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477 lines
17 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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<html><head>
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<!-- saved from http://www.win.tue.nl/~aeb/linux/lk/lk-10.html -->
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<meta name="GENERATOR" content="SGML-Tools 1.0.9"><title>The Linux kernel: Processes</title>
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</head>
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<body>
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<hr>
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<h2><a name="s10">10. Processes</a></h2>
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<p>Before looking at the Linux implementation, first a general Unix
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description of threads, processes, process groups and sessions.
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</p><p>A session contains a number of process groups, and a process group
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contains a number of processes, and a process contains a number
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of threads.
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</p><p>A session can have a controlling tty.
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At most one process group in a session can be a foreground process group.
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An interrupt character typed on a tty ("Teletype", i.e., terminal)
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causes a signal to be sent to all members of the foreground process group
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in the session (if any) that has that tty as controlling tty.
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</p><p>All these objects have numbers, and we have thread IDs, process IDs,
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process group IDs and session IDs.
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</p><p>
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</p><h2><a name="ss10.1">10.1 Processes</a>
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</h2>
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<p>
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</p><h3>Creation</h3>
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<p>A new process is traditionally started using the <code>fork()</code>
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system call:
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</p><blockquote>
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<pre>pid_t p;
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p = fork();
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if (p == (pid_t) -1)
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/* ERROR */
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else if (p == 0)
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/* CHILD */
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else
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/* PARENT */
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</pre>
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</blockquote>
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<p>This creates a child as a duplicate of its parent.
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Parent and child are identical in almost all respects.
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In the code they are distinguished by the fact that the parent
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learns the process ID of its child, while <code>fork()</code>
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returns 0 in the child. (It can find the process ID of its
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parent using the <code>getppid()</code> system call.)
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</p><p>
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</p><h3>Termination</h3>
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<p>Normal termination is when the process does
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</p><blockquote>
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<pre>exit(n);
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</pre>
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</blockquote>
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or
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<blockquote>
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<pre>return n;
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</pre>
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</blockquote>
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from its <code>main()</code> procedure. It returns the single byte <code>n</code>
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to its parent.
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<p>Abnormal termination is usually caused by a signal.
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</p><p>
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</p><h3>Collecting the exit code. Zombies</h3>
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<p>The parent does
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</p><blockquote>
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<pre>pid_t p;
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int status;
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p = wait(&status);
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</pre>
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</blockquote>
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and collects two bytes:
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<p>
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<figure>
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<eps file="absent">
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<img src="ctty_files/exit_status.png">
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</eps>
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</figure></p><p>A process that has terminated but has not yet been waited for
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is a <i>zombie</i>. It need only store these two bytes:
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exit code and reason for termination.
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</p><p>On the other hand, if the parent dies first, <code>init</code> (process 1)
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inherits the child and becomes its parent.
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</p><p>
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</p><h3>Signals</h3>
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<p>
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</p><h3>Stopping</h3>
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<p>Some signals cause a process to stop:
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<code>SIGSTOP</code> (stop!),
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<code>SIGTSTP</code> (stop from tty: probably ^Z was typed),
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<code>SIGTTIN</code> (tty input asked by background process),
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<code>SIGTTOU</code> (tty output sent by background process, and this was
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disallowed by <code>stty tostop</code>).
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</p><p>Apart from ^Z there also is ^Y. The former stops the process
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when it is typed, the latter stops it when it is read.
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</p><p>Signals generated by typing the corresponding character on some tty
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are sent to all processes that are in the foreground process group
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of the session that has that tty as controlling tty. (Details below.)
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</p><p>If a process is being traced, every signal will stop it.
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</p><p>
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</p><h3>Continuing</h3>
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<p><code>SIGCONT</code>: continue a stopped process.
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</p><p>
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</p><h3>Terminating</h3>
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<p><code>SIGKILL</code> (die! now!),
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<code>SIGTERM</code> (please, go away),
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<code>SIGHUP</code> (modem hangup),
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<code>SIGINT</code> (^C),
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<code>SIGQUIT</code> (^\), etc.
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Many signals have as default action to kill the target.
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(Sometimes with an additional core dump, when such is
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allowed by rlimit.)
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The signals <code>SIGCHLD</code> and <code>SIGWINCH</code>
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are ignored by default.
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All except <code>SIGKILL</code> and <code>SIGSTOP</code> can be
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caught or ignored or blocked.
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For details, see <code>signal(7)</code>.
