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4 <meta name="GENERATOR" content="SGML-Tools 1.0.9"><title>The Linux kernel: Processes</title>
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6<body>
7<hr>
8<h2><a name="s10">10. Processes</a></h2>
9
10<p>Before looking at the Linux implementation, first a general Unix
11description of threads, processes, process groups and sessions.
12</p><p>
13(See also <a href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap11.html">General Terminal Interface</a>)
14</p><p>A session contains a number of process groups, and a process group
15contains a number of processes, and a process contains a number
16of threads.
17</p><p>A session can have a controlling tty.
18At most one process group in a session can be a foreground process group.
19An interrupt character typed on a tty ("Teletype", i.e., terminal)
20causes a signal to be sent to all members of the foreground process group
21in the session (if any) that has that tty as controlling tty.
22</p><p>All these objects have numbers, and we have thread IDs, process IDs,
23process group IDs and session IDs.
24</p><p>
25</p><h2><a name="ss10.1">10.1 Processes</a>
26</h2>
27
28<p>
29</p><h3>Creation</h3>
30
31<p>A new process is traditionally started using the <code>fork()</code>
32system call:
33</p><blockquote>
34<pre>pid_t p;
35
36p = fork();
37if (p == (pid_t) -1)
38 /* ERROR */
39else if (p == 0)
40 /* CHILD */
41else
42 /* PARENT */
43</pre>
44</blockquote>
45<p>This creates a child as a duplicate of its parent.
46Parent and child are identical in almost all respects.
47In the code they are distinguished by the fact that the parent
48learns the process ID of its child, while <code>fork()</code>
49returns 0 in the child. (It can find the process ID of its
50parent using the <code>getppid()</code> system call.)
51</p><p>
52</p><h3>Termination</h3>
53
54<p>Normal termination is when the process does
55</p><blockquote>
56<pre>exit(n);
57</pre>
58</blockquote>
59
60or
61<blockquote>
62<pre>return n;
63</pre>
64</blockquote>
65
66from its <code>main()</code> procedure. It returns the single byte <code>n</code>
67to its parent.
68<p>Abnormal termination is usually caused by a signal.
69</p><p>
70</p><h3>Collecting the exit code. Zombies</h3>
71
72<p>The parent does
73</p><blockquote>
74<pre>pid_t p;
75int status;
76
77p = wait(&amp;status);
78</pre>
79</blockquote>
80
81and collects two bytes:
82<p>
83<figure>
84<eps file="absent">
85<img src="ctty_files/exit_status.png">
86</eps>
87</figure></p><p>A process that has terminated but has not yet been waited for
88is a <i>zombie</i>. It need only store these two bytes:
89exit code and reason for termination.
90</p><p>On the other hand, if the parent dies first, <code>init</code> (process 1)
91inherits the child and becomes its parent.
92</p><p>
93</p><h3>Signals</h3>
94
95<p>
96</p><h3>Stopping</h3>
97
98<p>Some signals cause a process to stop:
99<code>SIGSTOP</code> (stop!),
100<code>SIGTSTP</code> (stop from tty: probably ^Z was typed),
101<code>SIGTTIN</code> (tty input asked by background process),
102<code>SIGTTOU</code> (tty output sent by background process, and this was
103disallowed by <code>stty tostop</code>).
104</p><p>Apart from ^Z there also is ^Y. The former stops the process
105when it is typed, the latter stops it when it is read.
106</p><p>Signals generated by typing the corresponding character on some tty
107are sent to all processes that are in the foreground process group
108of the session that has that tty as controlling tty. (Details below.)
109</p><p>If a process is being traced, every signal will stop it.
110</p><p>
111</p><h3>Continuing</h3>
112
113<p><code>SIGCONT</code>: continue a stopped process.
114</p><p>
115</p><h3>Terminating</h3>
116
117<p><code>SIGKILL</code> (die! now!),
118<code>SIGTERM</code> (please, go away),
119<code>SIGHUP</code> (modem hangup),
120<code>SIGINT</code> (^C),
121<code>SIGQUIT</code> (^\), etc.
122Many signals have as default action to kill the target.
123(Sometimes with an additional core dump, when such is
124allowed by rlimit.)
125The signals <code>SIGCHLD</code> and <code>SIGWINCH</code>
126are ignored by default.
127All except <code>SIGKILL</code> and <code>SIGSTOP</code> can be
128caught or ignored or blocked.
129For details, see <code>signal(7)</code>.
130</p><p>
131</p><h2><a name="ss10.2">10.2 Process groups</a>
132</h2>
133
134<p>Every process is member of a unique <i>process group</i>,
135identified by its <i>process group ID</i>.
