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| <h4 class="subsection">24.3.2 Advanced Signal Handling</h4> |
| |
| <p><a name="index-g_t_0040code_007bsigaction_007d-function-2918"></a> |
| The <code>sigaction</code> function has the same basic effect as |
| <code>signal</code>: to specify how a signal should be handled by the process. |
| However, <code>sigaction</code> offers more control, at the expense of more |
| complexity. In particular, <code>sigaction</code> allows you to specify |
| additional flags to control when the signal is generated and how the |
| handler is invoked. |
| |
| <p>The <code>sigaction</code> function is declared in <samp><span class="file">signal.h</span></samp>. |
| <a name="index-signal_002eh-2919"></a> |
| <!-- signal.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Data Type: <b>struct sigaction</b><var><a name="index-struct-sigaction-2920"></a></var><br> |
| <blockquote><p>Structures of type <code>struct sigaction</code> are used in the |
| <code>sigaction</code> function to specify all the information about how to |
| handle a particular signal. This structure contains at least the |
| following members: |
| |
| <dl> |
| <dt><code>sighandler_t sa_handler</code><dd>This is used in the same way as the <var>action</var> argument to the |
| <code>signal</code> function. The value can be <code>SIG_DFL</code>, |
| <code>SIG_IGN</code>, or a function pointer. See <a href="Basic-Signal-Handling.html#Basic-Signal-Handling">Basic Signal Handling</a>. |
| |
| <br><dt><code>sigset_t sa_mask</code><dd>This specifies a set of signals to be blocked while the handler runs. |
| Blocking is explained in <a href="Blocking-for-Handler.html#Blocking-for-Handler">Blocking for Handler</a>. Note that the |
| signal that was delivered is automatically blocked by default before its |
| handler is started; this is true regardless of the value in |
| <code>sa_mask</code>. If you want that signal not to be blocked within its |
| handler, you must write code in the handler to unblock it. |
| |
| <br><dt><code>int sa_flags</code><dd>This specifies various flags which can affect the behavior of |
| the signal. These are described in more detail in <a href="Flags-for-Sigaction.html#Flags-for-Sigaction">Flags for Sigaction</a>. |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- signal.h --> |
| <!-- POSIX.1 --> |
| <div class="defun"> |
| — Function: int <b>sigaction</b> (<var>int signum, const struct sigaction *restrict action, struct sigaction *restrict old-action</var>)<var><a name="index-sigaction-2921"></a></var><br> |
| <blockquote><p>The <var>action</var> argument is used to set up a new action for the signal |
| <var>signum</var>, while the <var>old-action</var> argument is used to return |
| information about the action previously associated with this symbol. |
| (In other words, <var>old-action</var> has the same purpose as the |
| <code>signal</code> function's return value—you can check to see what the |
| old action in effect for the signal was, and restore it later if you |
| want.) |
| |
| <p>Either <var>action</var> or <var>old-action</var> can be a null pointer. If |
| <var>old-action</var> is a null pointer, this simply suppresses the return |
| of information about the old action. If <var>action</var> is a null pointer, |
| the action associated with the signal <var>signum</var> is unchanged; this |
| allows you to inquire about how a signal is being handled without changing |
| that handling. |
| |
| <p>The return value from <code>sigaction</code> is zero if it succeeds, and |
| <code>-1</code> on failure. The following <code>errno</code> error conditions are |
| defined for this function: |
| |
| <dl> |
| <dt><code>EINVAL</code><dd>The <var>signum</var> argument is not valid, or you are trying to |
| trap or ignore <code>SIGKILL</code> or <code>SIGSTOP</code>. |
| </dl> |
| </p></blockquote></div> |
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