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