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WAIT(2)                   FreeBSD System Calls Manual                  WAIT(2)

NAME
     wait, waitid, waitpid, wait3, wait4, wait6 - wait for processes to change
     status

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #include <sys/wait.h>

     pid_t
     wait(int *status);

     pid_t
     waitpid(pid_t wpid, int *status, int options);

     #include <sys/signal.h>

     int
     waitid(idtype_t idtype, id_t id, siginfo_t *info, int options);

     #include <sys/time.h>
     #include <sys/resource.h>

     pid_t
     wait3(int *status, int options, struct rusage *rusage);

     pid_t
     wait4(pid_t wpid, int *status, int options, struct rusage *rusage);

     pid_t
     wait6(idtype_t idtype, id_t id, int *status, int options,
         struct __wrusage *wrusage, siginfo_t *infop);

DESCRIPTION
     The wait() function suspends execution of its calling process until
     status information is available for a terminated child process, or a
     signal is received.  On return from a successful wait() call, the status
     area contains termination information about the process that exited as
     defined below.  The wait() call is the same as wait4() with a wpid value
     of -1, with an options value of zero, and a rusage value of zero.

     The wait4() system call provides a more general interface for programs
     that need to wait for certain child processes, that need resource
     utilization statistics accumulated by child processes, or that require
     options.

     The broadest interface of all functions in this family is wait6() which
     is otherwise very much like wait4() but with a few very important
     distinctions.  To wait for exited processes, the option flag WEXITED need
     to be explicitly specified.  This allows for waiting for processes which
     have experienced other status changes without having to handle also the
     exit status from the terminated processes.  Instead of the traditional
     rusage argument, a pointer to a new structure

     struct __wrusage {
             struct rusage   wru_self;
             struct rusage   wru_children;
     };
     can be passed.  This allows the calling process to collect resource usage
     statistics from both its own child process as well as from its grand
     children.  When no resource usage statistics are needed this pointer can
     be NULL.  The last argument infop must be either NULL or a pointer to a
     siginfo_t structure.  When specified, the structure is filled the same as
     for SIGNCHLD signal, delivered at the process state change.
     The process, which state is queried, is specified by two arguments idtype
     and id.  The separate idtype and id arguments allows to support many
     other types of IDs  as well in addition to PID and PGID.

           +o   If idtype is P_PID, waitid() and wait6() wait for the child
               process with a process ID equal to (pid_t)id.

           +o   If idtype is P_PGID, waitid() and wait6() wait for the child
               process with a process group ID equal to (pid_t)id.

           +o   If idtype is P_ALL, waitid() and wait6() wait for any child
               process and the id is ignored.

           +o   If idtype is P_PID or P_PGID and the id is zero, waitid() and
               wait6() wait for any child process in the same process group as
               the caller.

     Non-standard specifiers for the process to wait for, supported by this
     implementation of waitid() and wait6(), are:

           +o   The idtype value P_UID waits for processes which effective UID
               is equal to (uid_t)id.

           +o   The idtype value P_GID waits for processes which effective GID
               is equal to (gid_t)id.

           +o   The idtype value P_SID waits for processes which session ID is
               equal to id.  In case the child process started its own new
               session, SID will be the same as its own PID.  Otherwise the
               SID of a child process will match the caller's SID.

           +o   The idtype value P_JAILID waits for processes within a jail
               which jail identifier is equal to id.

     For wait(), wait3(), and wait4() functions, the single wpid argument
     specifies the set of child processes for which to wait.

           +o   If wpid is -1, the call waits for any child process.

           +o   If wpid is 0, the call waits for any child process in the
               process group of the caller.

           +o   If wpid is greater than zero, the call waits for the process
               with process id wpid.

           +o   If wpid is less than -1, the call waits for any process whose
               process group id equals the absolute value of wpid.

     The status argument is defined below.

     The options argument contains the bitwise OR of any of the following
     options.

     WCONTINUED
             indicates that children of the current process that have
             continued from a job control stop, by receiving a SIGCONT signal,
             should also have their status reported.

     WNOHANG
             is used to indicate that the call should not block when there are
             no processes wishing to report status.

     WUNTRACED
             indicates that children of the current process which are stopped
             due to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal shall have
             their status reported.

     WSTOPPED
             is an alias for WUNTRACED.

     WTRAPPED
             allows waiting for processes which have trapped or reached a
             breakpoint.

     WEXITED
             indicates that the caller is wants to receive status reports from
             terminated processes.  This flag is implicitly set for the
             functions wait(), waitpid(), wait3(), and wait4().
             For the waitid() and wait6() functions, the flag has to be
             explicitly included in the options, if status reports from
             terminated processes are expected.

     WNOWAIT
             keeps the process whose status is returned in a waitable state.
             The process may be waited for again after this call completes.

     For the waitid() and wait6() functions, at least one of the options
     WEXITED, WUNTRACED, WSTOPPED, WTRAPPED, or WCONTINUED must be specified.
     Otherwise there will be no events for the call to report.  To avoid
     hanging indefinitely in such a case these functions return -1 with errno
     set to EINVAL.

     If rusage is non-NULL, a summary of the resources used by the terminated
     process and all its children is returned.

     If wrusage argument is non-NULL, a resource usage statistics from both
     its own child process as well as from its grand children is returned.

