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| <h4 class="subsection">27.6.5 Stopped and Terminated Jobs</h4> |
| |
| <p><a name="index-stopped-jobs_002c-detecting-3222"></a><a name="index-terminated-jobs_002c-detecting-3223"></a>When a foreground process is launched, the shell must block until all of |
| the processes in that job have either terminated or stopped. It can do |
| this by calling the <code>waitpid</code> function; see <a href="Process-Completion.html#Process-Completion">Process Completion</a>. Use the <code>WUNTRACED</code> option so that status is reported |
| for processes that stop as well as processes that terminate. |
| |
| <p>The shell must also check on the status of background jobs so that it |
| can report terminated and stopped jobs to the user; this can be done by |
| calling <code>waitpid</code> with the <code>WNOHANG</code> option. A good place to |
| put a such a check for terminated and stopped jobs is just before |
| prompting for a new command. |
| |
| <p><a name="index-g_t_0040code_007bSIGCHLD_007d_002c-handling-of-3224"></a>The shell can also receive asynchronous notification that there is |
| status information available for a child process by establishing a |
| handler for <code>SIGCHLD</code> signals. See <a href="Signal-Handling.html#Signal-Handling">Signal Handling</a>. |
| |
| <p>In the sample shell program, the <code>SIGCHLD</code> signal is normally |
| ignored. This is to avoid reentrancy problems involving the global data |
| structures the shell manipulates. But at specific times when the shell |
| is not using these data structures—such as when it is waiting for |
| input on the terminal—it makes sense to enable a handler for |
| <code>SIGCHLD</code>. The same function that is used to do the synchronous |
| status checks (<code>do_job_notification</code>, in this case) can also be |
| called from within this handler. |
| |
| <p>Here are the parts of the sample shell program that deal with checking |
| the status of jobs and reporting the information to the user. |
| |
| <pre class="smallexample"> /* <span class="roman">Store the status of the process </span><var>pid</var><span class="roman"> that was returned by waitpid.</span> |
| <span class="roman">Return 0 if all went well, nonzero otherwise.</span> */ |
| |
| int |
| mark_process_status (pid_t pid, int status) |
| { |
| job *j; |
| process *p; |
| |
| if (pid > 0) |
| { |
| /* <span class="roman">Update the record for the process.</span> */ |
| for (j = first_job; j; j = j->next) |
| for (p = j->first_process; p; p = p->next) |
| if (p->pid == pid) |
| { |
| p->status = status; |
| if (WIFSTOPPED (status)) |
| p->stopped = 1; |
| else |
| { |
| p->completed = 1; |
| if (WIFSIGNALED (status)) |
| fprintf (stderr, "%d: Terminated by signal %d.\n", |
| (int) pid, WTERMSIG (p->status)); |
| } |
| return 0; |
| } |
| fprintf (stderr, "No child process %d.\n", pid); |
| return -1; |
| } |
| else if (pid == 0 || errno == ECHILD) |
| /* <span class="roman">No processes ready to report.</span> */ |
| return -1; |
| else { |
| /* <span class="roman">Other weird errors.</span> */ |
| perror ("waitpid"); |
| return -1; |
| } |
| } |
| |
| /* <span class="roman">Check for processes that have status information available,</span> |
| <span class="roman">without blocking.</span> */ |
| |
| void |
| update_status (void) |
| { |
| int status; |
| pid_t pid; |
| |
| do |
| pid = waitpid (WAIT_ANY, &status, WUNTRACED|WNOHANG); |
| while (!mark_process_status (pid, status)); |
| } |
| |
| /* <span class="roman">Check for processes that have status information available,</span> |
| <span class="roman">blocking until all processes in the given job have reported.</span> */ |
| |
| void |
| wait_for_job (job *j) |
| { |
| int status; |
| pid_t pid; |
| |
| do |
| pid = waitpid (WAIT_ANY, &status, WUNTRACED); |
| while (!mark_process_status (pid, status) |
| && !job_is_stopped (j) |
| && !job_is_completed (j)); |
| } |
| |
| /* <span class="roman">Format information about job status for the user to look at.</span> */ |
| |
| void |
| format_job_info (job *j, const char *status) |
| { |
| fprintf (stderr, "%ld (%s): %s\n", (long)j->pgid, status, j->command); |
| } |
| |
| /* <span class="roman">Notify the user about stopped or terminated jobs.</span> |
| <span class="roman">Delete terminated jobs from the active job list.</span> */ |
| |
| void |
| do_job_notification (void) |
| { |
| job *j, *jlast, *jnext; |
| process *p; |
| |
| /* <span class="roman">Update status information for child processes.</span> */ |
| update_status (); |
| |
| jlast = NULL; |
| for (j = first_job; j; j = jnext) |
| { |
| jnext = j->next; |
| |
| /* <span class="roman">If all processes have completed, tell the user the job has</span> |
| <span class="roman">completed and delete it from the list of active jobs.</span> */ |
| if (job_is_completed (j)) { |
| format_job_info (j, "completed"); |
| if (jlast) |
| jlast->next = jnext; |
| else |
| first_job = jnext; |
| free_job (j); |
| } |
| |
| /* <span class="roman">Notify the user about stopped jobs,</span> |
| <span class="roman">marking them so that we won't do this more than once.</span> */ |
| else if (job_is_stopped (j) && !j->notified) { |
| format_job_info (j, "stopped"); |
| j->notified = 1; |
| jlast = j; |
| } |
| |
| /* <span class="roman">Don't say anything about jobs that are still running.</span> */ |
| else |
| jlast = j; |
| } |
| } |
| </pre> |
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