136(When the process is created, it becomes a member of the process group
137of its parent.)
138By convention, the process group ID of a process group
139equals the process ID of the first member of the process group,
140called the <i>process group leader</i>.
141A process finds the ID of its process group using the system call
142<code>getpgrp()</code>, or, equivalently, <code>getpgid(0)</code>.
143One finds the process group ID of process <code>p</code> using
144<code>getpgid(p)</code>.
145</p><p>One may use the command <code>ps j</code> to see PPID (parent process ID),
146PID (process ID), PGID (process group ID) and SID (session ID)
147of processes. With a shell that does not know about job control,
148like <code>ash</code>, each of its children will be in the same session
149and have the same process group as the shell. With a shell that knows
150about job control, like <code>bash</code>, the processes of one pipeline, like
151</p><blockquote>
152<pre>% cat paper | ideal | pic | tbl | eqn | ditroff &gt; out
153</pre>
154</blockquote>
155
156form a single process group.
157<p>
158</p><h3>Creation</h3>
159
160<p>A process <code>pid</code> is put into the process group <code>pgid</code> by
161</p><blockquote>
162<pre>setpgid(pid, pgid);
163</pre>
164</blockquote>
165
166If <code>pgid == pid</code> or <code>pgid == 0</code> then this creates
167a new process group with process group leader <code>pid</code>.
168Otherwise, this puts <code>pid</code> into the already existing
169process group <code>pgid</code>.
170A zero <code>pid</code> refers to the current process.
171The call <code>setpgrp()</code> is equivalent to <code>setpgid(0,0)</code>.
172<p>
173</p><h3>Restrictions on setpgid()</h3>
174
175<p>The calling process must be <code>pid</code> itself, or its parent,
176and the parent can only do this before <code>pid</code> has done
177<code>exec()</code>, and only when both belong to the same session.
178It is an error if process <code>pid</code> is a session leader
179(and this call would change its <code>pgid</code>).
180</p><p>
181</p><h3>Typical sequence</h3>
182
183<p>
184</p><blockquote>
185<pre>p = fork();
186if (p == (pid_t) -1) {
187 /* ERROR */
188} else if (p == 0) { /* CHILD */
189 setpgid(0, pgid);
190 ...
191} else { /* PARENT */
192 setpgid(p, pgid);
193 ...
194}
195</pre>
196</blockquote>
197
198This ensures that regardless of whether parent or child is scheduled
199first, the process group setting is as expected by both.
200<p>
201</p><h3>Signalling and waiting</h3>
202
203<p>One can signal all members of a process group:
204</p><blockquote>
205<pre>killpg(pgrp, sig);
206</pre>
207</blockquote>
208<p>One can wait for children in ones own process group:
209</p><blockquote>
210<pre>waitpid(0, &amp;status, ...);
211</pre>
212</blockquote>
213
214or in a specified process group:
215<blockquote>
216<pre>waitpid(-pgrp, &amp;status, ...);
217</pre>
218</blockquote>
219<p>
220</p><h3>Foreground process group</h3>
221
222<p>Among the process groups in a session at most one can be
223the <i>foreground process group</i> of that session.
224The tty input and tty signals (signals generated by ^C, ^Z, etc.)
225go to processes in this foreground process group.
226</p><p>A process can determine the foreground process group in its session
227using <code>tcgetpgrp(fd)</code>, where <code>fd</code> refers to its
228controlling tty. If there is none, this returns a random value
229larger than 1 that is not a process group ID.
230</p><p>A process can set the foreground process group in its session
231using <code>tcsetpgrp(fd,pgrp)</code>, where <code>fd</code> refers to its
232controlling tty, and <code>pgrp</code> is a process group in
233its session, and this session still is associated to the controlling
234tty of the calling process.
235</p><p>How does one get <code>fd</code>? By definition, <code>/dev/tty</code>
236refers to the controlling tty, entirely independent of redirects
237of standard input and output. (There is also the function
238<code>ctermid()</code> to get the name of the controlling terminal.
239On a POSIX standard system it will return <code>/dev/tty</code>.)
240Opening the name of the
241controlling tty gives a file descriptor <code>fd</code>.
242</p><p>
243</p><h3>Background process groups</h3>
244
245<p>All process groups in a session that are not foreground
246process group are <i>background process groups</i>.
247Since the user at the keyboard is interacting with foreground
248processes, background processes should stay away from it.
