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412 lines
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<title>Using Signal APIs</title>
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<h2>Using Signal APIs</h2>
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<h3><a name="sigconcepts">Signal Concepts</a></h3>
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<p>An X/Open specification defines a "signal" as a mechanism by which a process
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may be notified of, or affected by, an event occurring in the system. The term
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signal is also used to refer to the event itself.</p>
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<p>A signal is said to be <strong>generated</strong> when the event that causes
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the signal first occurs. Examples of such events include the following:</p>
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<ul>
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<li>System-detected errors<br>
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<br>
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</li>
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<li>Timer expiration<br>
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<br>
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</li>
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<li>Terminal (work station) activity<br>
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<br>
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</li>
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<li>Calling an API such as the X/Open <strong>kill()</strong> function, the
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American National Standard C <strong>raise()</strong> function, or the ILE
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<strong>CEESGL</strong> (signal a condition) function.</li>
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</ul>
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<p>A <strong>synchronous signal</strong> is a signal that is generated by some
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action attributable to a program running within the thread, such as a
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system-detected error, <strong>raise()</strong>, or <strong>CEESGL</strong>. An
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<strong>asynchronous signal</strong> is a signal that is generated for the
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process by using the <strong>kill()</strong> function or by an asynchronous
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event such as terminal activity or an expired timer.</p>
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<p>The <strong>signal action vector</strong> is a list of signal-handling
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actions for each defined signal. The signal action vector is maintained
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separately for each process and is inherited from the parent process. The
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signal action vector specifies the signal-handling actions for both
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synchronously and asynchronously generated signals.</p>
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<p>A signal is said to be <strong>delivered</strong> to a process when the
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specified signal-handling action for the signal is taken. A signal is said to
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be <strong>accepted</strong> by a process when a signal is selected and
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returned by one of the <em>sigwait</em> functions.</p>
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<p>Signals generated for a process are delivered to or accepted by one thread
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in the process.</p>
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<p>A signal is said to be <strong>pending</strong> during the interval between
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the time the signal is generated and the time it is delivered or accepted.
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Ordinarily, this interval cannot be detected by an application. However, a
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signal can be <strong>blocked</strong> from being delivered to a thread. When a
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signal is blocked, the signal-handling action associated with the signal is not
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taken. If there are no threads in a call to a <em>sigwait</em> function
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selecting the signal and if all threads block delivery of the signal, the
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signal remains pending on the process. The signal remains pending until either
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a thread calls a <em>sigwait</em> function selecting the signal, a thread
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unblocks delivery of the signal, or the signal action associated with the
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signal is set to ignore the signal. The <strong>signal blocking mask</strong>
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defines the set of signals that are blocked from delivery to the thread. The
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signal blocking mask is maintained separately for each thread in the process
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and is inherited from the thread that created it.</p>
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<br>
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<h3><a name="sigmanagement">i5/OS Signal Management</a></h3>
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<p>The set of defined signals is determined by the system. The system specifies
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the attributes for each defined signal. These attributes consist of a signal
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number, the initial signal action, and the signal default action. The system
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also specifies an initial signal blocking mask. The set of defined signals, the
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signal attributes, and signal blocking mask are referred to as <strong>signal
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controls</strong>.</p>
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<p>A signal can be generated or delivered only to a process that has expressed
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an interest in signals. An error condition results under the following
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conditions:</p>
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<ul>
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<li>An attempt is made to generate a signal when the system signal controls
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have not been initialized.</li>
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<li>An attempt is made to generate a signal for a process that has not been
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enabled for signals.</li>
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</ul>
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<p>A process can express an interest in signals by calling the <strong>
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Qp0sEnableSignals()</strong> API. In addition, calling particular signal APIs
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implicitly enables the process for signals.</p>
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<p>If the process has not been enabled for signals, the process signal controls
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are set from signal controls established by the system during IPL (the system
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signal controls). An error condition results if an attempt is made to enable
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signals for the process before the system signal controls have been
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initialized.</p>
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<p>Once the process signal controls have been initialized, the user is
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permitted to change the signal controls for the process. For example, the
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signal blocking mask and the signal action for a signal are commonly changed.
