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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |VERSIONS |STANDARDS |HISTORY |NOTES |BUGS |EXAMPLES |SEE ALSO |COLOPHON | |
wait(2) System Calls Manualwait(2)wait, waitpid, waitid - wait for process to change state
Standard C library (libc,-lc)
#include <sys/wait.h>pid_t wait(int *_Nullablewstatus);pid_t waitpid(pid_tpid, int *_Nullablewstatus, intoptions);int waitid(idtype_tidtype, id_tid, siginfo_t *infop, intoptions); /* This is the glibc and POSIX interface; see VERSIONS for information on the raw system call. */ Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):waitid(): Since glibc 2.26: _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200809L glibc 2.25 and earlier: _XOPEN_SOURCE || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L || /* glibc <= 2.19: */ _BSD_SOURCE
All of these system calls are used to wait for state changes in a child of the calling process, and obtain information about the child whose state has changed. A state change is considered to be: the child terminated; the child was stopped by a signal; or the child was resumed by a signal. In the case of a terminated child, performing a wait allows the system to release the resources associated with the child; if a wait is not performed, then the terminated child remains in a "zombie" state (see NOTES below). If a child has already changed state, then these calls return immediately. Otherwise, they block until either a child changes state or a signal handler interrupts the call (assuming that system calls are not automatically restarted using theSA_RESTART flag ofsigaction(2)). In the remainder of this page, a child whose state has changed and which has not yet been waited upon by one of these system calls is termedwaitable.wait() and waitpid() Thewait() system call suspends execution of the calling thread until one of its children terminates. The callwait(&wstatus) is equivalent to: waitpid(-1, &wstatus, 0); Thewaitpid() system call suspends execution of the calling thread until a child specified bypid argument has changed state. By default,waitpid() waits only for terminated children, but this behavior is modifiable via theoptions argument, as described below. The value ofpid can be: <-1meaning wait for any child process whose process group ID is equal to the absolute value ofpid.-1meaning wait for any child process.0meaning wait for any child process whose process group ID is equal to that of the calling process at the time of the call towaitpid(). >0meaning wait for the child whose process ID is equal to the value ofpid. The value ofoptions is an OR of zero or more of the following constants:WNOHANG return immediately if no child has exited.WUNTRACED also return if a child has stopped (but not traced viaptrace(2)). Status fortraced children which have stopped is provided even if this option is not specified.WCONTINUED(since Linux 2.6.10) also return if a stopped child has been resumed by delivery ofSIGCONT. (For Linux-only options, see below.) Ifwstatus is not NULL,wait() andwaitpid() store status information in theint to which it points. This integer can be inspected with the following macros (which take the integer itself as an argument, not a pointer to it, as is done inwait() andwaitpid()!):WIFEXITED(wstatus) returns true if the child terminated normally, that is, by callingexit(3) or_exit(2), or by returning from main().WEXITSTATUS(wstatus) returns the exit status of the child. This consists of the least significant 8 bits of thestatus argument that the child specified in a call toexit(3) or_exit(2) or as the argument for a return statement in main(). This macro should be employed only ifWIFEXITEDreturned true.WIFSIGNALED(wstatus) returns true if the child process was terminated by a signal.WTERMSIG(wstatus) returns the number of the signal that caused the child process to terminate. This macro should be employed only ifWIFSIGNALEDreturned true.WCOREDUMP(wstatus) returns true if the child produced a core dump (seecore(5)). This macro should be employed only ifWIFSIGNALEDreturned true. This macro is not specified in POSIX.1-2001 and is not available on some UNIX implementations (e.g., AIX, SunOS). Therefore, enclose its use inside#ifdef WCOREDUMP ...