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| Next: <a rel="next" accesskey="n" href="Stopped-and-Terminated-Jobs.html#Stopped-and-Terminated-Jobs">Stopped and Terminated Jobs</a>, |
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| <h4 class="subsection">27.6.4 Foreground and Background</h4> |
| |
| <p>Now let's consider what actions must be taken by the shell when it |
| launches a job into the foreground, and how this differs from what |
| must be done when a background job is launched. |
| |
| <p><a name="index-foreground-job_002c-launching-3220"></a>When a foreground job is launched, the shell must first give it access |
| to the controlling terminal by calling <code>tcsetpgrp</code>. Then, the |
| shell should wait for processes in that process group to terminate or |
| stop. This is discussed in more detail in <a href="Stopped-and-Terminated-Jobs.html#Stopped-and-Terminated-Jobs">Stopped and Terminated Jobs</a>. |
| |
| <p>When all of the processes in the group have either completed or stopped, |
| the shell should regain control of the terminal for its own process |
| group by calling <code>tcsetpgrp</code> again. Since stop signals caused by |
| I/O from a background process or a SUSP character typed by the user |
| are sent to the process group, normally all the processes in the job |
| stop together. |
| |
| <p>The foreground job may have left the terminal in a strange state, so the |
| shell should restore its own saved terminal modes before continuing. In |
| case the job is merely stopped, the shell should first save the current |
| terminal modes so that it can restore them later if the job is |
| continued. The functions for dealing with terminal modes are |
| <code>tcgetattr</code> and <code>tcsetattr</code>; these are described in |
| <a href="Terminal-Modes.html#Terminal-Modes">Terminal Modes</a>. |
| |
| <p>Here is the sample shell's function for doing all of this. |
| |
| <pre class="smallexample"> /* <span class="roman">Put job </span><var>j</var><span class="roman"> in the foreground. If </span><var>cont</var><span class="roman"> is nonzero,</span> |
| <span class="roman">restore the saved terminal modes and send the process group a</span> |
| <code>SIGCONT</code><span class="roman"> signal to wake it up before we block.</span> */ |
| |
| void |
| put_job_in_foreground (job *j, int cont) |
| { |
| /* <span class="roman">Put the job into the foreground.</span> */ |
| tcsetpgrp (shell_terminal, j->pgid); |
| |
| /* <span class="roman">Send the job a continue signal, if necessary.</span> */ |
| if (cont) |
| { |
| tcsetattr (shell_terminal, TCSADRAIN, &j->tmodes); |
| if (kill (- j->pgid, SIGCONT) < 0) |
| perror ("kill (SIGCONT)"); |
| } |
| |
| /* <span class="roman">Wait for it to report.</span> */ |
| wait_for_job (j); |
| |
| /* <span class="roman">Put the shell back in the foreground.</span> */ |
| tcsetpgrp (shell_terminal, shell_pgid); |
| |
| /* <span class="roman">Restore the shell's terminal modes.</span> */ |
| tcgetattr (shell_terminal, &j->tmodes); |
| tcsetattr (shell_terminal, TCSADRAIN, &shell_tmodes); |
| } |
| </pre> |
| <p><a name="index-background-job_002c-launching-3221"></a>If the process group is launched as a background job, the shell should |
| remain in the foreground itself and continue to read commands from |
| the terminal. |
| |
| <p>In the sample shell, there is not much that needs to be done to put |
| a job into the background. Here is the function it uses: |
| |
| <pre class="smallexample"> /* <span class="roman">Put a job in the background. If the cont argument is true, send</span> |
| <span class="roman">the process group a </span><code>SIGCONT</code><span class="roman"> signal to wake it up.</span> */ |
| |
| void |
| put_job_in_background (job *j, int cont) |
| { |
| /* <span class="roman">Send the job a continue signal, if necessary.</span> */ |
| if (cont) |
| if (kill (-j->pgid, SIGCONT) < 0) |
| perror ("kill (SIGCONT)"); |
| } |
| </pre> |
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