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155 lines
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<TITLE>"PTHREAD_GETCONCURRENCY"(P) manual page</TITLE>
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<H4>POSIX Threads for Windows – REFERENCE - <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A></H4>
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<P><A HREF="index.html">Reference Index</A></P>
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<P><A HREF="#toc">Table of Contents</A></P>
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<H2><A HREF="#toc0" NAME="sect0">Name</A></H2>
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<P>pthread_getconcurrency, pthread_setconcurrency - get and set the
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level of concurrency
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</P>
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<H2><A HREF="#toc1" NAME="sect1">Synopsis</A></H2>
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<P><B>#include <pthread.h> </B>
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</P>
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<P><B>int pthread_getconcurrency(void);</B> <BR><B>int
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pthread_setconcurrency(int</B> <I>new_level</I><B>); </B>
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</P>
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<H2><A HREF="#toc2" NAME="sect2">Description</A></H2>
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<P>Unbound threads in a process may or may not be required to be
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simultaneously active. By default, the threads implementation ensures
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that a sufficient number of threads are active so that the process
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can continue to make progress. While this conserves system resources,
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it may not produce the most effective level of concurrency.
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</P>
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<P>The <B>pthread_setconcurrency</B> function allows an application
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to inform the threads implementation of its desired concurrency
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level, <I>new_level</I>. The actual level of concurrency provided by
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the implementation as a result of this function call is unspecified.
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</P>
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<P>If <I>new_level</I> is zero, it causes the implementation to
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maintain the concurrency level at its discretion as if
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<B>pthread_setconcurrency</B> had never been called.
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</P>
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<P>The <B>pthread_getconcurrency</B> function shall return the value
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set by a previous call to the <B>pthread_setconcurrency</B> function.
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If the <B>pthread_setconcurrency</B> function was not previously
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called, this function shall return zero to indicate that the
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implementation is maintaining the concurrency level.
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</P>
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<P>A call to <B>pthread_setconcurrency</B> shall inform the
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implementation of its desired concurrency level. The implementation
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shall use this as a hint, not a requirement.
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</P>
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<P>If an implementation does not support multiplexing of user threads
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on top of several kernel-scheduled entities, the
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<B>pthread_setconcurrency</B> and <B>pthread_getconcurrency</B>
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functions are provided for source code compatibility but they shall
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have no effect when called. To maintain the function semantics, the
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<I>new_level</I> parameter is saved when <B>pthread_setconcurrency</B>
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is called so that a subsequent call to <B>pthread_getconcurrency</B>
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shall return the same value.
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</P>
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<P><B>Pthreads-w32</B> provides these routines for source code
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compatibility only, as described in the previous paragraph.</P>
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<H2><A HREF="#toc3" NAME="sect3">Return Value</A></H2>
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<P>If successful, the <B>pthread_setconcurrency</B> function shall
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return zero; otherwise, an error number shall be returned to indicate
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the error.
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</P>
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<P>The <B>pthread_getconcurrency</B> function shall always return the
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concurrency level set by a previous call to <B>pthread_setconcurrency</B>
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. If the <B>pthread_setconcurrency</B> function has never been
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called, <B>pthread_getconcurrency</B> shall return zero.
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</P>
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<H2><A HREF="#toc4" NAME="sect4">Errors</A></H2>
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<P>The <B>pthread_setconcurrency</B> function shall fail if:
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</P>
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<DL>
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<DT><B>EINVAL</B>
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</DT><DD>
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The value specified by <I>new_level</I> is negative.
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</DD><DT>
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<B>EAGAIN</B>
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</DT><DD STYLE="margin-bottom: 0.5cm">
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The value specific by <I>new_level</I> would cause a system resource
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to be exceeded.
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</DD></DL>
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<P>
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These functions shall not return an error code of [EINTR].
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</P>
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<P><I>The following sections are informative.</I>
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</P>
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<H2><A HREF="#toc5" NAME="sect5">Examples</A></H2>
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<P>None.
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</P>
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<H2><A HREF="#toc6" NAME="sect6">Application Usage</A></H2>
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<P>Use of these functions changes the state of the underlying
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concurrency upon which the application depends. Library developers
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are advised to not use the <B>pthread_getconcurrency</B> and
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<B>pthread_setconcurrency</B> functions since their use may conflict
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with an applications use of these functions.
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</P>
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<H2><A HREF="#toc7" NAME="sect7">Rationale</A></H2>
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<P>None.
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</P>
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<H2><A HREF="#toc8" NAME="sect8">Future Directions</A></H2>
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<P>None.
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</P>
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<H2><A HREF="#toc9" NAME="sect9">See Also</A></H2>
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<P>The Base Definitions volume of IEEE Std 1003.1-2001,
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<I><pthread.h></I>
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</P>
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<H2><A HREF="#toc10" NAME="sect10">Copyright</A></H2>
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<P>Portions of this text are reprinted and reproduced in electronic
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form from IEEE Std 1003.1, 2003 Edition, Standard for Information
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Technology -- Portable Operating System Interface (POSIX), The Open
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Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the
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Institute of Electrical and Electronics Engineers, Inc and The Open
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Group. In the event of any discrepancy between this version and the
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original IEEE and The Open Group Standard, the original IEEE and The
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Open Group Standard is the referee document. The original Standard
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can be obtained online at <A HREF="http://www.opengroup.org/unix/online.html">http://www.opengroup.org/unix/online.html</A>
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.
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</P>
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<P>Modified by Ross Johnson for use with <A HREF="http://sources.redhat.com/pthreads-win32">Pthreads-w32</A>.</P>
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<HR>
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<P><A NAME="toc"></A><B>Table of Contents</B></P>
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<UL>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect0" NAME="toc0">Name</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect1" NAME="toc1">Synopsis</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect2" NAME="toc2">Description</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect3" NAME="toc3">Return
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Value</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect4" NAME="toc4">Errors</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect5" NAME="toc5">Examples</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect6" NAME="toc6">Application
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Usage</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect7" NAME="toc7">Rationale</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect8" NAME="toc8">Future
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Directions</A>
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</P>
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<LI><P STYLE="margin-bottom: 0cm"><A HREF="#sect9" NAME="toc9">See
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Also</A>
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</P>
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<LI><P><A HREF="#sect10" NAME="toc10">Copyright</A>
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</P>
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</UL>
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