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| <h3 class="section">24.9 Using a Separate Signal Stack</h3> |
| |
| <p>A signal stack is a special area of memory to be used as the execution |
| stack during signal handlers. It should be fairly large, to avoid any |
| danger that it will overflow in turn; the macro <code>SIGSTKSZ</code> is |
| defined to a canonical size for signal stacks. You can use |
| <code>malloc</code> to allocate the space for the stack. Then call |
| <code>sigaltstack</code> or <code>sigstack</code> to tell the system to use that |
| space for the signal stack. |
| |
| <p>You don't need to write signal handlers differently in order to use a |
| signal stack. Switching from one stack to the other happens |
| automatically. (Some non-GNU debuggers on some machines may get |
| confused if you examine a stack trace while a handler that uses the |
| signal stack is running.) |
| |
| <p>There are two interfaces for telling the system to use a separate signal |
| stack. <code>sigstack</code> is the older interface, which comes from 4.2 |
| BSD. <code>sigaltstack</code> is the newer interface, and comes from 4.4 |
| BSD. The <code>sigaltstack</code> interface has the advantage that it does |
| not require your program to know which direction the stack grows, which |
| depends on the specific machine and operating system. |
| |
| <!-- signal.h --> |
| <!-- XPG --> |
| <div class="defun"> |
| — Data Type: <b>stack_t</b><var><a name="index-stack_005ft-2983"></a></var><br> |
| <blockquote><p>This structure describes a signal stack. It contains the following members: |
| |
| <dl> |
| <dt><code>void *ss_sp</code><dd>This points to the base of the signal stack. |
| |
| <br><dt><code>size_t ss_size</code><dd>This is the size (in bytes) of the signal stack which ‘<samp><span class="samp">ss_sp</span></samp>’ points to. |
| You should set this to however much space you allocated for the stack. |
| |
| <p>There are two macros defined in <samp><span class="file">signal.h</span></samp> that you should use in |
| calculating this size: |
| |
| <dl> |
| <dt><code>SIGSTKSZ</code><a name="index-SIGSTKSZ-2984"></a><dd>This is the canonical size for a signal stack. It is judged to be |
| sufficient for normal uses. |
| |
| <br><dt><code>MINSIGSTKSZ</code><a name="index-MINSIGSTKSZ-2985"></a><dd>This is the amount of signal stack space the operating system needs just |
| to implement signal delivery. The size of a signal stack <strong>must</strong> |
| be greater than this. |
| |
| <p>For most cases, just using <code>SIGSTKSZ</code> for <code>ss_size</code> is |
| sufficient. But if you know how much stack space your program's signal |
| handlers will need, you may want to use a different size. In this case, |
| you should allocate <code>MINSIGSTKSZ</code> additional bytes for the signal |
| stack and increase <code>ss_size</code> accordingly. |
| </dl> |
| |
| <br><dt><code>int ss_flags</code><dd>This field contains the bitwise <span class="sc">or</span> of these flags: |
| |
| <dl> |
| <dt><code>SS_DISABLE</code><a name="index-SS_005fDISABLE-2986"></a><dd>This tells the system that it should not use the signal stack. |
| |
| <br><dt><code>SS_ONSTACK</code><a name="index-SS_005fONSTACK-2987"></a><dd>This is set by the system, and indicates that the signal stack is |
| currently in use. If this bit is not set, then signals will be |
| delivered on the normal user stack. |
| </dl> |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- signal.h --> |
| <!-- XPG --> |
| <div class="defun"> |
| — Function: int <b>sigaltstack</b> (<var>const stack_t *restrict stack, stack_t *restrict oldstack</var>)<var><a name="index-sigaltstack-2988"></a></var><br> |
| <blockquote><p>The <code>sigaltstack</code> function specifies an alternate stack for use |
| during signal handling. When a signal is received by the process and |
| its action indicates that the signal stack is used, the system arranges |
| a switch to the currently installed signal stack while the handler for |
| that signal is executed. |
| |
| <p>If <var>oldstack</var> is not a null pointer, information about the currently |
| installed signal stack is returned in the location it points to. If |
| <var>stack</var> is not a null pointer, then this is installed as the new |
| stack for use by signal handlers. |
| |
| <p>The return value is <code>0</code> on success and <code>-1</code> on failure. If |
| <code>sigaltstack</code> fails, it sets <code>errno</code> to one of these values: |
| |
| <dl> |
| <dt><code>EINVAL</code><dd>You tried to disable a stack that was in fact currently in use. |
| |
| <br><dt><code>ENOMEM</code><dd>The size of the alternate stack was too small. |
| It must be greater than <code>MINSIGSTKSZ</code>. |
| </dl> |
| </p></blockquote></div> |
| |
| <p>Here is the older <code>sigstack</code> interface. You should use |
| <code>sigaltstack</code> instead on systems that have it. |
| |
| <!-- signal.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Data Type: <b>struct sigstack</b><var><a name="index-struct-sigstack-2989"></a></var><br> |
| <blockquote><p>This structure describes a signal stack. It contains the following members: |
| |
| <dl> |
| <dt><code>void *ss_sp</code><dd>This is the stack pointer. If the stack grows downwards on your |
| machine, this should point to the top of the area you allocated. If the |
| stack grows upwards, it should point to the bottom. |
| |
| <br><dt><code>int ss_onstack</code><dd>This field is true if the process is currently using this stack. |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- signal.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>sigstack</b> (<var>const struct sigstack *stack, struct sigstack *oldstack</var>)<var><a name="index-sigstack-2990"></a></var><br> |
| <blockquote><p>The <code>sigstack</code> function specifies an alternate stack for use during |
| signal handling. When a signal is received by the process and its |
| action indicates that the signal stack is used, the system arranges a |
| switch to the currently installed signal stack while the handler for |
| that signal is executed. |
| |
| <p>If <var>oldstack</var> is not a null pointer, information about the currently |
| installed signal stack is returned in the location it points to. If |
| <var>stack</var> is not a null pointer, then this is installed as the new |
| stack for use by signal handlers. |
| |
| <p>The return value is <code>0</code> on success and <code>-1</code> on failure. |
| </p></blockquote></div> |
| |
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