man2/wait.2

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   3 .\" Copyright (c) 1993 by Thomas Koenig <ig25@rz.uni-karlsruhe.de>
   4 .\" and Copyright (c) 2004 by Michael Kerrisk <mtk.manpages@gmail.com>
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  27 .\" Modified Sat Jul 24 13:30:06 1993 by Rik Faith <faith@cs.unc.edu>
  28 .\" Modified Sun Aug 21 17:42:42 1994 by Rik Faith <faith@cs.unc.edu>
  29 .\"          (Thanks to Koen Holtman <koen@win.tue.nl>)
  30 .\" Modified Wed May 17 15:54:12 1995 by Rik Faith <faith@cs.unc.edu>
  31 .\"           To remove *'s from status in macros (Thanks to Michael Shields).
  32 .\" Modified as suggested by Nick Duffek <nsd@bbc.com>, aeb, 960426
  33 .\" Modified Mon Jun 23 14:09:52 1997 by aeb - add EINTR.
  34 .\" Modified Thu Nov 26 02:12:45 1998 by aeb - add SIGCHLD stuff.
  35 .\" Modified Mon Jul 24 21:37:38 2000 by David A. Wheeler
  36 .\"          <dwheeler@dwheeler.com> - noted thread issues.
  37 .\" Modified 26 Jun 01 by Michael Kerrisk
  38 .\"          Added __WCLONE, __WALL, and __WNOTHREAD descriptions
  39 .\" Modified 2001-09-25, aeb
  40 .\" Modified 26 Jun 01 by Michael Kerrisk, <mtk.manpages@gmail.com>
  41 .\"     Updated notes on setting disposition of SIGCHLD to SIG_IGN
  42 .\" 2004-11-11, mtk
  43 .\"     Added waitid(2); added WCONTINUED and WIFCONTINUED()
  44 .\"     Added text on SA_NOCLDSTOP
  45 .\"     Updated discussion of SA_NOCLDWAIT to reflect 2.6 behavior
  46 .\"     Much other text rewritten
  47 .\" 2005-05-10, mtk, __W* flags can't be used with waitid()
  48 .\"
  49 .TH WAIT 2  2007-07-26 "Linux" "Linux Programmer's Manual"
  50 .SH NAME
  51 wait, waitpid, waitid \- wait for process to change state
  52 .SH SYNOPSIS
  53 .B #include <sys/types.h>
  54 .br
  55 .B #include <sys/wait.h>
  56 .sp
  57 .BI "pid_t wait(int *" "status" );
  58 .br
  59 .BI "pid_t waitpid(pid_t " pid ", int *" status ", int " options );
  60 .br
  61 .BI "int waitid(idtype_t " idtype ", id_t " id \
  62 ", siginfo_t *" infop ", int " options );
  63 .sp
  64 .in -4n
  65 Feature Test Macro Requirements for glibc (see
  66 .BR feature_test_macros (7)):
  67 .in
  68 .sp
  69 .BR waitid ():
  70 _SVID_SOURCE || _XOPEN_SOURCE
  71 .SH DESCRIPTION
  72 All of these system calls are used to wait for state changes
  73 in a child of the calling process, and obtain information
  74 about the child whose state has changed.
  75 A state change is considered to be: the child terminated;
  76 the child was stopped by a signal; or the child was resumed by a signal.
  77 In the case of a terminated child, performing a wait allows
  78 the system to release the resources associated with the child;
  79 if a wait is not performed, then terminated the child remains in
  80 a "zombie" state (see NOTES below).
  81
  82 If a child has already changed state, then these calls return immediately.
  83 Otherwise they block until either a child changes state or
  84 a signal handler interrupts the call (assuming that system calls
  85 are not automatically restarted using the
  86 .B SA_RESTART
  87 flag of
  88 .BR sigaction (2)).
  89 In the remainder of this page, a child whose state has changed
  90 and which has not yet been waited upon by one of these system
  91 calls is termed
  92 .IR waitable .
  93 .SS "wait() and waitpid()"
  94 The
  95 .BR wait ()
  96 system call suspends execution of the current process until one of its
  97 children terminates.