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Some signal controls, such as the number of defined signals and the signal
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default action for a signal, cannot be changed at the process level.</p>
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<p>The attributes for each defined signal are stored in an object called a
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<strong>signal monitor</strong>. The system supports a maximum of 63 signal
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monitors for each process. The process signal action vector is a list of signal
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monitors, one for each defined signal. The signal monitor contains, but is not
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limited to, the following information:</p>
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<ul>
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<li>Signal action</li>
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<li>Signal default action</li>
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<li>Signal options</li>
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</ul>
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<p>The <strong>signal action</strong> defines the action to be taken by the
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system when a process receives an unblocked signal. The user can change the
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signal action for a process signal monitor. The possible signal actions
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are:</p>
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<ul>
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<li>Handle using signal default action (SIG_DFL)
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<p>The <em>handle using signal default action</em> signal action indicates that
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the system is to take the action specified by the signal default action field
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when the signal is eligible to be delivered.</p>
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</li>
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<li>Ignore the signal (SIG_IGN)
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<p>The <em>ignore the signal</em> signal action indicates that the user is not
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interested in handling the signal. When an ignored signal is generated for the
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process, the system automatically discards the signal, regardless of the
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blocked or unblocked state of the signal monitor.</p>
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</li>
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<li>Handle the signal by running signal-catching function
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<p>The <em>handle the signal by running signal-catching function</em> signal
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action causes the system to call the signal-catching function when a signal is
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received for the signal monitor. The signal-catching function is set to point
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to a procedure within an active activation group.</p>
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</li>
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</ul>
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<p>The signal default action field defines the action to be taken by the system
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when the signal action is set to <em>handle using signal default action</em>.
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The signal default action for a signal monitor is set in the system signal
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controls and cannot be changed for a process signal monitor. The possible
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signal default actions are:</p>
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<ul>
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<li>Terminate the process
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<p>The <em>terminate the process</em> action puts the process in a phase that
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ends the process, allowing cancel handlers to be called. If the process is
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already in the end phase, the <em>terminate the process</em> action is
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ignored.</p>
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</li>
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<li>End the request
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<p>The <em>end the request</em> action results in the cancelation of all calls
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up to the nearest call that has a call status of request processor. If a call
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with a status of request processor is not present or the job is capable of
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having multiple threads, the <em>terminate the process</em> action is
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taken.</p>
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</li>
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<li>Ignore the signal
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<p>The <em>ignore the signal</em> action causes the system to discard the
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signal. A signal is discarded for a signal monitor in the blocked state when
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the signal action is <em>handle using signal default action</em> and the
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default signal action is <em>ignore the signal</em>.</p>
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</li>
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<li>Stop the process
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<p>The <em>stop the process</em> action causes the system to place the process
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in the stopped state. When a process is in the <strong>stopped</strong> state,
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it is temporarily suspended until a signal is generated for the process that
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has <em>continue the process if stopped</em> as its signal default action. When
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a process is in the stopped state, the normal process control functions remain
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in effect (the process can be suspended, resumed, or ended). When a signal is
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generated for a signal monitor that has <em>stop the process</em> as its signal
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default action, the system removes any pending signals for signal monitors that
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have <em>continue the process if stopped</em> as their default action.</p>
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</li>
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<li>Continue the process if stopped
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<p>The <em>continue the process if stopped</em> action causes the system to
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resume running the process that is in the stopped state, even if the signal
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monitor with the signal default action of <em>continue the process if
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stopped</em> is in the blocked state or has a signal action of <em>ignore the
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signal</em>. When a signal is generated for a signal monitor that has <em>
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continue the process if stopped</em> as its signal default action, the system
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removes any pending signals for signal monitors that have <em>stop the
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process</em> as their signal default action.</p>
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</li>
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<li>Signal exception
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<p>The <em>signal exception</em> action causes the system to send the MCH7603
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escape message to the process.</p>
