release/src/router/busybox/docs/ctty.htm

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   8 <h2><a name="s10">10. Processes</a></h2>
   9
  10 <p>Before looking at the Linux implementation, first a general Unix
  11 description 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
  15 contains a number of processes, and a process contains a number
  16 of threads.
  17 </p><p>A session can have a controlling tty.
  18 At most one process group in a session can be a foreground process group.
  19 An interrupt character typed on a tty ("Teletype", i.e., terminal)
  20 causes a signal to be sent to all members of the foreground process group
  21 in 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,
  23 process 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>
  32 system call:
  33 </p><blockquote>
  34 <pre>pid_t p;
  35
  36 p = fork();
  37 if (p == (pid_t) -1)
  38         /* ERROR */
  39 else if (p == 0)
  40         /* CHILD */
  41 else
  42         /* PARENT */
  43 </pre>
  44 </blockquote>
  45 <p>This creates a child as a duplicate of its parent.
  46 Parent and child are identical in almost all respects.
  47 In the code they are distinguished by the fact that the parent
  48 learns the process ID of its child, while <code>fork()</code>
  49 returns 0 in the child. (It can find the process ID of its
  50 parent 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
  60 or
  61 <blockquote>
  62 <pre>return n;
  63 </pre>
  64 </blockquote>
  65
  66 from its <code>main()</code> procedure. It returns the single byte <code>n</code>
  67 to 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;
  75 int status;
  76
  77 p = wait(&amp;status);
  78 </pre>
  79 </blockquote>
  80
  81 and 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
  88 is a <i>zombie</i>. It need only store these two bytes:
  89 exit code and reason for termination.
  90 </p><p>On the other hand, if the parent dies first, <code>init</code> (process 1)
  91 inherits 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
 103 disallowed by <code>stty tostop</code>).
 104 </p><p>Apart from ^Z there also is ^Y. The former stops the process
 105 when it is typed, the latter stops it when it is read.
 106 </p><p>Signals generated by typing the corresponding character on some tty
 107 are sent to all processes that are in the foreground process group
 108 of 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.
 122 Many signals have as default action to kill the target.
 123 (Sometimes with an additional core dump, when such is
 124 allowed by rlimit.)
 125 The signals <code>SIGCHLD</code> and <code>SIGWINCH</code>
 126 are ignored by default.
 127 All except <code>SIGKILL</code> and <code>SIGSTOP</code> can be
 128 caught or ignored or blocked.
 129 For 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>,
 135 identified by its <i>process group ID</i>.
 136 (When the process is created, it becomes a member of the process group
 137 of its parent.)
 138 By convention, the process group ID of a process group
 139 equals the process ID of the first member of the process group,
 140 called the <i>process group leader</i>.
 141 A process finds the ID of its process group using the system call
 142 <code>getpgrp()</code>, or, equivalently, <code>getpgid(0)</code>.
 143 One 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),
 146 PID (process ID), PGID (process group ID) and SID (session ID)
 147 of processes. With a shell that does not know about job control,
 148 like <code>ash</code>, each of its children will be in the same session
 149 and have the same process group as the shell. With a shell that knows
 150 about 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
 156 form 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
 166 If <code>pgid == pid</code> or <code>pgid == 0</code> then this creates
 167 a new process group with process group leader <code>pid</code>.
 168 Otherwise, this puts <code>pid</code> into the already existing
 169 process group <code>pgid</code>.
 170 A zero <code>pid</code> refers to the current process.
 171 The 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,
 176 and 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.
 178 It 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();
 186 if (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
 198 This ensures that regardless of whether parent or child is scheduled
 199 first, 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
 214 or 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
 223 the <i>foreground process group</i> of that session.
 224 The tty input and tty signals (signals generated by ^C, ^Z, etc.)
 225 go to processes in this foreground process group.
 226 </p><p>A process can determine the foreground process group in its session
 227 using <code>tcgetpgrp(fd)</code>, where <code>fd</code> refers to its
 228 controlling tty. If there is none, this returns a random value
 229 larger than 1 that is not a process group ID.
 230 </p><p>A process can set the foreground process group in its session
 231 using <code>tcsetpgrp(fd,pgrp)</code>, where <code>fd</code> refers to its
 232 controlling tty, and <code>pgrp</code> is a process group in
 233 its session, and this session still is associated to the controlling
 234 tty of the calling process.
 235 </p><p>How does one get <code>fd</code>? By definition, <code>/dev/tty</code>
 236 refers to the controlling tty, entirely independent of redirects
 237 of standard input and output. (There is also the function
 238 <code>ctermid()</code> to get the name of the controlling terminal.
 239 On a POSIX standard system it will return <code>/dev/tty</code>.)
 240 Opening the name of the
 241 controlling 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
 246 process group are <i>background process groups</i>.
 247 Since the user at the keyboard is interacting with foreground
 248 processes, background processes should stay away from it.
 249 When a background process reads from the terminal it gets
 250 a SIGTTIN signal. Normally, that will stop it, the job control shell
 251 notices and tells the user, who can say <code>fg</code> to continue
 252 this background process as a foreground process, and then this
 253 process can read from the terminal. But if the background process
 254 ignores or blocks the SIGTTIN signal, or if its process group
 255 is orphaned (see below), then the read() returns an EIO error,
 256 and no signal is sent. (Indeed, the idea is to tell the process
 257 that reading from the terminal is not allowed right now.