  98 The call
  99 .I wait(&status)
 100 is equivalent to:
 101 .nf
 102
 103     waitpid(\-1, &status, 0);
 104 .fi
 105
 106 The
 107 .BR waitpid ()
 108 system call suspends execution of the current process until a
 109 child specified by
 110 .I pid
 111 argument has changed state.
 112 By default,
 113 .BR waitpid ()
 114 waits only for terminated children, but this behavior is modifiable
 115 via the
 116 .I options
 117 argument, as described below.
 118
 119 The value of
 120 .I pid
 121 can be:
 122 .IP "< \-1"
 123 meaning wait for any child process whose process group ID is
 124 equal to the absolute value of
 125 .IR pid .
 126 .IP \-1
 127 meaning wait for any child process.
 128 .IP 0
 129 meaning wait for any child process whose process group ID is
 130 equal to that of the calling process.
 131 .IP "> 0"
 132 meaning wait for the child whose process ID is equal to the
 133 value of
 134 .IR pid .
 135 .PP
 136 The value of
 137 .I options
 138 is an OR of zero or more of the following constants:
 139 .TP
 140 .B WNOHANG
 141 return immediately if no child has exited.
 142 .TP
 143 .B WUNTRACED
 144 also return if a child has stopped
 145 (but not traced via
 146 .BR ptrace (2)).
 147 Status for
 148 .I traced
 149 children which have stopped is provided
 150 even if this option is not specified.
 151 .TP
 152 .B WCONTINUED
 153 (Since Linux 2.6.10)
 154 also return if a stopped child has been resumed by delivery of
 155 .BR SIGCONT .
 156 .PP
 157 (For Linux-only options, see below.)
 158 .PP
 159 The
 160 .B WUNTRACED
 161 and
 162 .B WCONTINUED
 163 options are only effective if the
 164 .B SA_NOCLDSTOP
 165 flag has not been set for the
 166 .B SIGCHLD
 167 signal (see
 168 .BR sigaction (2)).
 169 .PP
 170 If
 171 .I status
 172 is not NULL,
 173 .BR wait ()
 174 and
 175 .BR waitpid ()
 176 store status information in the \fIint\fP to which it points.
 177 This integer can be inspected with the following macros (which
 178 take the integer itself as an argument, not a pointer to it,
 179 as is done in
 180 .BR wait ()
 181 and
 182 .BR waitpid ()!):
 183 .TP
 184 .BI WIFEXITED( status )
 185 returns true if the child terminated normally, that is,
 186 by calling
 187 .BR exit (3)
 188 or
 189 .BR _exit (2),
 190 or by returning from main().
 191 .TP
 192 .BI WEXITSTATUS( status )
 193 returns the exit status of the child.
 194 This consists of the least significant 8 bits of the
 195 .I status
 196 argument that the child specified in a call to
 197 .BR exit (3)
 198 or
 199 .BR _exit (2)
 200 or as the argument for a return statement in main().
 201 This macro should only be employed if
 202 .B WIFEXITED
 203 returned true.
 204 .TP
 205 .BI WIFSIGNALED( status )
 206 returns true if the child process was terminated by a signal.
 207 .TP
 208 .BI WTERMSIG( status )
 209 returns the number of the signal that caused the child process to
 210 terminate.
 211 This macro should only be employed if
 212 .B WIFSIGNALED
 213 returned true.
 214 .TP
 215 .BI WCOREDUMP( status )
 216 returns true if the child produced a core dump.
 217 This macro should only be employed if
 218 .B WIFSIGNALED
 219 returned true.
 220 This macro is not specified in POSIX.1-2001 and is not available on
 221 some Unix implementations (e.g., AIX, SunOS).
 222 Only use this enclosed in #ifdef WCOREDUMP ... #endif.
 223 .TP
 224 .BI WIFSTOPPED( status )
 225 returns true if the child process was stopped by delivery of a signal;
 226 this is only possible if the call was done using
 227 .BR WUNTRACED
 228 or when the child is being traced (see
 229 .BR ptrace (2)).
 230 .TP
 231 .BI WSTOPSIG( status )
 232 returns the number of the signal which caused the child to stop.