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</li>
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</ul>
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<p>The <strong>signal options</strong> specify an additional set of attributes
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for the signal monitor. The primary use of these options is to specify an
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additional set of actions to be taken by the system when a signal-catching
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function is called.</p>
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<p>A signal is generated by sending a request to a signal monitor. Scheduling
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of the signal-handling action is controlled separately for each signal monitor
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through the <strong>signal blocking mask</strong>. The signal blocking mask is
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a bit mask that defines the set of signals to be blocked from delivery to the
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thread. The blocked or unblocked option specified for the <em>nth</em> bit
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position in the signal blocking mask is applied to the <em>nth</em> signal
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monitor defined for the process. When <samp>signal is unblocked</samp> is
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specified, the signal-handling action is eligible to be scheduled. When <samp>
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signal is blocked</samp> is specified, the signal-handling action is blocked
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from delivery.</p>
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<p>The process to receive the signal is identified by a <strong>process
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ID</strong>. The process ID is used to indicate whether the signal should be
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sent to an individual process or to a group of processes (known as a process
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group). The process ID is a 4-byte binary number that is used to locate an
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entry in the system-managed process table. A process table entry contains the
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following information relating to the process:</p>
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<ul>
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<li>Parent process ID</li>
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<li>Process group ID</li>
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<li>Status information</li>
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</ul>
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<p>The <strong>parent process</strong> is the logical creator of the process. A
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<strong>process group</strong> represents a collection of processes that are
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bound together for some common purpose. An error condition results if the
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process ID specified when a signal is sent does not represent a valid process
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or process group.</p>
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<p>The process sending a signal must have the appropriate authority to the
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receiving process. The parent process is allowed to send a signal to a child
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process (the parent process ID of the receiving process is equal to the process
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ID of the process sending the signal). A child process is allowed to send a
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signal to its parent process (the process ID of the receiving process is equal
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to the parent process ID of the process sending the signal). A process can send
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a signal to another process if the sending process has *JOBCTL authority
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defined for the current process user profile or in an adopted user profile.
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Otherwise, the real or effective user ID of the sending process must match the
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real or effective user ID of the receiving process. An error condition results
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if the process does not have authority to send the signal to a receiving
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process.</p>
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<br>
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<h3><a name="sigdifferences">Differences from Signals on UNIX Systems</a></h3>
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<p>The i5/OS<SUP>(TM)</SUP> support for signals does differ from the usual behavior of
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signals on UNIX<SUP>(R)</SUP> systems:</p>
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<ul>
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<li>Integration of American National Standard C signal model and X/Open signal
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model
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<p>On UNIX systems, the standard C signal functions (as defined by American
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National Standards Institute (ANSI)) and the UNIX signal functions interact.
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That is, the standard C <strong>signal()</strong> function operates on the
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process signal action vector. Likewise, when a signal is generated for a
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process using the standard C <strong>raise()</strong> function, the process
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signal blocking mask and the signal action vector are used to determine the
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action to be taken.</p>
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<p>On i5/OS, the behavior of the standard C signal functions depends on a
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compiler option. When the compiler option <strong>
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SYSIFCOPT(*ASYNCSIGNAL)</strong> is specified, the standard C <strong>
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signal()</strong> and <strong>raise()</strong> functions operate like the UNIX
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signal functions by operating on the process signal action vector and the
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process signal blocking mask. However, if the <strong>
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SYSIFCOPT(*ASYNCSIGNAL)</strong> is not specified the standard C signal
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functions do not operate like the UNIX signal functions. Although the default C
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signal model does not interact with the UNIX signal functions, the UNIX signal
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functions <strong>sigaction()</strong> and <strong>kill()</strong> provide the
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same type of capability as the standard C <strong>signal()</strong> and
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<strong>raise()</strong> functions. For more information, see <a href=
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"sigactn.htm">sigaction()--Examine and Change Signal Action</a> and <a href=
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"sigkill.htm">kill()--Send Signal to Process or Group of Processes</a>.</p>
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</li>
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<li>Scope of signal action vector, signal-blocking mask, and pending signals
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<p>On most UNIX systems, a process consists of a single thread of control. When
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the program in control needs to perform a task that is contained in another
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program, the program uses the <strong>fork()</strong> and <strong>
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exec()</strong> functions to start a child process that runs the other program.
