Man

Command Section
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 <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 thread until status
     information is available for a child process or a signal is received.  On
     return from a successful wait() call, the status area contains
     information about the process that reported a status change as defined
     below.

     The wait4() and wait6() system calls provide a more general interface for
     programs that need to wait for specific child processes, that need
     resource utilization statistics accumulated by child processes, or that
     require options.  The other wait functions are implemented using either
     wait4() or wait6().

     The wait6() function is the most general function in this family and its
     distinct features are:

     All of the desired process statuses to be waited on must be explicitly
     specified in options.  The wait(), waitpid(), wait3(), and wait4()
     functions all implicitly wait for exited and trapped processes, but the
     waitid() and wait6() functions require the corresponding WEXITED and
     WTRAPPED flags to be explicitly specified.  This allows waiting for
     processes which have experienced other status changes without having to
     also handle the exit status from terminated processes.

     The wait6() function accepts a wrusage argument which points to a
     structure defined as:

     struct __wrusage {
             struct rusage   wru_self;
             struct rusage   wru_children;
     };

     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.  If non-NULL, the structure is filled with the same data as
     for a SIGCHLD signal delivered when the process changed state.

     The set of child processes to be queried is specified by the arguments
     idtype and id.  The separate idtype and id arguments support many other
     types of identifiers in addition to process IDs and process group IDs.

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

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

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

              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 identifier types supported by this implementation of
     waitid() and wait6() are:

     P_UID     Wait for processes whose effective user ID is equal to (uid_t)
               id.

     P_GID     Wait for processes whose effective group ID is equal to (gid_t)
               id.

     P_SID     Wait for processes whose session ID is equal to id.  If the
               child process started its own session, its session ID will be
               the same as its process ID.  Otherwise the session ID of a
               child process will match the caller's session ID.

     P_JAILID  Waits for processes within a jail whose jail identifier is
               equal to id.

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

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

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

              If wpid is greater than zero, the call waits for the process
               with process ID wpid.

              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  Report the status of selected processes that have continued
                 from a job control stop by receiving a SIGCONT signal.

     WNOHANG     Do not block when there are no processes wishing to report
                 status.

     WUNTRACED   Report the status of selected processes which are stopped due
                 to a SIGTTIN, SIGTTOU, SIGTSTP, or SIGSTOP signal.

     WSTOPPED    An alias for WUNTRACED.

     WTRAPPED    Report the status of selected processes which are being
                 traced via ptrace(2) and have trapped or reached a
                 breakpoint.  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 options if status reports from trapped
                 processes are expected.

     WEXITED     Report the status of selected processes which have
                 terminated.  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 options if status reports from
                 terminated processes are expected.

     WNOWAIT     Keep 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 is non-NULL, separate summaries are returned for the resources
     used by the terminated process and the resources used by all its
     children.

     If infop is non-NULL, a siginfo_t structure is returned with the si_signo
     field set to SIGCHLD and the si_pid field set to the process ID of the
     process reporting status.  For the exited process, the si_status field of
     the siginfo_t structure contains the full 32 bit exit status passed to
     _exit(2); the status argument of other calls only returns 8 lowest bits
     of the exit status.

     When the WNOHANG option is specified and no processes wish to report
     status, waitid() sets the si_signo and si_pid fields in infop to zero.
     Checking these fields is the only way to know if a status change was
     reported.

     When the WNOHANG option is specified and no processes wish to report
     status, wait4() and wait6() return a process id of 0.

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

     The following macros may be used to test the current status of the
     process.  Exactly one of the following 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 and 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.  If an error is detected, a value of -1 is
     returned and errno is set to indicate the error.  If WNOHANG is specified
     and there are no stopped, continued or exited children, 0 is returned.
     The si_signo and si_pid fields of infop must be checked against zero to
     determine if a process reported status.

     wait() called with -1 to wait for any child process will ignore a child
     that is referenced by a process descriptor (see pdfork(2)).  Specific
     processes can still be waited on by specifying the process ID or
     descriptor (see pdwait(4)).

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 is an extension to the POSIX interface.

     The ability to use the WNOWAIT flag with waitpid() is an extension; POSIX
     only permits this flag with waitid().

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

FreeBSD 11.1-RELEASE-p4          June 1, 2016          FreeBSD 11.1-RELEASE-p4
Command Section