 233 This macro should only be employed if
 234 .B WIFSTOPPED
 235 returned true.
 236 .TP
 237 .BI WIFCONTINUED( status )
 238 (Since Linux 2.6.10)
 239 returns true if the child process was resumed by delivery of
 240 .BR SIGCONT .
 241 .SS "waitid()"
 242 The
 243 .BR waitid ()
 244 system call (available since Linux 2.6.9) provides more precise
 245 control over which child state changes to wait for.
 246
 247 The
 248 .I idtype
 249 and
 250 .I id
 251 arguments select the child(ren) to wait for, as follows:
 252 .IP "\fIidtype\fP == \fBP_PID\fP"
 253 Wait for the child whose process ID matches
 254 .IR id .
 255 .IP "\fIidtype\fP == \fBP_PGID\fP"
 256 Wait for any child whose process group ID matches
 257 .IR id .
 258 .IP "\fIidtype\fP == \fBP_ALL\fP"
 259 Wait for any child;
 260 .I id
 261 is ignored.
 262 .PP
 263 The child state changes to wait for are specified by ORing
 264 one or more of the following flags in
 265 .IR options :
 266 .TP
 267 .B WEXITED
 268 Wait for children that have terminated.
 269 .TP
 270 .B WSTOPPED
 271 Wait for children that have been stopped by delivery of a signal.
 272 .TP
 273 .B WCONTINUED
 274 Wait for (previously stopped) children that have been
 275 resumed by delivery of
 276 .BR SIGCONT .
 277 .PP
 278 The following flags may additionally be ORed in
 279 .IR options :
 280 .TP
 281 .B WNOHANG
 282 As for
 283 .BR waitpid ().
 284 .TP
 285 .B WNOWAIT
 286 Leave the child in a waitable state; a later wait call
 287 can be used to again retrieve the child status information.
 288 .PP
 289 Upon successful return,
 290 .BR waitid ()
 291 fills in the following fields of the
 292 .I siginfo_t
 293 structure pointed to by
 294 .IR infop :
 295 .IP \fIsi_pid\fP
 296 The process ID of the child.
 297 .IP \fIsi_uid\fP
 298 The real user ID of the child.
 299 (This field is not set on most other implementations.)
 300 .IP \fIsi_signo\fP
 301 Always set to
 302 .BR SIGCHLD .
 303 .IP \fIsi_status\fP
 304 Either the exit status of the child, as given to
 305 .BR _exit (2)
 306 (or
 307 .BR exit (3)),
 308 or the signal that caused the child to terminate, stop, or continue.
 309 The
 310 .I si_code
 311 field can be used to determine how to interpret this field.
 312 .IP \fIsi_code\fP
 313 Set to one of:
 314 .B CLD_EXITED
 315 (child called
 316 .BR _exit (2));
 317 .B CLD_KILLED
 318 (child killed by signal);
 319 .B CLD_STOPPED
 320 (child stopped by signal); or
 321 .B CLD_CONTINUED
 322 (child continued by
 323 .BR SIGCONT ).
 324 .PP
 325 If
 326 .B WNOHANG
 327 was specified in
 328 .I options
 329 and there were no children in a waitable state, then
 330 .BR waitid ()
 331 returns 0 immediately and
 332 the state of the
 333 .I siginfo_t
 334 structure pointed to by
 335 .I infop
 336 is unspecified.
 337 .\" POSIX.1-2001 leaves this possibility unspecified; most
 338 .\" implementations (including Linux) zero out the structure
 339 .\" in this case, but at at least one implementation (AIX 5.1)
 340 .\" does not -- MTK Nov 04
 341 To distinguish this case from that where a child was in a
 342 waitable state, zero out the
 343 .I si_pid
 344 field before the call and check for a non-zero value in this field
 345 after the call returns.
 346 .SH "RETURN VALUE"
 347 .BR wait ():
 348 on success, returns the process ID of the terminated child;
 349 on error, \-1 is returned.