#endif.WIFSTOPPED(wstatus) returns true if the child process was stopped by delivery of a signal; this is possible only if the call was done usingWUNTRACEDor when the child is being traced (seeptrace(2)).WSTOPSIG(wstatus) returns the number of the signal which caused the child to stop. This macro should be employed only ifWIFSTOPPED returned true.WIFCONTINUED(wstatus) (since Linux 2.6.10) returns true if the child process was resumed by delivery ofSIGCONT.waitid() Thewaitid() system call (available since Linux 2.6.9) provides more precise control over which child state changes to wait for. Theidtype andid arguments select the child(ren) to wait for, as follows:idtype ==P_PID Wait for the child whose process ID matchesid.idtype ==P_PIDFD(since Linux 5.4) Wait for the child referred to by the PID file descriptor specified inid. (Seepidfd_open(2) for further information on PID file descriptors.)idtype ==P_PGID Wait for any child whose process group ID matchesid. Since Linux 5.4, ifid is zero, then wait for any child that is in the same process group as the caller's process group at the time of the call.idtype ==P_ALL Wait for any child;id is ignored. The child state changes to wait for are specified by ORing one or more of the following flags inoptions:WEXITED Wait for children that have terminated.WSTOPPED Wait for children that have been stopped by delivery of a signal.WCONTINUED Wait for (previously stopped) children that have been resumed by delivery ofSIGCONT. The following flags may additionally be ORed inoptions:WNOHANG As forwaitpid().WNOWAIT Leave the child in a waitable state; a later wait call can be used to again retrieve the child status information. Upon successful return,waitid() fills in the following fields of thesiginfo_t structure pointed to byinfop:si_pid The process ID of the child.si_uid The real user ID of the child. (This field is not set on most other implementations.)si_signo Always set toSIGCHLD.si_status Either the exit status of the child, as given to_exit(2) (orexit(3)), or the signal that caused the child to terminate, stop, or continue. Thesi_code field can be used to determine how to interpret this field.si_code Set to one of:CLD_EXITED(child called_exit(2));CLD_KILLED(child killed by signal);CLD_DUMPED(child killed by signal, and dumped core);CLD_STOPPED(child stopped by signal);CLD_TRAPPED(traced child has trapped); orCLD_CONTINUED(child continued bySIGCONT). IfWNOHANGwas specified inoptions and there were no children in a waitable state, thenwaitid() returns 0 immediately and the state of thesiginfo_t structure pointed to byinfop depends on the implementation. To (portably) distinguish this case from that where a child was in a waitable state, zero out thesi_pid field before the call and check for a nonzero value in this field after the call returns. POSIX.1-2008 Technical Corrigendum 1 (2013) adds the requirement that whenWNOHANGis specified inoptions and there were no children in a waitable state, thenwaitid() should zero out thesi_pid andsi_signo fields of the structure. On Linux and other implementations that adhere to this requirement, it is not necessary to zero out thesi_pid field before callingwaitid(). However, not all implementations follow the POSIX.1 specification on this point.
wait(): on success, returns the process ID of the terminated child; on failure, -1 is returned.waitpid(): on success, returns the process ID of the child whose state has changed; ifWNOHANGwas specified and one or more child(ren) specified bypid exist, but have not yet changed state, then 0 is returned. On failure, -1 is returned.waitid(): returns 0 on success or ifWNOHANGwas specified and no child(ren) specified byid has yet changed state; on failure, -1 is returned. On failure, each of these calls setserrno to indicate the error.
EAGAINThe PID file descriptor specified inid is nonblocking and the process that it refers to has not terminated.ECHILD(forwait()) The calling process does not have any unwaited-for children.ECHILD(forwaitpid() orwaitid()) The process specified bypid (waitpid()) oridtype andid (waitid()) does not exist or is not a child of the calling process. (This can happen for one's own child if the action forSIGCHLDis set toSIG_IGN. See also theLinux Notes section about threads.)EINTR WNOHANGwas not set and an unblocked signal or aSIGCHLD was caught; seesignal(7).EINVALTheoptions argument was invalid.ESRCH(forwait() orwaitpid())pid is equal toINT_MIN.
C library/kernel differenceswait() is actually a library function that (in glibc) is implemented as a call towait4(2). On some architectures, there is nowaitpid() system call; instead, this interface is implemented via a C library wrapper function that callswait4(2). The rawwaitid() system call takes a fifth argument, of typestruct rusage *. If this argument is non-NULL, then it is used to return resource usage information about the child, in the same manner aswait4(2). Seegetrusage(2) for details.
POSIX.1-2008.
SVr4, 4.3BSD, POSIX.1-2001.
