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<h3 class="section">23.4 Complete Context Control</h3>

<p>The Unix standard one more set of function to control the execution path
and these functions are more powerful than those discussed in this
chapter so far.  These function were part of the original System&nbsp;V<!-- /@w -->
API and by this route were added to the Unix API.  Beside on branded
Unix implementations these interfaces are not widely available.  Not all
platforms and/or architectures the GNU C Library is available on provide
this interface.  Use <samp><span class="file">configure</span></samp> to detect the availability.

   <p>Similar to the <code>jmp_buf</code> and <code>sigjmp_buf</code> types used for the
variables to contain the state of the <code>longjmp</code> functions the
interfaces of interest here have an appropriate type as well.  Objects
of this type are normally much larger since more information is
contained.  The type is also used in a few more places as we will see. 
The types and functions described in this section are all defined and
declared respectively in the <samp><span class="file">ucontext.h</span></samp> header file.

<!-- ucontext.h -->
<!-- SVID -->
<div class="defun">
&mdash; Data Type: <b>ucontext_t</b><var><a name="index-ucontext_005ft-2805"></a></var><br>
<blockquote>
        <p>The <code>ucontext_t</code> type is defined as a structure with as least the
following elements:

          <dl>
<dt><code>ucontext_t *uc_link</code><dd>This is a pointer to the next context structure which is used if the
context described in the current structure returns.

          <br><dt><code>sigset_t uc_sigmask</code><dd>Set of signals which are blocked when this context is used.

          <br><dt><code>stack_t uc_stack</code><dd>Stack used for this context.  The value need not be (and normally is
not) the stack pointer.  See <a href="Signal-Stack.html#Signal-Stack">Signal Stack</a>.

          <br><dt><code>mcontext_t uc_mcontext</code><dd>This element contains the actual state of the process.  The
<code>mcontext_t</code> type is also defined in this header but the definition
should be treated as opaque.  Any use of knowledge of the type makes
applications less portable.

        </dl>
        </p></blockquote></div>

   <p>Objects of this type have to be created by the user.  The initialization
and modification happens through one of the following functions:

<!-- ucontext.h -->
<!-- SVID -->
<div class="defun">
&mdash; Function: int <b>getcontext</b> (<var>ucontext_t *ucp</var>)<var><a name="index-getcontext-2806"></a></var><br>
<blockquote><p>The <code>getcontext</code> function initializes the variable pointed to by
<var>ucp</var> with the context of the calling thread.  The context contains
the content of the registers, the signal mask, and the current stack. 
Executing the contents would start at the point where the
<code>getcontext</code> call just returned.

        <p>The function returns <code>0</code> if successful.  Otherwise it returns
<code>-1</code> and sets <var>errno</var> accordingly. 
</p></blockquote></div>

   <p>The <code>getcontext</code> function is similar to <code>setjmp</code> but it does
not provide an indication of whether the function returns for the first
time or whether the initialized context was used and the execution is
resumed at just that point.  If this is necessary the user has to take
determine this herself.  This must be done carefully since the context
contains registers which might contain register variables.  This is a
good situation to define variables with <code>volatile</code>.

   <p>Once the context variable is initialized it can be used as is or it can
be modified.  The latter is normally done to implement co-routines or
similar constructs.  The <code>makecontext</code> function is what has to be
used to do that.

<!-- ucontext.h -->
<!-- SVID -->
<div class="defun">
&mdash; Function: void <b>makecontext</b> (<var>ucontext_t *ucp, void </var>(<var>*func</var>) (<var>void</var>)<var>, int argc, <small class="dots">...</small></var>)<var><a name="index-makecontext-2807"></a></var><br>
<blockquote>
        <p>The <var>ucp</var> parameter passed to the <code>makecontext</code> shall be
initialized by a call to <code>getcontext</code>.  The context will be
modified to in a way so that if the context is resumed it will start by
calling the function <code>func</code> which gets <var>argc</var> integer arguments
passed.  The integer arguments which are to be passed should follow the
<var>argc</var> parameter in the call to <code>makecontext</code>.

        <p>Before the call to this function the <code>uc_stack</code> and <code>uc_link</code>
element of the <var>ucp</var> structure should be initialized.  The
<code>uc_stack</code> element describes the stack which is used for this
context.  No two contexts which are used at the same time should use the
same memory region for a stack.