 350
 351 .BR waitpid ():
 352 on success, returns the process ID of the child whose state has changed;
 353 on error, \-1 is returned;
 354 if
 355 .B WNOHANG
 356 was specified and no child(ren) specified by
 357 .I pid
 358 has yet changed state, then 0 is returned.
 359
 360 .BR waitid ():
 361 returns 0 on success or
 362 if
 363 .B WNOHANG
 364 was specified and no child(ren) specified by
 365 .I id
 366 has yet changed state;
 367 on error, \-1 is returned.
 368
 369 Each of these calls sets
 370 .I errno
 371 to an appropriate value in the case of an error.
 372 .SH ERRORS
 373 .TP
 374 .BR ECHILD
 375 (for
 376 .BR wait ())
 377 The calling process does not have any unwaited-for children.
 378 .TP
 379 .BR ECHILD
 380 (for
 381 .BR waitpid ()
 382 or
 383 .BR waitid ())
 384 The process specified by
 385 .I pid
 386 .RB ( waitpid ())
 387 or
 388 .I idtype
 389 and
 390 .I id
 391 .RB ( waitid ())
 392 does not exist or is not a child of the calling process.
 393 (This can happen for one's own child if the action for
 394 .B SIGCHLD
 395 is set to
 396 .BR SIG_IGN .
 397 See also the \fILinux Notes\fP section about threads.)
 398 .TP
 399 .B EINTR
 400 .B WNOHANG
 401 was not set and an unblocked signal or a
 402 .B SIGCHLD
 403 was caught.
 404 .TP
 405 .B EINVAL
 406 The
 407 .I options
 408 argument was invalid.
 409 .SH "CONFORMING TO"
 410 SVr4, 4.3BSD, POSIX.1-2001.
 411 .SH NOTES
 412 A child that terminates, but has not been waited for becomes a "zombie".
 413 The kernel maintains a minimal set of information about the zombie
 414 process (PID, termination status, resource usage information)
 415 in order to allow the parent to later perform a wait to obtain
 416 information about the child.
 417 As long as a zombie is not removed from the system via a wait,
 418 it will consume a slot in the kernel process table, and if
 419 this table fills, it will not be possible to create further processes.
 420 If a parent process terminates, then its "zombie" children (if any)
 421 are adopted by
 422 .BR init (8),
 423 which automatically performs a wait to remove the zombies.
 424
 425 POSIX.1-2001 specifies that if the disposition of
 426 .B SIGCHLD
 427 is set to
 428 .B SIG_IGN
 429 or the
 430 .B SA_NOCLDWAIT
 431 flag is set for
 432 .BR SIGCHLD
 433 (see
 434 .BR sigaction (2)),
 435 then children that terminate do not become zombies and a call to
 436 .BR wait ()
 437 or
 438 .BR waitpid ()
 439 will block until all children have terminated, and then fail with
 440 .I errno
 441 set to
 442 .BR ECHILD .
 443 (The original POSIX standard left the behavior of setting
 444 .B SIGCHLD
 445 to
 446 .B SIG_IGN
 447 unspecified.)
 448 Linux 2.6 conforms to this specification.
 449 However, Linux 2.4 (and earlier) does not:
 450 if a
 451 .BR wait ()
 452 or
 453 .BR waitpid ()
 454 call is made while
 455 .B SIGCHLD
 456 is being ignored, the call behaves just as though
 457 .B SIGCHLD
 458 were not being ignored, that is, the call blocks until the next child
 459 terminates and then returns the process ID and status of that child.
 460 .SS Linux Notes
 461 In the Linux kernel, a kernel-scheduled thread is not a distinct
 462 construct from a process.
 463 Instead, a thread is simply a process
 464 that is created using the Linux-unique
 465 .BR clone (2)
 466 system call; other routines such as the portable
 467 .BR pthread_create (3)
 468 call are implemented using
 469 .BR clone (2).
 470 Before Linux 2.4, a thread was just a special case of a process,
 471 and as a consequence one thread could not wait on the children
 472 of another thread, even when the latter belongs to the same thread group.
 473 However, POSIX prescribes such functionality, and since Linux 2.4
 474 a thread can, and by default will, wait on children of other threads
 475 in the same thread group.
