1WAIT(2)                    Linux Programmer's Manual                   WAIT(2)
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NAME

6       wait, waitpid, waitid - wait for process to change state
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SYNOPSIS

9       #include <sys/wait.h>
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11       pid_t wait(int *wstatus);
12       pid_t waitpid(pid_t pid, int *wstatus, int options);
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14       int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);
15                       /* This is the glibc and POSIX interface; see
16                          NOTES for information on the raw system call. */
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18   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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20       waitid():
21           Since glibc 2.26:
22               _XOPEN_SOURCE >= 500 || _POSIX_C_SOURCE >= 200809L
23           Glibc 2.25 and earlier:
24               _XOPEN_SOURCE
25                   || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
26                   || /* Glibc <= 2.19: */ _BSD_SOURCE
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DESCRIPTION

29       All of these system calls are used to wait for state changes in a child
30       of the calling process, and obtain information about  the  child  whose
31       state  has changed.  A state change is considered to be: the child ter‐
32       minated; the child was stopped by a signal; or the child was resumed by
33       a  signal.  In the case of a terminated child, performing a wait allows
34       the system to release the resources associated with  the  child;  if  a
35       wait  is not performed, then the terminated child remains in a "zombie"
36       state (see NOTES below).
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38       If a child has already changed state, then these calls  return  immedi‐
39       ately.   Otherwise,  they block until either a child changes state or a
40       signal handler interrupts the call (assuming that system calls are  not
41       automatically restarted using the SA_RESTART flag of sigaction(2)).  In
42       the remainder of this page, a child whose state has changed  and  which
43       has  not  yet  been  waited upon by one of these system calls is termed
44       waitable.
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46   wait() and waitpid()
47       The wait() system call suspends execution of the calling  thread  until
48       one  of its children terminates.  The call wait(&wstatus) is equivalent
49       to:
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51           waitpid(-1, &wstatus, 0);
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53       The waitpid() system call suspends execution of the calling thread  un‐
54       til  a  child specified by pid argument has changed state.  By default,
55       waitpid() waits only for terminated children, but this behavior is mod‐
56       ifiable via the options argument, as described below.
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58       The value of pid can be:
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60       < -1   meaning  wait  for  any  child process whose process group ID is
61              equal to the absolute value of pid.
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63       -1     meaning wait for any child process.
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65       0      meaning wait for any child process whose  process  group  ID  is
66              equal  to that of the calling process at the time of the call to
67              waitpid().
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69       > 0    meaning wait for the child whose process  ID  is  equal  to  the
70              value of pid.
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72       The  value  of  options  is an OR of zero or more of the following con‐