'\"
'\" The contents of this file are subject to the AOLserver Public License
'\" Version 1.1 (the "License"); you may not use this file except in
'\" compliance with the License. You may obtain a copy of the License at
'\" http://aolserver.com/.
'\"
'\" Software distributed under the License is distributed on an "AS IS"
'\" basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
'\" the License for the specific language governing rights and limitations
'\" under the License.
'\"
'\" The Original Code is AOLserver Code and related documentation
'\" distributed by AOL.
'\" 
'\" The Initial Developer of the Original Code is America Online,
'\" Inc. Portions created by AOL are Copyright (C) 1999 America Online,
'\" Inc. All Rights Reserved.
'\"
'\" Alternatively, the contents of this file may be used under the terms
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'\" GPL and not to allow others to use your version of this file under the
'\" License, indicate your decision by deleting the provisions above and
'\" replace them with the notice and other provisions required by the GPL.
'\" If you do not delete the provisions above, a recipient may use your
'\" version of this file under either the License or the GPL.
'\" 
'\"
'\" $Header: /cvsroot/aolserver/aolserver/doc/Ns_CritSec.3,v 1.5 2003/04/10 22:00:37 shmooved Exp $
'\"
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.TH Ns_CritSec 3aolserver 4.0 AOLserver "AOLserver Library Procedures"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
, Ns_CsDestroy, Ns_CsEnter, Ns_CsInit, Ns_CsLeave \- Manage and use critical section locks
.SH SYNOPSIS
.nf
\fB#include "ns.h"\fR
.sp
void
\fBNs_CsDestroy\fR(\fINs_Cs *csPtr\fR)
.sp
void
\fBNs_CsEnter\fR(\fINs_Cs *csPtr\fR)
.sp
void
\fBNs_CsInit\fR(\fINs_Cs *csPtr\fR)
.sp
void
\fBNs_CsLeave\fR(\fINs_Cs *csPtr\fR)
.BE

.SH DESCRIPTION
.PP
Critical section locks are used to prevent more than one thread from
executing a specific section of code at one time. They are implemented
as "objects", which simply means that memory is allocated to hold the
lock state. They can also be called "sychronization objects".
.PP
While a thread is executing a critical section of code, all other
threads that want to execute that same section of code must wait until
the lock surrounding that critical section has been released.
.PP
This is crucial to prevent race conditions which could put the server
into an unknown state. For example, if a section of code frees a
pointer and then decrements a counter that stores how many pointers
exist, it is possible that the counter value and the actual number of
pointers may be different. If another section of the server relies on
this counter and reads it when the pointer has been freed, but the
counter has not yet been decremented, it could crash the server or put
it into an unknown state.
.PP
Critical section locks should be used sparingly as they will adversely
impact the performance of the server or module. They essentially cause
the section of code they enclose into behaving in a single-threaded
manner. If a critical section executes slowly or blocks, other threads
that must execute that section of code will begin to block as well
until the critical section lock is released.
.PP
You will normally want to wrap sections of code that are used to both
read and write values, create and destroy pointers and structures or
otherwise look at or modify data in the system. Use the same named
lock for both read and write operations on the same data.
.PP
Threads that are waiting for a critical section lock to be released do
not have to poll the lock. The critical section lock functions use
thread condition functions to signal when a lock is released.
.TP
\fBNs_CsDestroy\fR(\fIcsPtr\fR)

Destroy a critical section object.  Note that you would almost never
need to call this function as synchronization objects are typically
created at startup and exist until the server exits.

The underlying objects in the critical section are destroyed and the
critical section memory returned to the heap.
.TP
\fBNs_CsEnter\fR(\fIcsPtr\fR)

Lock a critical section object, initializing it first if needed.  If
the critical section is in use by another thread, the calling thread
will block until it is no longer so.

Note that critical sections are recursive and must be exited the same
number of times as they were entered.
.TP
\fBNs_CsInit\fR(\fIcsPtr\fR)

Initialize a critical section object. Memory will be allocated to hold
the object's state.

.TP
\fBNs_CsLeave\fR(\fIcsPtr\fR)

Unlock a critical section once. A count of threads waiting to enter
the critical section is kept, and a condition is signaled if this is
the final unlock of the critical section so that other threads may
enter the critical section.
.SH "SEE ALSO"
nsd(1), info(n), Ns_MasterLock(3), Ns_MasterUnlock(3),
Ns_CondDestroy(3), Ns_CondSignal(3), Ns_CondWait(3), Ns_MutexLock(3),
Ns_MutexUnlock(3)

.SH KEYWORDS