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</p><p>
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</p><h2><a name="ss10.2">10.2 Process groups</a>
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</h2>
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<p>Every process is member of a unique <i>process group</i>,
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identified by its <i>process group ID</i>.
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(When the process is created, it becomes a member of the process group
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of its parent.)
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By convention, the process group ID of a process group
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equals the process ID of the first member of the process group,
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called the <i>process group leader</i>.
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A process finds the ID of its process group using the system call
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<code>getpgrp()</code>, or, equivalently, <code>getpgid(0)</code>.
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One finds the process group ID of process <code>p</code> using
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<code>getpgid(p)</code>.
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</p><p>One may use the command <code>ps j</code> to see PPID (parent process ID),
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PID (process ID), PGID (process group ID) and SID (session ID)
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of processes. With a shell that does not know about job control,
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like <code>ash</code>, each of its children will be in the same session
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and have the same process group as the shell. With a shell that knows
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about job control, like <code>bash</code>, the processes of one pipeline. like
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</p><blockquote>
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<pre>% cat paper | ideal | pic | tbl | eqn | ditroff > out
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</pre>
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</blockquote>
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form a single process group.
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<p>
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</p><h3>Creation</h3>
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<p>A process <code>pid</code> is put into the process group <code>pgid</code> by
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</p><blockquote>
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<pre>setpgid(pid, pgid);
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</pre>
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</blockquote>
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If <code>pgid == pid</code> or <code>pgid == 0</code> then this creates
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a new process group with process group leader <code>pid</code>.
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Otherwise, this puts <code>pid</code> into the already existing
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process group <code>pgid</code>.
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A zero <code>pid</code> refers to the current process.
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The call <code>setpgrp()</code> is equivalent to <code>setpgid(0,0)</code>.
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<p>
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</p><h3>Restrictions on setpgid()</h3>
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<p>The calling process must be <code>pid</code> itself, or its parent,
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and the parent can only do this before <code>pid</code> has done
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<code>exec()</code>, and only when both belong to the same session.
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It is an error if process <code>pid</code> is a session leader
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(and this call would change its <code>pgid</code>).
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</p><p>
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</p><h3>Typical sequence</h3>
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<p>
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</p><blockquote>
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<pre>p = fork();
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if (p == (pid_t) -1) {
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/* ERROR */
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} else if (p == 0) { /* CHILD */
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setpgid(0, pgid);
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...
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} else { /* PARENT */
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setpgid(p, pgid);
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...
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}
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</pre>
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</blockquote>
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This ensures that regardless of whether parent or child is scheduled
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first, the process group setting is as expected by both.
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<p>
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</p><h3>Signalling and waiting</h3>
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<p>One can signal all members of a process group:
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</p><blockquote>
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<pre>killpg(pgrp, sig);
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</pre>
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</blockquote>
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<p>One can wait for children in ones own process group:
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</p><blockquote>
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<pre>waitpid(0, &status, ...);
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</pre>
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</blockquote>
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or in a specified process group:
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<blockquote>
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<pre>waitpid(-pgrp, &status, ...);
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</pre>
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</blockquote>
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<p>
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</p><h3>Foreground process group</h3>
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<p>Among the process groups in a session at most one can be
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the <i>foreground process group</i> of that session.
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The tty input and tty signals (signals generated by ^C, ^Z, etc.)
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go to processes in this foreground process group.
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</p><p>A process can determine the foreground process group in its session
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using <code>tcgetpgrp(fd)</code>, where <code>fd</code> refers to its
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controlling tty. If there is none, this returns a random value
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larger than 1 that is not a process group ID.
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</p><p>A process can set the foreground process group in its session
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using <code>tcsetpgrp(fd,pgrp)</code>, where <code>fd</code> refers to its
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controlling tty, and <code>pgrp</code> is a process group in the
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its session, and this session still is associated to the controlling
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tty of the calling process.
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</p><p>How does one get <code>fd</code>? By definition, <code>/dev/tty</code>
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refers to the controlling tty, entirely independent of redirects
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of standard input and output. (There is also the function
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<code>ctermid()</code> to get the name of the controlling terminal.
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On a POSIX standard system it will return <code>/dev/tty</code>.)
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Opening the name of the
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controlling tty gives a file descriptor <code>fd</code>.
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</p><p>
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</p><h3>Background process groups</h3>
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<p>All process groups in a session that are not foreground
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process group are <i>background process groups</i>.
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Since the user at the keyboard is interacting with foreground
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processes, background processes should stay away from it.