A child that terminates, but has not been waited for becomes a "zombie". The kernel maintains a minimal set of information about the zombie process (PID, termination status, resource usage information) in order to allow the parent to later perform a wait to obtain information about the child. As long as a zombie is not removed from the system via a wait, it will consume a slot in the kernel process table, and if this table fills, it will not be possible to create further processes. If a parent process terminates, then its "zombie" children (if any) are adopted byinit(1), (or by the nearest "subreaper" process as defined through the use of theprctl(2)PR_SET_CHILD_SUBREAPERoperation);init(1) automatically performs a wait to remove the zombies. POSIX.1-2001 specifies that if the disposition ofSIGCHLDis set toSIG_IGNor theSA_NOCLDWAITflag is set forSIGCHLD(seesigaction(2)), then children that terminate do not become zombies and a call towait() orwaitpid() will block until all children have terminated, and then fail witherrno set toECHILD. (The original POSIX standard left the behavior of settingSIGCHLDtoSIG_IGNunspecified. Note that even though the default disposition ofSIGCHLDis "ignore", explicitly setting the disposition toSIG_IGNresults in different treatment of zombie process children.) Linux 2.6 conforms to the POSIX requirements. However, Linux 2.4 (and earlier) does not: if await() orwaitpid() call is made whileSIGCHLDis being ignored, the call behaves just as thoughSIGCHLDwere not being ignored, that is, the call blocks until the next child terminates and then returns the process ID and status of that child.Linux notes In the Linux kernel, a kernel-scheduled thread is not a distinct construct from a process. Instead, a thread is simply a process that is created using the Linux-uniqueclone(2) system call; other routines such as the portablepthread_create(3) call are implemented usingclone(2). Before Linux 2.4, a thread was just a special case of a process, and as a consequence one thread could not wait on the children of another thread, even when the latter belongs to the same thread group. However, POSIX prescribes such functionality, and since Linux 2.4 a thread can, and by default will, wait on children of other threads in the same thread group. The following Linux-specificoptions are for use with children created usingclone(2); they can also, since Linux 4.7, be used withwaitid():__WCLONE Wait for "clone" children only. If omitted, then wait for "non-clone" children only. (A "clone" child is one which delivers no signal, or a signal other thanSIGCHLDto its parent upon termination.) This option is ignored if__WALL is also specified.__WALL(since Linux 2.4) Wait for all children, regardless of type ("clone" or "non- clone").__WNOTHREAD(since Linux 2.4) Do not wait for children of other threads in the same thread group. This was the default before Linux 2.4. Since Linux 4.7, the__WALLflag is automatically implied if the child is being ptraced.According to POSIX.1-2008, an application callingwaitid() must ensure thatinfop points to asiginfo_t structure (i.e., that it is a non-null pointer). On Linux, ifinfop is NULL,waitid() succeeds, and returns the process ID of the waited-for child. Applications should avoid relying on this inconsistent, nonstandard, and unnecessary feature.
The following program demonstrates the use offork(2) andwaitpid(). The program creates a child process. If no command- line argument is supplied to the program, then the child suspends its execution usingpause(2), to allow the user to send signals to the child. Otherwise, if a command-line argument is supplied, then the child exits immediately, using the integer supplied on the command line as the exit status. The parent process executes a loop that monitors the child usingwaitpid(), and uses the W*() macros described above to analyze the wait status value. The following shell session demonstrates the use of the program: $./a.out & Child PID is 32360 [1] 32359 $kill -STOP 32360 stopped by signal 19 $kill -CONT 32360 continued $kill -TERM 32360 killed by signal 15 [1]+ Done ./a.out $Program source #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <sys/types.h> #include <sys/wait.h> #include <unistd.h> int main(int argc, char *argv[]) { int wstatus; pid_t cpid, w; cpid = fork(); if (cpid == -1) { perror("fork"); exit(EXIT_FAILURE); } if (cpid == 0) { /* Code executed by child */ printf("Child PID is %jd\n", (intmax_t) getpid()); if (argc == 1) pause(); /* Wait for signals */ _exit(atoi(argv[1])); } else { /* Code executed by parent */ do { w = waitpid(cpid, &wstatus, WUNTRACED | WCONTINUED); if (w == -1) { perror("waitpid"); exit(EXIT_FAILURE); } if (WIFEXITED(wstatus)) { printf("exited, status=%d\n", WEXITSTATUS(wstatus)); } else if (WIFSIGNALED(wstatus)) { printf("killed by signal %d\n", WTERMSIG(wstatus)); } else if (WIFSTOPPED(wstatus)) { printf("stopped by signal %d\n", WSTOPSIG(wstatus)); } else if (WIFCONTINUED(wstatus)) { printf("continued\n"); } } while (!WIFEXITED(wstatus) && !WIFSIGNALED(wstatus)); exit(EXIT_SUCCESS); } }_exit(2),clone(2),fork(2),kill(2),ptrace(2),sigaction(2),signal(2),wait4(2),pthread_create(3),core(5),credentials(7),signal(7)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17wait(2)Pages that refer to this page:intro(1), waitpid(1), clone(2), _exit(2), fork(2), getrusage(2), io_uring_enter2(2), io_uring_enter(2), kill(2), pidfd_open(2), PR_SET_CHILD_SUBREAPER(2const), ptrace(2), reboot(2), seccomp(2), seccomp_unotify(2), sigaction(2), syscalls(2), times(2), vfork(2), wait4(2), clock(3), exit(3), ibv_fork_init(3), id_t(3type), io_uring_prep_waitid(3), __pmprocessexec(3), __pmprocesspipe(3), pmrecord(3), posix_spawn(3), pthread_exit(3), sd-event(3), sd_event_add_child(3), sd_event_add_inotify(3), system(3), proc_pid_stat(5), systemd.exec(5), credentials(7), man-pages(7), pthreads(7), signal(7), signal-safety(7), user_namespaces(7)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |