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<title>avr-libc: <setjmp.h>: Non-local goto</title>
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<h1><setjmp.h>: Non-local goto</h1><hr><a name="_details"></a><h2>Detailed Description</h2>
While the C language has the dreaded <code>goto</code> statement, it can only be used to jump to a label in the same (local) function. In order to jump directly to another (non-local) function, the C library provides the <a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a> and <a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp()</a> functions. <a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a> and <a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp()</a> are useful for dealing with errors and interrupts encountered in a low-level subroutine of a program.<p>
<dl class="note" compact><dt><b>Note:</b></dt><dd><a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a> and <a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp()</a> make programs hard to understand and maintain. If possible, an alternative should be used.<p>
<a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp()</a> can destroy changes made to global register variables (see <a class="el" href="FAQ.html#faq_regbind">How to permanently bind a variable to a register?</a>).</dd></dl>
For a very detailed discussion of <a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a>/longjmp(), see Chapter 7 of <em>Advanced Programming in the UNIX Environment</em>, by W. Richard Stevens.<p>
Example:<p>
<div class="fragment"><pre class="fragment"><span class="preprocessor"> #include <<a class="code" href="setjmp_8h.html">setjmp.h</a>></span>
jmp_buf env;
<span class="keywordtype">int</span> main (<span class="keywordtype">void</span>)
{
<span class="keywordflow">if</span> (<a class="code" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp</a> (env))
{
... handle error ...
}
<span class="keywordflow">while</span> (1)
{
... main processing loop which calls foo() some where ...
}
}
...
<span class="keywordtype">void</span> foo (<span class="keywordtype">void</span>)
{
... blah, blah, blah ...
<span class="keywordflow">if</span> (err)
{
<a class="code" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp</a> (env, 1);
}
}
</pre></div>
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<tr><td colspan="2"><br><h2>Functions</h2></td></tr>
<tr><td class="memItemLeft" nowrap align="right" valign="top">int </td><td class="memItemRight" valign="bottom"><a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6">setjmp</a> (jmp_buf __jmpb)</td></tr>
<tr><td class="memItemLeft" nowrap align="right" valign="top">void </td><td class="memItemRight" valign="bottom"><a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf">longjmp</a> (jmp_buf __jmpb, int __ret) __ATTR_NORETURN__</td></tr>
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<hr><h2>Function Documentation</h2>
<a class="anchor" name="g87f44eafaab5ec0ef8f5a11a8b853acf"></a><!-- doxytag: member="setjmp.h::longjmp" ref="g87f44eafaab5ec0ef8f5a11a8b853acf" args="(jmp_buf __jmpb, int __ret) __ATTR_NORETURN__" -->
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<td class="memname">void longjmp </td>
<td>(</td>
<td class="paramtype">jmp_buf </td>
<td class="paramname"> <em>__jmpb</em>, </td>
</tr>
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<td class="paramkey"></td>
<td></td>
<td class="paramtype">int </td>
<td class="paramname"> <em>__ret</em></td><td> </td>
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<td></td>
<td>)</td>
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<p>
Non-local jump to a saved stack context.
<p>
<div class="fragment"><pre class="fragment"><span class="preprocessor"> #include <<a class="code" href="setjmp_8h.html">setjmp.h</a>></span>
</pre></div><p>
<a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp()</a> restores the environment saved by the last call of <a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a> with the corresponding <em>__jmpb</em> argument. After <a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp()</a> is completed, program execution continues as if the corresponding call of <a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a> had just returned the value <em>__ret</em>.<p>
<dl class="note" compact><dt><b>Note:</b></dt><dd><a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp()</a> cannot cause 0 to be returned. If <a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp()</a> is invoked with a second argument of 0, 1 will be returned instead.</dd></dl>
<dl compact><dt><b>Parameters:</b></dt><dd>
<table border="0" cellspacing="2" cellpadding="0">
<tr><td valign="top"></td><td valign="top"><em>__jmpb</em> </td><td>Information saved by a previous call to <a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a>. </td></tr>
<tr><td valign="top"></td><td valign="top"><em>__ret</em> </td><td>Value to return to the caller of <a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a>.</td></tr>
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</dl>
<dl class="return" compact><dt><b>Returns:</b></dt><dd>This function never returns. </dd></dl>
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</div><p>
<a class="anchor" name="g2687c5ef7a3f376db90908999a9a7fc6"></a><!-- doxytag: member="setjmp.h::setjmp" ref="g2687c5ef7a3f376db90908999a9a7fc6" args="(jmp_buf __jmpb)" -->
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<td class="memname">int setjmp </td>
<td>(</td>
<td class="paramtype">jmp_buf </td>
<td class="paramname"> <em>__jmpb</em> </td>
<td> ) </td>
<td></td>
</tr>
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</div>
<div class="memdoc">
<p>
Save stack context for non-local goto.
<p>
<div class="fragment"><pre class="fragment"><span class="preprocessor"> #include <<a class="code" href="setjmp_8h.html">setjmp.h</a>></span>
</pre></div><p>
<a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a> saves the stack context/environment in <em>__jmpb</em> for later use by <a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp()</a>. The stack context will be invalidated if the function which called <a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a> returns.<p>
<dl compact><dt><b>Parameters:</b></dt><dd>
<table border="0" cellspacing="2" cellpadding="0">
<tr><td valign="top"></td><td valign="top"><em>__jmpb</em> </td><td>Variable of type <code>jmp_buf</code> which holds the stack information such that the environment can be restored.</td></tr>
</table>
</dl>
<dl class="return" compact><dt><b>Returns:</b></dt><dd><a class="el" href="group__setjmp.html#g2687c5ef7a3f376db90908999a9a7fc6" title="Save stack context for non-local goto.">setjmp()</a> returns 0 if returning directly, and non-zero when returning from <a class="el" href="group__setjmp.html#g87f44eafaab5ec0ef8f5a11a8b853acf" title="Non-local jump to a saved stack context.">longjmp()</a> using the saved context. </dd></dl>
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