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When a background process reads from the terminal it gets
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a SIGTTIN signal. Normally, that will stop it, the job control shell
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notices and tells the user, who can say <code>fg</code> to continue
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this background process as a foreground process, and then this
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process can read from the terminal. But if the background process
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ignores or blocks the SIGTTIN signal, or if its process group
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is orphaned (see below), then the read() returns an EIO error,
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and no signal is sent. (Indeed, the idea is to tell the process
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that reading from the terminal is not allowed right now.
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If it wouldn't see the signal, then it will see the error return.)
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</p><p>When a background process writes to the terminal, it may get
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a SIGTTOU signal. May: namely, when the flag that this must happen
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is set (it is off by default). One can set the flag by
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</p><blockquote>
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<pre>% stty tostop
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</pre>
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</blockquote>
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and clear it again by
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<blockquote>
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<pre>% stty -tostop
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</pre>
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</blockquote>
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and inspect it by
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<blockquote>
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<pre>% stty -a
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</pre>
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</blockquote>
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Again, if TOSTOP is set but the background process ignores or blocks
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the SIGTTOU signal, or if its process group is orphaned (see below),
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then the write() returns an EIO error, and no signal is sent.
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<p>
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</p><h3>Orphaned process groups</h3>
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<p>The process group leader is the first member of the process group.
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It may terminate before the others, and then the process group is
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without leader.
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</p><p>A process group is called <i>orphaned</i> when <i>the
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parent of every member is either in the process group
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or outside the session</i>.
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In particular, the process group of the session leader
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is always orphaned.
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</p><p>If termination of a process causes a process group to become
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orphaned, and some member is stopped, then all are sent first SIGHUP
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and then SIGCONT.
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</p><p>The idea is that perhaps the parent of the process group leader
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is a job control shell. (In the same session but a different
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process group.) As long as this parent is alive, it can
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handle the stopping and starting of members in the process group.
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When it dies, there may be nobody to continue stopped processes.
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Therefore, these stopped processes are sent SIGHUP, so that they
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die unless they catch or ignore it, and then SIGCONT to continue them.
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</p><p>Note that the process group of the session leader is already
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orphaned, so no signals are sent when the session leader dies.
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</p><p>Note also that a process group can become orphaned in two ways
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by termination of a process: either it was a parent and not itself
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in the process group, or it was the last element of the process group
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with a parent outside but in the same session.
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Furthermore, that a process group can become orphaned
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other than by termination of a process, namely when some
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member is moved to a different process group.
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</p><p>
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</p><h2><a name="ss10.3">10.3 Sessions</a>
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</h2>
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<p>Every process group is in a unique <i>session</i>.
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(When the process is created, it becomes a member of the session
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of its parent.)
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By convention, the session ID of a session
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equals the process ID of the first member of the session,
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called the <i>session leader</i>.
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A process finds the ID of its session using the system call
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<code>getsid()</code>.
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</p><p>Every session may have a <i>controlling tty</i>,
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that then also is called the controlling tty of each of
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its member processes.
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A file descriptor for the controlling tty is obtained by
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opening <code>/dev/tty</code>. (And when that fails, there was no
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controlling tty.) Given a file descriptor for the controlling tty,
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one may obtain the SID using <code>tcgetsid(fd)</code>.
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</p><p>A session is often set up by a login process. The terminal
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on which one is logged in then becomes the controlling tty
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of the session. All processes that are descendants of the
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login process will in general be members of the session.
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</p><p>
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</p><h3>Creation</h3>
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<p>A new session is created by
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</p><blockquote>
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<pre>pid = setsid();
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</pre>
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</blockquote>
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This is allowed only when the current process is not a process group leader.
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In order to be sure of that we fork first:
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<blockquote>
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<pre>p = fork();
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if (p) exit(0);
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pid = setsid();
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</pre>
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</blockquote>
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The result is that the current process (with process ID <code>pid</code>)
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becomes session leader of a new session with session ID <code>pid</code>.
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Moreover, it becomes process group leader of a new process group.
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Both session and process group contain only the single process <code>pid</code>.
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Furthermore, this process has no controlling tty.
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<p>The restriction that the current process must not be a process group leader
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is needed: otherwise its PID serves as PGID of some existing process group
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and cannot be used as the PGID of a new process group.
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</p><p>
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</p><h3>Getting a controlling tty</h3>
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<p>How does one get a controlling terminal? Nobody knows,
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this is a great mystery.
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</p><p>The System V approach is that the first tty opened by the process
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becomes its controlling tty.