249When a background process reads from the terminal it gets
250a SIGTTIN signal. Normally, that will stop it, the job control shell
251notices and tells the user, who can say <code>fg</code> to continue
252this background process as a foreground process, and then this
253process can read from the terminal. But if the background process
254ignores or blocks the SIGTTIN signal, or if its process group
255is orphaned (see below), then the read() returns an EIO error,
256and no signal is sent. (Indeed, the idea is to tell the process
257that reading from the terminal is not allowed right now.
258If it wouldn't see the signal, then it will see the error return.)
259</p><p>When a background process writes to the terminal, it may get
260a SIGTTOU signal. May: namely, when the flag that this must happen
261is set (it is off by default). One can set the flag by
262</p><blockquote>
263<pre>% stty tostop
264</pre>
265</blockquote>
266
267and clear it again by
268<blockquote>
269<pre>% stty -tostop
270</pre>
271</blockquote>
272
273and inspect it by
274<blockquote>
275<pre>% stty -a
276</pre>
277</blockquote>
278
279Again, if TOSTOP is set but the background process ignores or blocks
280the SIGTTOU signal, or if its process group is orphaned (see below),
281then the write() returns an EIO error, and no signal is sent.
282[vda: correction. SUS says that if SIGTTOU is blocked/ignored, write succeeds. ]
283<p>
284</p><h3>Orphaned process groups</h3>
285
286<p>The process group leader is the first member of the process group.
287It may terminate before the others, and then the process group is
288without leader.
289</p><p>A process group is called <i>orphaned</i> when <i>the
290parent of every member is either in the process group
291or outside the session</i>.
292In particular, the process group of the session leader
293is always orphaned.
294</p><p>If termination of a process causes a process group to become
295orphaned, and some member is stopped, then all are sent first SIGHUP
296and then SIGCONT.
297</p><p>The idea is that perhaps the parent of the process group leader
298is a job control shell. (In the same session but a different
299process group.) As long as this parent is alive, it can
300handle the stopping and starting of members in the process group.
301When it dies, there may be nobody to continue stopped processes.
302Therefore, these stopped processes are sent SIGHUP, so that they
303die unless they catch or ignore it, and then SIGCONT to continue them.
304</p><p>Note that the process group of the session leader is already
305orphaned, so no signals are sent when the session leader dies.
306</p><p>Note also that a process group can become orphaned in two ways
307by termination of a process: either it was a parent and not itself
308in the process group, or it was the last element of the process group
309with a parent outside but in the same session.
310Furthermore, that a process group can become orphaned
311other than by termination of a process, namely when some
312member is moved to a different process group.
313</p><p>
314</p><h2><a name="ss10.3">10.3 Sessions</a>
315</h2>
316
317<p>Every process group is in a unique <i>session</i>.
318(When the process is created, it becomes a member of the session
319of its parent.)
320By convention, the session ID of a session
321equals the process ID of the first member of the session,
322called the <i>session leader</i>.
323A process finds the ID of its session using the system call
324<code>getsid()</code>.
325</p><p>Every session may have a <i>controlling tty</i>,
326that then also is called the controlling tty of each of
327its member processes.
328A file descriptor for the controlling tty is obtained by
329opening <code>/dev/tty</code>. (And when that fails, there was no
330controlling tty.) Given a file descriptor for the controlling tty,
331one may obtain the SID using <code>tcgetsid(fd)</code>.
332</p><p>A session is often set up by a login process. The terminal
333on which one is logged in then becomes the controlling tty
334of the session. All processes that are descendants of the
335login process will in general be members of the session.
336</p><p>
337</p><h3>Creation</h3>
338
339<p>A new session is created by
340</p><blockquote>
341<pre>pid = setsid();
342</pre>
343</blockquote>
344
345This is allowed only when the current process is not a process group leader.
346In order to be sure of that we fork first:
347<blockquote>
348<pre>p = fork();
349if (p) exit(0);
350pid = setsid();
351</pre>
352</blockquote>
353
354The result is that the current process (with process ID <code>pid</code>)
355becomes session leader of a new session with session ID <code>pid</code>.
356Moreover, it becomes process group leader of a new process group.
357Both session and process group contain only the single process <code>pid</code>.
358Furthermore, this process has no controlling tty.
359<p>The restriction that the current process must not be a process group leader
360is needed: otherwise its PID serves as PGID of some existing process group
361and cannot be used as the PGID of a new process group.
362</p><p>
363</p><h3>Getting a controlling tty</h3>
364
365<p>How does one get a controlling terminal? Nobody knows,
366this is a great mystery.
367</p><p>The System V approach is that the first tty opened by the process
368becomes its controlling tty.