        <p>The <code>uc_link</code> element of the object pointed to by <var>ucp</var> should
be a pointer to the context to be executed when the function <var>func</var>
returns or it should be a null pointer.  See <code>setcontext</code> for more
information about the exact use. 
</p></blockquote></div>

   <p>While allocating the memory for the stack one has to be careful.  Most
modern processors keep track of whether a certain memory region is
allowed to contain code which is executed or not.  Data segments and
heap memory is normally not tagged to allow this.  The result is that
programs would fail.  Examples for such code include the calling
sequences the GNU C compiler generates for calls to nested functions. 
Safe ways to allocate stacks correctly include using memory on the
original threads stack or explicitly allocate memory tagged for
execution using (see <a href="Memory_002dmapped-I_002fO.html#Memory_002dmapped-I_002fO">Memory-mapped I/O</a>).

   <p><strong>Compatibility note</strong>: The current Unix standard is very imprecise
about the way the stack is allocated.  All implementations seem to agree
that the <code>uc_stack</code> element must be used but the values stored in
the elements of the <code>stack_t</code> value are unclear.  The GNU C library
and most other Unix implementations require the <code>ss_sp</code> value of
the <code>uc_stack</code> element to point to the base of the memory region
allocated for the stack and the size of the memory region is stored in
<code>ss_size</code>.  There are implements out there which require
<code>ss_sp</code> to be set to the value the stack pointer will have (which
can depending on the direction the stack grows be different).  This
difference makes the <code>makecontext</code> function hard to use and it
requires detection of the platform at compile time.

<!-- ucontext.h -->
<!-- SVID -->
<div class="defun">
&mdash; Function: int <b>setcontext</b> (<var>const ucontext_t *ucp</var>)<var><a name="index-setcontext-2808"></a></var><br>
<blockquote>
        <p>The <code>setcontext</code> function restores the context described by
<var>ucp</var>.  The context is not modified and can be reused as often as
wanted.

        <p>If the context was created by <code>getcontext</code> execution resumes with
the registers filled with the same values and the same stack as if the
<code>getcontext</code> call just returned.

        <p>If the context was modified with a call to <code>makecontext</code> execution
continues with the function passed to <code>makecontext</code> which gets the
specified parameters passed.  If this function returns execution is
resumed in the context which was referenced by the <code>uc_link</code>
element of the context structure passed to <code>makecontext</code> at the
time of the call.  If <code>uc_link</code> was a null pointer the application
terminates in this case.

        <p>Since the context contains information about the stack no two threads
should use the same context at the same time.  The result in most cases
would be disastrous.

        <p>The <code>setcontext</code> function does not return unless an error occurred
in which case it returns <code>-1</code>. 
</p></blockquote></div>

   <p>The <code>setcontext</code> function simply replaces the current context with
the one described by the <var>ucp</var> parameter.  This is often useful but
there are situations where the current context has to be preserved.

<!-- ucontext.h -->
<!-- SVID -->
<div class="defun">
&mdash; Function: int <b>swapcontext</b> (<var>ucontext_t *restrict oucp, const ucontext_t *restrict ucp</var>)<var><a name="index-swapcontext-2809"></a></var><br>
<blockquote>
        <p>The <code>swapcontext</code> function is similar to <code>setcontext</code> but
instead of just replacing the current context the latter is first saved
in the object pointed to by <var>oucp</var> as if this was a call to
<code>getcontext</code>.  The saved context would resume after the call to
<code>swapcontext</code>.

        <p>Once the current context is saved the context described in <var>ucp</var> is
installed and execution continues as described in this context.

        <p>If <code>swapcontext</code> succeeds the function does not return unless the
context <var>oucp</var> is used without prior modification by
<code>makecontext</code>.  The return value in this case is <code>0</code>.  If the
function fails it returns <code>-1</code> and set <var>errno</var> accordingly. 
</p></blockquote></div>

<h3 class="heading">Example for SVID Context Handling</h3>

<p>The easiest way to use the context handling functions is as a
replacement for <code>setjmp</code> and <code>longjmp</code>.  The context contains
on most platforms more information which might lead to less surprises
but this also means using these functions is more expensive (beside
being less portable).

<pre class="smallexample">     int
     random_search (int n, int (*fp) (int, ucontext_t *))
     {
       volatile int cnt = 0;
       ucontext_t uc;
     
       /* <span class="roman">Safe current context.</span>  */
       if (getcontext (&amp;uc) &lt; 0)
         return -1;
     
       /* <span class="roman">If we have not tried </span><var>n</var><span class="roman"> times try again.</span>  */
       if (cnt++ &lt; n)
         /* <span class="roman">Call the function with a new random number</span>
            <span class="roman">and the context</span>.  */
         if (fp (rand (), &amp;uc) != 0)
           /* <span class="roman">We found what we were looking for.</span>  */
           return 1;
     
       /* <span class="roman">Not found.</span>  */
       return 0;
     }
</pre>
   <p>Using contexts in such a way enables emulating exception handling.  The
search functions passed in the <var>fp</var> parameter could be very large,
nested, and complex which would make it complicated (or at least would
require a lot of code) to leave the function with an error value which
has to be passed down to the caller.  By using the context it is
possible to leave the search function in one step and allow restarting
the search which also has the nice side effect that it can be
significantly faster.