     If infop is non-NULL, it must point to a siginfo_t structure which is
     filled on return such that the si_signo field is always SIGCHLD and the
     field si_pid if be non-zero, if there is a status change to report.  If
     there are no status changes to report and WNOHANG is applied, both of
     these fields are returned zero.  When using the waitid() function with
     the WNOHANG option set, checking these fields is the only way to know
     whether there were any status changes to report, because the return value
     from waitid() is be zero as it is for any successful return from
     waitid().

     When the WNOHANG option is specified and no processes wish to report
     status, wait4() returns a process id of 0.

     The waitpid() function is identical to wait4() with an rusage value of
     zero.  The older wait3() call is the same as wait4() with a wpid value of
     -1.  The wait6() call, with the bits WEXITED and WTRAPPED set in the
     options and with infop set to NULL, is similar to wait4().

     The following macros may be used to test the manner of exit of the
     process.  One of the first four macros will evaluate to a non-zero (true)
     value:

     WIFCONTINUED(status)
             True if the process has not terminated, and has continued after a
             job control stop.  This macro can be true only if the wait call
             specified the WCONTINUED option).

     WIFEXITED(status)
             True if the process terminated normally by a call to _exit(2) or
             exit(3).

     WIFSIGNALED(status)
             True if the process terminated due to receipt of a signal.

     WIFSTOPPED(status)
             True if the process has not terminated, but has stopped and can
             be restarted.  This macro can be true only if the wait call
             specified the WUNTRACED option or if the child process is being
             traced (see ptrace(2)).

     Depending on the values of those macros, the following macros produce the
     remaining status information about the child process:

     WEXITSTATUS(status)
             If WIFEXITED(status) is true, evaluates to the low-order 8 bits
             of the argument passed to _exit(2) or exit(3) by the child.

     WTERMSIG(status)
             If WIFSIGNALED(status) is true, evaluates to the number of the
             signal that caused the termination of the process.

     WCOREDUMP(status)
             If WIFSIGNALED(status) is true, evaluates as true if the
             termination of the process was accompanied by the creation of a
             core file containing an image of the process when the signal was
             received.

     WSTOPSIG(status)
             If WIFSTOPPED(status) is true, evaluates to the number of the
             signal that caused the process to stop.

NOTES
     See sigaction(2) for a list of termination signals.  A status of 0
     indicates normal termination.

     If a parent process terminates without waiting for all of its child
     processes to terminate, the remaining child processes are assigned the
     parent process 1 ID (the init process ID).

     If a signal is caught while any of the wait() calls are pending, the call
     may be interrupted or restarted when the signal-catching routine returns,
     depending on the options in effect for the signal; see discussion of
     SA_RESTART in sigaction(2).

     The implementation queues one SIGCHLD signal for each child process whose
     status has changed, if wait() returns because the status of a child
     process is available, the pending SIGCHLD signal associated with the
     process ID of the child process will be discarded.  Any other pending
     SIGCHLD signals remain pending.

     If SIGCHLD is blocked, wait() returns because the status of a child
     process is available, the pending SIGCHLD signal will be cleared unless
     another status of the child process is available.

RETURN VALUES
     If wait() returns due to a stopped, continued, or terminated child
     process, the process ID of the child is returned to the calling process.
     Otherwise, a value of -1 is returned and errno is set to indicate the
     error.

     If wait6(), wait4(), wait3(), or waitpid() returns due to a stopped,
     continued, or terminated child process, the process ID of the child is
     returned to the calling process.  If there are no children not previously
     awaited, -1 is returned with errno set to ECHILD.  Otherwise, if WNOHANG
     is specified and there are no stopped, continued or exited children, 0 is
     returned.  If an error is detected or a caught signal aborts the call, a
     value of -1 is returned and errno is set to indicate the error.

     If waitid() returns because one or more processes have a state change to
     report, 0 is returned.  To indicate an error, -1 will be returned and
     errno set to an appropriate value.  If WNOHANG was used, 0 can be
     returned indicating no error, but no processes may have changed state
     either, if si_signo and/or si_pid are zero.

ERRORS
     The wait() function will fail and return immediately if:

     [ECHILD]           The calling process has no existing unwaited-for child
                        processes.

     [ECHILD]           No status from the terminated child process is
                        available because the calling process has asked the
                        system to discard such status by ignoring the signal
                        SIGCHLD or setting the flag SA_NOCLDWAIT for that
                        signal.

     [EFAULT]           The status or rusage argument points to an illegal
                        address.  (May not be detected before exit of a child
                        process.)

     [EINTR]            The call was interrupted by a caught signal, or the
                        signal did not have the SA_RESTART flag set.

     [EINVAL]           An invalid value was specified for options, or idtype
                        and id do not specify a valid set of processes.

SEE ALSO
     _exit(2), ptrace(2), sigaction(2), exit(3), siginfo(3)

STANDARDS
     The wait(), waitpid(), and waitid() functions are defined by POSIX;
     wait6(), wait4(), and wait3() are not specified by POSIX.  The
     WCOREDUMP() macro and the ability to restart a pending wait() call are
     extensions to the POSIX interface.

HISTORY
     The wait() function appeared in Version 6 AT&T UNIX.

FreeBSD 11.0-PRERELEASE        November 10, 2012       FreeBSD 11.0-PRERELEASE

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | NOTES | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY

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