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 <h3 class="section">23.1 Introduction to Non-Local Exits</h3>

 <p>As an example of a situation where a non-local exit can be useful,
 suppose you have an interactive program that has a &ldquo;main loop&rdquo; that
 prompts for and executes commands.  Suppose the &ldquo;read&rdquo; command reads
 input from a file, doing some lexical analysis and parsing of the input
 while processing it.  If a low-level input error is detected, it would
 be useful to be able to return immediately to the &ldquo;main loop&rdquo; instead
 of having to make each of the lexical analysis, parsing, and processing
 phases all have to explicitly deal with error situations initially
 detected by nested calls.

    <p>(On the other hand, if each of these phases has to do a substantial
 amount of cleanup when it exits&mdash;such as closing files, deallocating
 buffers or other data structures, and the like&mdash;then it can be more
 appropriate to do a normal return and have each phase do its own
 cleanup, because a non-local exit would bypass the intervening phases and
 their associated cleanup code entirely.  Alternatively, you could use a
 non-local exit but do the cleanup explicitly either before or after
 returning to the &ldquo;main loop&rdquo;.)

    <p>In some ways, a non-local exit is similar to using the &lsquo;<samp><span class="samp">return</span></samp>&rsquo;
 statement to return from a function.  But while &lsquo;<samp><span class="samp">return</span></samp>&rsquo; abandons
 only a single function call, transferring control back to the point at
 which it was called, a non-local exit can potentially abandon many
 levels of nested function calls.

    <p>You identify return points for non-local exits by calling the function
 <code>setjmp</code>.  This function saves information about the execution
 environment in which the call to <code>setjmp</code> appears in an object of
 type <code>jmp_buf</code>.  Execution of the program continues normally after
 the call to <code>setjmp</code>, but if an exit is later made to this return
 point by calling <code>longjmp</code> with the corresponding <code>jmp_buf</code><!-- /@w -->
 object, control is transferred back to the point where <code>setjmp</code> was
 called.  The return value from <code>setjmp</code> is used to distinguish
 between an ordinary return and a return made by a call to
 <code>longjmp</code>, so calls to <code>setjmp</code> usually appear in an &lsquo;<samp><span class="samp">if</span></samp>&rsquo;
 statement.

    <p>Here is how the example program described above might be set up:

 <pre class="smallexample">     #include &lt;setjmp.h&gt;
      #include &lt;stdlib.h&gt;
      #include &lt;stdio.h&gt;

      jmp_buf main_loop;

      void
      abort_to_main_loop (int status)
      {
        longjmp (main_loop, status);
      }

      int
      main (void)
      {
        while (1)
          if (setjmp (main_loop))
            puts ("Back at main loop....");
          else
            do_command ();
      }


      void
      do_command (void)
      {
        char buffer[128];
        if (fgets (buffer, 128, stdin) == NULL)
          abort_to_main_loop (-1);
        else
          exit (EXIT_SUCCESS);
      }
 </pre>
    <p>The function <code>abort_to_main_loop</code> causes an immediate transfer of
 control back to the main loop of the program, no matter where it is
 called from.

    <p>The flow of control inside the <code>main</code> function may appear a little
 mysterious at first, but it is actually a common idiom with
 <code>setjmp</code>.  A normal call to <code>setjmp</code> returns zero, so the
 &ldquo;else&rdquo; clause of the conditional is executed.  If
 <code>abort_to_main_loop</code> is called somewhere within the execution of
 <code>do_command</code>, then it actually appears as if the <em>same</em> call
 to <code>setjmp</code> in <code>main</code> were returning a second time with a value
 of <code>-1</code>.

    <p>So, the general pattern for using <code>setjmp</code> looks something like:

 <pre class="smallexample">     if (setjmp (<var>buffer</var>))
        /* <span class="roman">Code to clean up after premature return.</span> */
        ...
      else
        /* <span class="roman">Code to be executed normally after setting up the return point.</span> */
        ...
 </pre>
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	<h3 class="section">23.1 Introduction to Non-Local Exits</h3>

	<p>As an example of a situation where a non-local exit can be useful,
	suppose you have an interactive program that has a “main loop” that
	prompts for and executes commands. Suppose the “read” command reads
	input from a file, doing some lexical analysis and parsing of the input
	while processing it. If a low-level input error is detected, it would
	be useful to be able to return immediately to the “main loop” instead
	of having to make each of the lexical analysis, parsing, and processing
	phases all have to explicitly deal with error situations initially
	detected by nested calls.

	<p>(On the other hand, if each of these phases has to do a substantial
	amount of cleanup when it exits—such as closing files, deallocating
	buffers or other data structures, and the like—then it can be more
	appropriate to do a normal return and have each phase do its own
	cleanup, because a non-local exit would bypass the intervening phases and
	their associated cleanup code entirely. Alternatively, you could use a
	non-local exit but do the cleanup explicitly either before or after
	returning to the “main loop”.)

	<p>In some ways, a non-local exit is similar to using the ‘<samp><span class="samp">return</span></samp>’
	statement to return from a function. But while ‘<samp><span class="samp">return</span></samp>’ abandons
	only a single function call, transferring control back to the point at
	which it was called, a non-local exit can potentially abandon many
	levels of nested function calls.

	<p>You identify return points for non-local exits by calling the function
	<code>setjmp</code>. This function saves information about the execution
	environment in which the call to <code>setjmp</code> appears in an object of
	type <code>jmp_buf</code>. Execution of the program continues normally after
	the call to <code>setjmp</code>, but if an exit is later made to this return
	point by calling <code>longjmp</code> with the corresponding <code>jmp_buf</code><!-- /@w -->
	object, control is transferred back to the point where <code>setjmp</code> was
	called. The return value from <code>setjmp</code> is used to distinguish
	between an ordinary return and a return made by a call to
	<code>longjmp</code>, so calls to <code>setjmp</code> usually appear in an ‘<samp><span class="samp">if</span></samp>’
	statement.

	<p>Here is how the example program described above might be set up:

	<pre class="smallexample"> #include <setjmp.h>
	#include <stdlib.h>
	#include <stdio.h>

	jmp_buf main_loop;

	void
	abort_to_main_loop (int status)
	{
	longjmp (main_loop, status);
	}

	int
	main (void)
	{
	while (1)
	if (setjmp (main_loop))
	puts ("Back at main loop....");
	else
	do_command ();
	}


	void
	do_command (void)
	{
	char buffer[128];
	if (fgets (buffer, 128, stdin) == NULL)
	abort_to_main_loop (-1);
	else
	exit (EXIT_SUCCESS);
	}
	</pre>
	<p>The function <code>abort_to_main_loop</code> causes an immediate transfer of
	control back to the main loop of the program, no matter where it is
	called from.

	<p>The flow of control inside the <code>main</code> function may appear a little
	mysterious at first, but it is actually a common idiom with
	<code>setjmp</code>. A normal call to <code>setjmp</code> returns zero, so the
	“else” clause of the conditional is executed. If
	<code>abort_to_main_loop</code> is called somewhere within the execution of
	<code>do_command</code>, then it actually appears as if the <em>same</em> call
	to <code>setjmp</code> in <code>main</code> were returning a second time with a value
	of <code>-1</code>.

	<p>So, the general pattern for using <code>setjmp</code> looks something like:

	<pre class="smallexample"> if (setjmp (<var>buffer</var>))
	/* <span class="roman">Code to clean up after premature return.</span> */
	...
	else
	/* <span class="roman">Code to be executed normally after setting up the return point.</span> */
	...
	</pre>
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