title: What is PTHREAD_MUTEX_ADAPTIVE_NP
body: <p>Where can I find documentation for "adaptive" pthread mutexes?   The symbol PTHREAD_MUTEX_ADAPTIVE_NP is defined on my system, but the <a href="http://elias.rhi.hi.is/libc/Mutexes.html">only documentation</a> I can find online says nothing about what an adaptive mutex is, or when it's appropriate to use.</p>
  
  <p>So... what is it, and when should I use it?</p>
  
  <p>For reference, my version of libc is:</p>
  
  <pre><code>GNU C Library (Ubuntu EGLIBC 2.15-0ubuntu10.5) stable release version 2.15, by Roland McGrath et al.
  Copyright (C) 2012 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.
  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
  PARTICULAR PURPOSE.
  Compiled by GNU CC version 4.6.3.
  Compiled on a Linux 3.2.50 system on 2013-09-30.
  Available extensions:
      crypt add-on version 2.1 by Michael Glad and others
      GNU Libidn by Simon Josefsson
      Native POSIX Threads Library by Ulrich Drepper et al
      BIND-8.2.3-T5B
  libc ABIs: UNIQUE IFUNC
  For bug reporting instructions, please see:
  &lt;http://www.debian.org/Bugs/&gt;.
  </code></pre>
  
  <p>and "uname -a" gives</p>
  
  <pre><code>Linux desktop 3.2.0-55-generic #85-Ubuntu SMP Wed Oct 2 12:29:27 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
  </code></pre>