 258 If 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
 260 a SIGTTOU signal. May: namely, when the flag that this must happen
 261 is set (it is off by default). One can set the flag by
 262 </p><blockquote>
 263 <pre>% stty tostop
 264 </pre>
 265 </blockquote>
 266
 267 and clear it again by
 268 <blockquote>
 269 <pre>% stty -tostop
 270 </pre>
 271 </blockquote>
 272
 273 and inspect it by
 274 <blockquote>
 275 <pre>% stty -a
 276 </pre>
 277 </blockquote>
 278
 279 Again, if TOSTOP is set but the background process ignores or blocks
 280 the SIGTTOU signal, or if its process group is orphaned (see below),
 281 then 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.
 287 It may terminate before the others, and then the process group is
 288 without leader.
 289 </p><p>A process group is called <i>orphaned</i> when <i>the
 290 parent of every member is either in the process group
 291 or outside the session</i>.
 292 In particular, the process group of the session leader
 293 is always orphaned.
 294 </p><p>If termination of a process causes a process group to become
 295 orphaned, and some member is stopped, then all are sent first SIGHUP
 296 and then SIGCONT.
 297 </p><p>The idea is that perhaps the parent of the process group leader
 298 is a job control shell. (In the same session but a different
 299 process group.) As long as this parent is alive, it can
 300 handle the stopping and starting of members in the process group.
 301 When it dies, there may be nobody to continue stopped processes.
 302 Therefore, these stopped processes are sent SIGHUP, so that they
 303 die 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
 305 orphaned, 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
 307 by termination of a process: either it was a parent and not itself
 308 in the process group, or it was the last element of the process group
 309 with a parent outside but in the same session.
 310 Furthermore, that a process group can become orphaned
 311 other than by termination of a process, namely when some
 312 member 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
 319 of its parent.)
 320 By convention, the session ID of a session
 321 equals the process ID of the first member of the session,
 322 called the <i>session leader</i>.
 323 A 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>,
 326 that then also is called the controlling tty of each of
 327 its member processes.
 328 A file descriptor for the controlling tty is obtained by
 329 opening <code>/dev/tty</code>. (And when that fails, there was no
 330 controlling tty.) Given a file descriptor for the controlling tty,
 331 one may obtain the SID using <code>tcgetsid(fd)</code>.
 332 </p><p>A session is often set up by a login process. The terminal
 333 on which one is logged in then becomes the controlling tty
 334 of the session. All processes that are descendants of the
 335 login 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
 345 This is allowed only when the current process is not a process group leader.
 346 In order to be sure of that we fork first:
 347 <blockquote>
 348 <pre>p = fork();
 349 if (p) exit(0);
 350 pid = setsid();
 351 </pre>
 352 </blockquote>
 353
 354 The result is that the current process (with process ID <code>pid</code>)
 355 becomes session leader of a new session with session ID <code>pid</code>.
 356 Moreover, it becomes process group leader of a new process group.
 357 Both session and process group contain only the single process <code>pid</code>.
 358 Furthermore, this process has no controlling tty.
 359 <p>The restriction that the current process must not be a process group leader
 360 is needed: otherwise its PID serves as PGID of some existing process group
 361 and 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,
 366 this is a great mystery.
 367 </p><p>The System V approach is that the first tty opened by the process
 368 becomes 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
 375 to get a controlling tty.
 376 <p>Linux tries to be compatible with both, as always, and this
 377 results in a very obscure complex of conditions. Roughly:
 378 </p><p>The <code>TIOCSCTTY</code> ioctl will give us a controlling tty,
 379 provided that (i) the current process is a session leader,
 380 and (ii) it does not yet have a controlling tty, and
 381 (iii) maybe the tty should not already control some other session;
 382 if it does it is an error if we aren't root, or we steal the tty
 383 if we are all-powerful.
 384 [vda: correction: third parameter controls this: if 1, we steal tty from
 385 any such session, if 0, we don't steal]
 386 </p><p>Opening some terminal will give us a controlling tty,
 387 provided 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
 398 from its controlling tty. Above we saw that <code>setsid()</code>
 399 will remove the controlling tty. Also the ioctl TIOCNOTTY does this.
 400 Moreover, in order not to get a controlling tty again as soon as it
 401 opens a tty, the process has to fork once more, to assure that it
 402 is 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,
 415 then a SIGHUP is sent to the session leader.
 416 Any 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
 419 is closed (for the last time), then a SIGHUP is sent to the foreground
 420 process group of the slave side.
 421 </p><p>When the session leader dies, a SIGHUP is sent to all processes
 422 in the foreground process group. Moreover, the terminal stops being
 423 the controlling terminal of this session (so that it can become
 424 the controlling terminal of another session).
 425 </p><p>Thus, if the terminal goes away and the session leader is
 426 a job control shell, then it can handle things for its descendants,
 427 e.g. by sending them again a SIGHUP.
 428 If on the other hand the session leader is an innocent process
 429 that does not catch SIGHUP, it will die, and all foreground processes
 430 get 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
 436 as the parent thread) are started using the <code>clone</code> system
 437 call using the <code>CLONE_THREAD</code> flag. Threads are distinguished
 438 by a <i>thread ID</i> (TID). An ordinary process has a single thread
 439 with TID equal to PID. The system call <code>gettid()</code> returns the
 440 TID. 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
 452 int thread(void *p) {
 453         printf("thread: %d %d\n", gettid(), getpid());
 454 }
 455
 456 main() {
 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 }
 467 EOF
 468 % cc -o gettid-demo gettid-demo.c
 469 % ./gettid-demo
 470 clone returns 21826
 471 parent: 21825 21825
 472 thread: 21826 21825
 473 %
 474 </pre>
 475 <p>
 476 </p><p>
 477 </p><hr>
 478
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