcntl\fP(),
\fIfopen\fP(), \fImmap\fP(), \fIopen\fP(), \fIread\fP(), \fIwrite\fP(),
the Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<unistd.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
