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| <h3 class="section">23.2 Details of Non-Local Exits</h3> |
| |
| <p>Here are the details on the functions and data structures used for |
| performing non-local exits. These facilities are declared in |
| <samp><span class="file">setjmp.h</span></samp>. |
| <a name="index-setjmp_002eh-2797"></a> |
| <!-- setjmp.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Data Type: <b>jmp_buf</b><var><a name="index-jmp_005fbuf-2798"></a></var><br> |
| <blockquote><p>Objects of type <code>jmp_buf</code> hold the state information to |
| be restored by a non-local exit. The contents of a <code>jmp_buf</code> |
| identify a specific place to return to. |
| </p></blockquote></div> |
| |
| <!-- setjmp.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Macro: int <b>setjmp</b> (<var>jmp_buf state</var>)<var><a name="index-setjmp-2799"></a></var><br> |
| <blockquote><p>When called normally, <code>setjmp</code> stores information about the |
| execution state of the program in <var>state</var> and returns zero. If |
| <code>longjmp</code> is later used to perform a non-local exit to this |
| <var>state</var>, <code>setjmp</code> returns a nonzero value. |
| </p></blockquote></div> |
| |
| <!-- setjmp.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: void <b>longjmp</b> (<var>jmp_buf state, int value</var>)<var><a name="index-longjmp-2800"></a></var><br> |
| <blockquote><p>This function restores current execution to the state saved in |
| <var>state</var>, and continues execution from the call to <code>setjmp</code> that |
| established that return point. Returning from <code>setjmp</code> by means of |
| <code>longjmp</code> returns the <var>value</var> argument that was passed to |
| <code>longjmp</code>, rather than <code>0</code>. (But if <var>value</var> is given as |
| <code>0</code>, <code>setjmp</code> returns <code>1</code>). |
| </p></blockquote></div> |
| |
| <p>There are a lot of obscure but important restrictions on the use of |
| <code>setjmp</code> and <code>longjmp</code>. Most of these restrictions are |
| present because non-local exits require a fair amount of magic on the |
| part of the C compiler and can interact with other parts of the language |
| in strange ways. |
| |
| <p>The <code>setjmp</code> function is actually a macro without an actual |
| function definition, so you shouldn't try to ‘<samp><span class="samp">#undef</span></samp>’ it or take |
| its address. In addition, calls to <code>setjmp</code> are safe in only the |
| following contexts: |
| |
| <ul> |
| <li>As the test expression of a selection or iteration |
| statement (such as ‘<samp><span class="samp">if</span></samp>’, ‘<samp><span class="samp">switch</span></samp>’, or ‘<samp><span class="samp">while</span></samp>’). |
| |
| <li>As one operand of a equality or comparison operator that appears as the |
| test expression of a selection or iteration statement. The other |
| operand must be an integer constant expression. |
| |
| <li>As the operand of a unary ‘<samp><span class="samp">!</span></samp>’ operator, that appears as the |
| test expression of a selection or iteration statement. |
| |
| <li>By itself as an expression statement. |
| </ul> |
| |
| <p>Return points are valid only during the dynamic extent of the function |
| that called <code>setjmp</code> to establish them. If you <code>longjmp</code> to |
| a return point that was established in a function that has already |
| returned, unpredictable and disastrous things are likely to happen. |
| |
| <p>You should use a nonzero <var>value</var> argument to <code>longjmp</code>. While |
| <code>longjmp</code> refuses to pass back a zero argument as the return value |
| from <code>setjmp</code>, this is intended as a safety net against accidental |
| misuse and is not really good programming style. |
| |
| <p>When you perform a non-local exit, accessible objects generally retain |
| whatever values they had at the time <code>longjmp</code> was called. The |
| exception is that the values of automatic variables local to the |
| function containing the <code>setjmp</code> call that have been changed since |
| the call to <code>setjmp</code> are indeterminate, unless you have declared |
| them <code>volatile</code>. |
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