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| <h3 class="section">26.2 Process Creation Concepts</h3> |
| |
| <p>This section gives an overview of processes and of the steps involved in |
| creating a process and making it run another program. |
| |
| <p><a name="index-process-ID-3154"></a><a name="index-process-lifetime-3155"></a>Each process is named by a <dfn>process ID</dfn> number. A unique process ID |
| is allocated to each process when it is created. The <dfn>lifetime</dfn> of |
| a process ends when its termination is reported to its parent process; |
| at that time, all of the process resources, including its process ID, |
| are freed. |
| |
| <p><a name="index-creating-a-process-3156"></a><a name="index-forking-a-process-3157"></a><a name="index-child-process-3158"></a><a name="index-parent-process-3159"></a>Processes are created with the <code>fork</code> system call (so the operation |
| of creating a new process is sometimes called <dfn>forking</dfn> a process). |
| The <dfn>child process</dfn> created by <code>fork</code> is a copy of the original |
| <dfn>parent process</dfn>, except that it has its own process ID. |
| |
| <p>After forking a child process, both the parent and child processes |
| continue to execute normally. If you want your program to wait for a |
| child process to finish executing before continuing, you must do this |
| explicitly after the fork operation, by calling <code>wait</code> or |
| <code>waitpid</code> (see <a href="Process-Completion.html#Process-Completion">Process Completion</a>). These functions give you |
| limited information about why the child terminated—for example, its |
| exit status code. |
| |
| <p>A newly forked child process continues to execute the same program as |
| its parent process, at the point where the <code>fork</code> call returns. |
| You can use the return value from <code>fork</code> to tell whether the program |
| is running in the parent process or the child. |
| |
| <p><a name="index-process-image-3160"></a>Having several processes run the same program is only occasionally |
| useful. But the child can execute another program using one of the |
| <code>exec</code> functions; see <a href="Executing-a-File.html#Executing-a-File">Executing a File</a>. The program that the |
| process is executing is called its <dfn>process image</dfn>. Starting |
| execution of a new program causes the process to forget all about its |
| previous process image; when the new program exits, the process exits |
| too, instead of returning to the previous process image. |
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