 476 .LP
 477 The following Linux specific
 478 .I options
 479 are for use with children created using
 480 .BR clone (2);
 481 they cannot be used with
 482 .BR waitid ():
 483 .TP
 484 .B __WCLONE
 485 .\" since 0.99pl10
 486 Wait for "clone" children only.
 487 If omitted then wait for "non-clone" children only.
 488 (A "clone" child is one which delivers no signal, or a signal other than
 489 .B SIGCHLD
 490 to its parent upon termination.)
 491 This option is ignored if
 492 .B __WALL
 493 is also specified.
 494 .TP
 495 .B __WALL
 496 .\" since patch-2.3.48
 497 (Since Linux 2.4) Wait for all children, regardless of
 498 type ("clone" or "non-clone").
 499 .TP
 500 .B __WNOTHREAD
 501 .\" since patch-2.4.0-test8
 502 (Since Linux 2.4) Do not wait for children of other threads in
 503 the same thread group.
 504 This was the default before Linux 2.4.
 505 .SH EXAMPLE
 506 .\" fork.2 refers to this example program.
 507 The following program demonstrates the use of
 508 .BR fork (2)
 509 and
 510 .BR waitpid (2).
 511 The program creates a child process.
 512 If no command-line argument is supplied to the program,
 513 then the child suspends its execution using
 514 .BR pause (2),
 515 to allow the user to send signals to the child.
 516 Otherwise, if a command-line argument is supplied,
 517 then the child exits immediately,
 518 using the integer supplied on the command line as the exit status.
 519 The parent process executes a loop that monitors the child using
 520 .BR waitpid (2),
 521 and uses the W*() macros described above to analyze the wait status value.
 522
 523 The following shell session demonstrates the use of the program:
 524 .nf
 525
 526 $ ./a.out &
 527 Child PID is 32360
 528 [1] 32359
 529 $ kill \-STOP 32360
 530 stopped by signal 19
 531 $ kill \-CONT 32360
 532 continued
 533 $ kill \-TERM 32360
 534 killed by signal 15
 535 [1]+  Done                    ./a.out
 536 $
 537
 538 #include <sys/wait.h>
 539 #include <stdlib.h>
 540 #include <unistd.h>
 541 #include <stdio.h>
 542
 543 int
 544 main(int argc, char *argv[])
 545 {
 546     pid_t cpid, w;
 547     int status;
 548
 549     cpid = fork();
 550     if (cpid == \-1) {
 551         perror("fork");
 552         exit(EXIT_FAILURE);
 553     }
 554
 555     if (cpid == 0) {            /* Code executed by child */
 556         printf("Child PID is %ld\\n", (long) getpid());
 557         if (argc == 1)
 558             pause();                    /* Wait for signals */
 559         _exit(atoi(argv[1]));
 560
 561     } else {                    /* Code executed by parent */
 562         do {
 563             w = waitpid(cpid, &status, WUNTRACED | WCONTINUED);
 564             if (w == \-1) {
 565                 perror("waitpid");
 566                 exit(EXIT_FAILURE);
 567             }
 568
 569             if (WIFEXITED(status)) {
 570                 printf("exited, status=%d\\n", WEXITSTATUS(status));
 571             } else if (WIFSIGNALED(status)) {
 572                 printf("killed by signal %d\\n", WTERMSIG(status));
 573             } else if (WIFSTOPPED(status)) {
 574                 printf("stopped by signal %d\\n", WSTOPSIG(status));
 575             } else if (WIFCONTINUED(status)) {
 576                 printf("continued\\n");
 577             }
 578         } while (!WIFEXITED(status) && !WIFSIGNALED(status));
 579         exit(EXIT_SUCCESS);
 580     }
 581 }
 582 .fi
 583 .SH "SEE ALSO"
 584 .BR _exit (2),
 585 .BR clone (2),
 586 .BR fork (2),
 587 .BR kill (2),
 588 .BR ptrace (2),
 589 .BR sigaction (2),
 590 .BR signal (2),
 591 .BR wait4 (2),
 592 .BR pthread_create (3),
 593 .BR credentials (7),
 594 .BR signal (7)