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| <h3 class="section">26.4 Creating a Process</h3> |
| |
| <p>The <code>fork</code> function is the primitive for creating a process. |
| It is declared in the header file <samp><span class="file">unistd.h</span></samp>. |
| <a name="index-unistd_002eh-3166"></a> |
| <!-- unistd.h --> |
| <!-- POSIX.1 --> |
| |
| <div class="defun"> |
| — Function: pid_t <b>fork</b> (<var>void</var>)<var><a name="index-fork-3167"></a></var><br> |
| <blockquote><p>The <code>fork</code> function creates a new process. |
| |
| <p>If the operation is successful, there are then both parent and child |
| processes and both see <code>fork</code> return, but with different values: it |
| returns a value of <code>0</code> in the child process and returns the child's |
| process ID in the parent process. |
| |
| <p>If process creation failed, <code>fork</code> returns a value of <code>-1</code> in |
| the parent process. The following <code>errno</code> error conditions are |
| defined for <code>fork</code>: |
| |
| <dl> |
| <dt><code>EAGAIN</code><dd>There aren't enough system resources to create another process, or the |
| user already has too many processes running. This means exceeding the |
| <code>RLIMIT_NPROC</code> resource limit, which can usually be increased; |
| see <a href="Limits-on-Resources.html#Limits-on-Resources">Limits on Resources</a>. |
| |
| <br><dt><code>ENOMEM</code><dd>The process requires more space than the system can supply. |
| </dl> |
| </p></blockquote></div> |
| |
| <p>The specific attributes of the child process that differ from the |
| parent process are: |
| |
| <ul> |
| <li>The child process has its own unique process ID. |
| |
| <li>The parent process ID of the child process is the process ID of its |
| parent process. |
| |
| <li>The child process gets its own copies of the parent process's open file |
| descriptors. Subsequently changing attributes of the file descriptors |
| in the parent process won't affect the file descriptors in the child, |
| and vice versa. See <a href="Control-Operations.html#Control-Operations">Control Operations</a>. However, the file position |
| associated with each descriptor is shared by both processes; |
| see <a href="File-Position.html#File-Position">File Position</a>. |
| |
| <li>The elapsed processor times for the child process are set to zero; |
| see <a href="Processor-Time.html#Processor-Time">Processor Time</a>. |
| |
| <li>The child doesn't inherit file locks set by the parent process. |
| <!-- !!! flock locks shared --> |
| See <a href="Control-Operations.html#Control-Operations">Control Operations</a>. |
| |
| <li>The child doesn't inherit alarms set by the parent process. |
| See <a href="Setting-an-Alarm.html#Setting-an-Alarm">Setting an Alarm</a>. |
| |
| <li>The set of pending signals (see <a href="Delivery-of-Signal.html#Delivery-of-Signal">Delivery of Signal</a>) for the child |
| process is cleared. (The child process inherits its mask of blocked |
| signals and signal actions from the parent process.) |
| </ul> |
| |
| <!-- unistd.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: pid_t <b>vfork</b> (<var>void</var>)<var><a name="index-vfork-3168"></a></var><br> |
| <blockquote><p>The <code>vfork</code> function is similar to <code>fork</code> but on some systems |
| it is more efficient; however, there are restrictions you must follow to |
| use it safely. |
| |
| <p>While <code>fork</code> makes a complete copy of the calling process's address |
| space and allows both the parent and child to execute independently, |
| <code>vfork</code> does not make this copy. Instead, the child process |
| created with <code>vfork</code> shares its parent's address space until it |
| calls <code>_exit</code> or one of the <code>exec</code> functions. In the |
| meantime, the parent process suspends execution. |
| |
| <p>You must be very careful not to allow the child process created with |
| <code>vfork</code> to modify any global data or even local variables shared |
| with the parent. Furthermore, the child process cannot return from (or |
| do a long jump out of) the function that called <code>vfork</code>! This |
| would leave the parent process's control information very confused. If |
| in doubt, use <code>fork</code> instead. |
| |
| <p>Some operating systems don't really implement <code>vfork</code>. The GNU C |
| library permits you to use <code>vfork</code> on all systems, but actually |
| executes <code>fork</code> if <code>vfork</code> isn't available. If you follow |
| the proper precautions for using <code>vfork</code>, your program will still |
| work even if the system uses <code>fork</code> instead. |
| </p></blockquote></div> |
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