369</p><p>The BSD approach is that one has to explicitly call
370</p><blockquote>
371<pre>ioctl(fd, TIOCSCTTY, 0/1);
372</pre>
373</blockquote>
374
375to get a controlling tty.
376<p>Linux tries to be compatible with both, as always, and this
377results in a very obscure complex of conditions. Roughly:
378</p><p>The <code>TIOCSCTTY</code> ioctl will give us a controlling tty,
379provided that (i) the current process is a session leader,
380and (ii) it does not yet have a controlling tty, and
381(iii) maybe the tty should not already control some other session;
382if it does it is an error if we aren't root, or we steal the tty
383if we are all-powerful.
384[vda: correction: third parameter controls this: if 1, we steal tty from
385any such session, if 0, we don't steal]
386</p><p>Opening some terminal will give us a controlling tty,
387provided that (i) the current process is a session leader, and
388(ii) it does not yet have a controlling tty, and
389(iii) the tty does not already control some other session, and
390(iv) the open did not have the <code>O_NOCTTY</code> flag, and
391(v) the tty is not the foreground VT, and
392(vi) the tty is not the console, and
393(vii) maybe the tty should not be master or slave pty.
394</p><p>
395</p><h3>Getting rid of a controlling tty</h3>
396
397<p>If a process wants to continue as a daemon, it must detach itself
398from its controlling tty. Above we saw that <code>setsid()</code>
399will remove the controlling tty. Also the ioctl TIOCNOTTY does this.
400Moreover, in order not to get a controlling tty again as soon as it
401opens a tty, the process has to fork once more, to assure that it
402is not a session leader. Typical code fragment:
403</p><p>
404</p><pre> if ((fork()) != 0)
405 exit(0);
406 setsid();
407 if ((fork()) != 0)
408 exit(0);
409</pre>
410<p>See also <code>daemon(3)</code>.
411</p><p>
412</p><h3>Disconnect</h3>
413
414<p>If the terminal goes away by modem hangup, and the line was not local,
415then a SIGHUP is sent to the session leader.
416Any further reads from the gone terminal return EOF.
417(Or possibly -1 with <code>errno</code> set to EIO.)
418</p><p>If the terminal is the slave side of a pseudotty, and the master side
419is closed (for the last time), then a SIGHUP is sent to the foreground
420process group of the slave side.
421</p><p>When the session leader dies, a SIGHUP is sent to all processes
422in the foreground process group. Moreover, the terminal stops being
423the controlling terminal of this session (so that it can become
424the controlling terminal of another session).
425</p><p>Thus, if the terminal goes away and the session leader is
426a job control shell, then it can handle things for its descendants,
427e.g. by sending them again a SIGHUP.
428If on the other hand the session leader is an innocent process
429that does not catch SIGHUP, it will die, and all foreground processes
430get a SIGHUP.
431</p><p>
432</p><h2><a name="ss10.4">10.4 Threads</a>
433</h2>
434
435<p>A process can have several threads. New threads (with the same PID
436as the parent thread) are started using the <code>clone</code> system
437call using the <code>CLONE_THREAD</code> flag. Threads are distinguished
438by a <i>thread ID</i> (TID). An ordinary process has a single thread
439with TID equal to PID. The system call <code>gettid()</code> returns the
440TID. The system call <code>tkill()</code> sends a signal to a single thread.
441</p><p>Example: a process with two threads. Both only print PID and TID and exit.
442(Linux 2.4.19 or later.)
443</p><pre>% cat &lt;&lt; EOF &gt; gettid-demo.c
444#include &lt;unistd.h&gt;
445#include &lt;sys/types.h&gt;
446#define CLONE_SIGHAND 0x00000800
447#define CLONE_THREAD 0x00010000
448#include &lt;linux/unistd.h&gt;
449#include &lt;errno.h&gt;
450_syscall0(pid_t,gettid)
451
452int thread(void *p) {
453 printf("thread: %d %d\n", gettid(), getpid());
454}
455
456main() {
457 unsigned char stack[4096];
458 int i;
459
460 i = clone(thread, stack+2048, CLONE_THREAD | CLONE_SIGHAND, NULL);
461 if (i == -1)
462 perror("clone");
463 else
464 printf("clone returns %d\n", i);
465 printf("parent: %d %d\n", gettid(), getpid());
466}
467EOF
468% cc -o gettid-demo gettid-demo.c
469% ./gettid-demo
470clone returns 21826
471parent: 21825 21825
472thread: 21826 21825
473%
474</pre>
475<p>
476</p><p>
477</p><hr>
478
479</body></html>