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| <h4 class="subsection">24.1.2 Concepts of Signal Generation</h4> |
| |
| <p><a name="index-generation-of-signals-2811"></a> |
| In general, the events that generate signals fall into three major |
| categories: errors, external events, and explicit requests. |
| |
| <p>An error means that a program has done something invalid and cannot |
| continue execution. But not all kinds of errors generate signals—in |
| fact, most do not. For example, opening a nonexistent file is an error, |
| but it does not raise a signal; instead, <code>open</code> returns <code>-1</code>. |
| In general, errors that are necessarily associated with certain library |
| functions are reported by returning a value that indicates an error. |
| The errors which raise signals are those which can happen anywhere in |
| the program, not just in library calls. These include division by zero |
| and invalid memory addresses. |
| |
| <p>An external event generally has to do with I/O or other processes. |
| These include the arrival of input, the expiration of a timer, and the |
| termination of a child process. |
| |
| <p>An explicit request means the use of a library function such as |
| <code>kill</code> whose purpose is specifically to generate a signal. |
| |
| <p>Signals may be generated <dfn>synchronously</dfn> or <dfn>asynchronously</dfn>. A |
| synchronous signal pertains to a specific action in the program, and is |
| delivered (unless blocked) during that action. Most errors generate |
| signals synchronously, and so do explicit requests by a process to |
| generate a signal for that same process. On some machines, certain |
| kinds of hardware errors (usually floating-point exceptions) are not |
| reported completely synchronously, but may arrive a few instructions |
| later. |
| |
| <p>Asynchronous signals are generated by events outside the control of the |
| process that receives them. These signals arrive at unpredictable times |
| during execution. External events generate signals asynchronously, and |
| so do explicit requests that apply to some other process. |
| |
| <p>A given type of signal is either typically synchronous or typically |
| asynchronous. For example, signals for errors are typically synchronous |
| because errors generate signals synchronously. But any type of signal |
| can be generated synchronously or asynchronously with an explicit |
| request. |
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