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</p><p>The BSD approach is that one has to explicitly call
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</p><blockquote>
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<pre>ioctl(fd, TIOCSCTTY, 0/1);
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</pre>
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</blockquote>
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to get a controlling tty.
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<p>Linux tries to be compatible with both, as always, and this
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results in a very obscure complex of conditions. Roughly:
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</p><p>The <code>TIOCSCTTY</code> ioctl will give us a controlling tty,
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provided that (i) the current process is a session leader,
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and (ii) it does not yet have a controlling tty, and
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(iii) maybe the tty should not already control some other session;
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if it does it is an error if we aren't root, or we steal the tty
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if we are all-powerful.
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[vda: correction: third parameter controls this: if 1, we steal tty from
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any such session, if 0, we don't steal]
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</p><p>Opening some terminal will give us a controlling tty,
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provided that (i) the current process is a session leader, and
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(ii) it does not yet have a controlling tty, and
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(iii) the tty does not already control some other session, and
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(iv) the open did not have the <code>O_NOCTTY</code> flag, and
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(v) the tty is not the foreground VT, and
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(vi) the tty is not the console, and
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(vii) maybe the tty should not be master or slave pty.
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</p><p>
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</p><h3>Getting rid of a controlling tty</h3>
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<p>If a process wants to continue as a daemon, it must detach itself
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from its controlling tty. Above we saw that <code>setsid()</code>
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will remove the controlling tty. Also the ioctl TIOCNOTTY does this.
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Moreover, in order not to get a controlling tty again as soon as it
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opens a tty, the process has to fork once more, to assure that it
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is not a session leader. Typical code fragment:
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</p><p>
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</p><pre> if ((fork()) != 0)
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exit(0);
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setsid();
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if ((fork()) != 0)
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exit(0);
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</pre>
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<p>See also <code>daemon(3)</code>.
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</p><p>
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</p><h3>Disconnect</h3>
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<p>If the terminal goes away by modem hangup, and the line was not local,
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then a SIGHUP is sent to the session leader.
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Any further reads from the gone terminal return EOF.
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(Or possibly -1 with <code>errno</code> set to EIO.)
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</p><p>If the terminal is the slave side of a pseudotty, and the master side
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is closed (for the last time), then a SIGHUP is sent to the foreground
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process group of the slave side.
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</p><p>When the session leader dies, a SIGHUP is sent to all processes
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in the foreground process group. Moreover, the terminal stops being
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the controlling terminal of this session (so that it can become
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the controlling terminal of another session).
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</p><p>Thus, if the terminal goes away and the session leader is
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a job control shell, then it can handle things for its descendants,
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e.g. by sending them again a SIGHUP.
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If on the other hand the session leader is an innocent process
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that does not catch SIGHUP, it will die, and all foreground processes
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get a SIGHUP.
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</p><p>
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</p><h2><a name="ss10.4">10.4 Threads</a>
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</h2>
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<p>A process can have several threads. New threads (with the same PID
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as the parent thread) are started using the <code>clone</code> system
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call using the <code>CLONE_THREAD</code> flag. Threads are distinguished
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by a <i>thread ID</i> (TID). An ordinary process has a single thread
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with TID equal to PID. The system call <code>gettid()</code> returns the
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TID. The system call <code>tkill()</code> sends a signal to a single thread.
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</p><p>Example: a process with two threads. Both only print PID and TID and exit.
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(Linux 2.4.19 or later.)
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</p><pre>% cat << EOF > gettid-demo.c
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#include <unistd.h>
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#include <sys/types.h>
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#define CLONE_SIGHAND 0x00000800
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#define CLONE_THREAD 0x00010000
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#include <linux/unistd.h>
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#include <errno.h>
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_syscall0(pid_t,gettid)
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|
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|
int thread(void *p) {
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printf("thread: %d %d\n", gettid(), getpid());
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}
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|
|
|
main() {
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|
unsigned char stack[4096];
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int i;
|
|
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i = clone(thread, stack+2048, CLONE_THREAD | CLONE_SIGHAND, NULL);
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if (i == -1)
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|
perror("clone");
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|
else
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|
printf("clone returns %d\n", i);
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|
printf("parent: %d %d\n", gettid(), getpid());
|
|
}
|
|
EOF
|
|
% cc -o gettid-demo gettid-demo.c
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|
% ./gettid-demo
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|
clone returns 21826
|
|
parent: 21825 21825
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|
thread: 21826 21825
|
|
%
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</pre>
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<p>
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</p><p>
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</p><hr>
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</body></html>
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