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The signal controls for the child process are inherited from the parent
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process. Changes to the signal controls in either the parent or the child
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process are isolated to the process in which the change is made.</p>
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<p>On i5/OS, when a program needs to perform a task that is contained in
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another program, the program calls that program directly. The target program is
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run using the same process structure. As a result of this call and return
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mechanism, if a called program changes the process signal controls and does not
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restore the original signal controls when returning to its caller, the changed
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process signal controls remain in effect. The called program inherits the
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signal controls of its caller. However, there are some differences from what
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would be expected if <strong>fork()</strong> and <strong>exec()</strong> were
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used in a UNIX process:</p>
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<ul>
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<li>The set of pending signals is not cleared.</li>
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<li>Alarms are not reset.</li>
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<li>Signals set to be caught are not reset to the default action.</li>
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</ul>
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<p>Programs that use signals and change the signal controls of the process
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should restore the old actions or signal blocking mask (or both) when they
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return to their callers. Programs using signals should explicitly enable the
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process for signals when the program begins. If the process was not enabled for
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signals when the program was called, the program should also disable signals
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when it returns to the process. For more information, see <a href=
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"sigesig.htm">Qp0sEnableSignals()--Enable Process for Signals</a> and <a href=
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"sigdsig.htm">Qp0sDisableSignals()--Disable Process for Signals</a>.</p>
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</li>
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<li>Mapping system-detected errors to signals
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<p>On UNIX systems, system-detected errors are mapped to signal numbers. For
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example, a floating point error results in the SIGFPE signal being generated
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for the process. On i5/OS, the default C signal model presents system-detected
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errors to the user as escape messages which can be handled with C signal
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handlers established with the C <strong>signal()</strong> function or with ILE
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C exception-handling functions, but not with signal handlers established with
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the UNIX <strong>sigaction()</strong> function. When the compiler option
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<strong>SYSIFCOPT(*ASYNCSIGNAL)</strong> is specified, system-detected errors
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are mapped to signal numbers and can be handled with signal handlers
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established either with the C <strong>signal()</strong> function or the UNIX
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<strong>sigaction()</strong> function, but not with ILE C exception-handling
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functions.</p>
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</li>
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<li>Unexpected error handling in the signal-catching function
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<p>On UNIX systems, an unhandled error condition in a signal-catching function
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results in ending the process. On i5/OS, unhandled error conditions in the
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signal-catching function are implicitly handled. The signal-catching function
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is ended and the receiving program resumes running at the point at which it was
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interrupted. The error condition may be logged in the job log. Aside from the
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job log entry for the error, no further error notification takes place.</p>
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</li>
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<li>Termination action
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<p>i5/OS offers two types of termination actions. The termination action
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applied to most signals is to end the most recent request. This usually results
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in ending the current program, which is the expectation of most UNIX
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programmers. The second termination action is to end the process, which is more
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severe. The only signal with this action is SIGKILL.</p>
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</li>
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<li>Default actions
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<p>On i5/OS, some default actions for signals are different than on typical
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UNIX systems. For example, the i5/OS default action for the SIGPIPE signal is
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to ignore the signal.</p>
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</li>
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</ul>
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<hr>
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<center>
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<table cellpadding="2" cellspacing="2">
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<tr align="center">
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<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href=
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"unix.htm">UNIX-Type APIs</a> | <a href="aplist.htm">APIs by category</a></td>
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</tr>
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</table>
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</center>
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