   <p>Something which is harder to implement with <code>setjmp</code> and
<code>longjmp</code> is to switch temporarily to a different execution path
and then resume where execution was stopped.

<pre class="smallexample">     #include &lt;signal.h&gt;
     #include &lt;stdio.h&gt;
     #include &lt;stdlib.h&gt;
     #include &lt;ucontext.h&gt;
     #include &lt;sys/time.h&gt;
     
     /* <span class="roman">Set by the signal handler.</span> */
     static volatile int expired;
     
     /* <span class="roman">The contexts.</span> */
     static ucontext_t uc[3];
     
     /* <span class="roman">We do only a certain number of switches.</span> */
     static int switches;
     
     
     /* <span class="roman">This is the function doing the work.  It is just a
        skeleton, real code has to be filled in.</span> */
     static void
     f (int n)
     {
       int m = 0;
       while (1)
         {
           /* <span class="roman">This is where the work would be done.</span> */
           if (++m % 100 == 0)
             {
               putchar ('.');
               fflush (stdout);
             }
     
           /* <span class="roman">Regularly the </span><var>expire</var><span class="roman"> variable must be checked.</span> */
           if (expired)
             {
               /* <span class="roman">We do not want the program to run forever.</span> */
               if (++switches == 20)
                 return;
     
               printf ("\nswitching from %d to %d\n", n, 3 - n);
               expired = 0;
               /* <span class="roman">Switch to the other context, saving the current one.</span> */
               swapcontext (&amp;uc[n], &amp;uc[3 - n]);
             }
         }
     }
     
     /* <span class="roman">This is the signal handler which simply set the variable.</span> */
     void
     handler (int signal)
     {
       expired = 1;
     }
     
     
     int
     main (void)
     {
       struct sigaction sa;
       struct itimerval it;
       char st1[8192];
       char st2[8192];
     
       /* <span class="roman">Initialize the data structures for the interval timer.</span> */
       sa.sa_flags = SA_RESTART;
       sigfillset (&amp;sa.sa_mask);
       sa.sa_handler = handler;
       it.it_interval.tv_sec = 0;
       it.it_interval.tv_usec = 1;
       it.it_value = it.it_interval;
     
       /* <span class="roman">Install the timer and get the context we can manipulate.</span> */
       if (sigaction (SIGPROF, &amp;sa, NULL) &lt; 0
           || setitimer (ITIMER_PROF, &amp;it, NULL) &lt; 0
           || getcontext (&amp;uc[1]) == -1
           || getcontext (&amp;uc[2]) == -1)
         abort ();
     
       /* <span class="roman">Create a context with a separate stack which causes the
          function </span><code>f</code><span class="roman"> to be call with the parameter </span><code>1</code><span class="roman">.
          Note that the </span><code>uc_link</code><span class="roman"> points to the main context
          which will cause the program to terminate once the function
          return.</span> */
       uc[1].uc_link = &amp;uc[0];
       uc[1].uc_stack.ss_sp = st1;
       uc[1].uc_stack.ss_size = sizeof st1;
       makecontext (&amp;uc[1], (void (*) (void)) f, 1, 1);
     
       /* <span class="roman">Similarly, but </span><code>2</code><span class="roman"> is passed as the parameter to </span><code>f</code><span class="roman">.</span> */
       uc[2].uc_link = &amp;uc[0];
       uc[2].uc_stack.ss_sp = st2;
       uc[2].uc_stack.ss_size = sizeof st2;
       makecontext (&amp;uc[2], (void (*) (void)) f, 1, 2);
     
       /* <span class="roman">Start running.</span> */
       swapcontext (&amp;uc[0], &amp;uc[1]);
       putchar ('\n');
     
       return 0;
     }
</pre>
   <p>This an example how the context functions can be used to implement
co-routines or cooperative multi-threading.  All that has to be done is
to call every once in a while <code>swapcontext</code> to continue running a
different context.  It is not allowed to do the context switching from
the signal handler directly since neither <code>setcontext</code> nor
<code>swapcontext</code> are functions which can be called from a signal
handler.  But setting a variable in the signal handler and checking it
in the body of the functions which are executed.  Since
<code>swapcontext</code> is saving the current context it is possible to have
multiple different scheduling points in the code.  Execution will always
resume where it was left.

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