wait (2)

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(The comments found at the beginning of the groff file "man2/wait.2freebsd".)

NAME

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

LIBRARY

Lb libc

SYNOPSIS

In sys/types.h In sys/wait.h Ft pid_t Fn wait int *status Ft pid_t Fn waitpid pid_t wpid int *status int options In signal.h Ft int Fn waitid idtype_t idtype id_t id siginfo_t *info int options In sys/time.h In sys/resource.h Ft pid_t Fn wait3 int *status int options struct rusage *rusage Ft pid_t Fn wait4 pid_t wpid int *status int options struct rusage *rusage Ft pid_t Fn wait6 idtype_t idtype id_t id int *status int options struct __wrusage *wrusage siginfo_t *infop

DESCRIPTION

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

The Fn 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 Fn wait6 which is otherwise very much like Fn 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 Fa infop must be either NULL or a pointer to a Fa 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 Fa idtype and Fa id . The separate Fa idtype and Fa id arguments allows to support many other types of IDs as well in addition to PID and PGID.

• If Fa idtype is P_PID Fn waitid and Fn wait6 wait for the child process with a process ID equal to (pid_t)id
• If Fa idtype is P_PGID Fn waitid and Fn wait6 wait for the child process with a process group ID equal to (pid_t)id
• If Fa idtype is P_ALL Fn waitid and Fn wait6 wait for any child process and the id is ignored.
• If Fa idtype is P_PID or P_PGID and the id is zero, Fn waitid and Fn 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 Fn waitid and Fn wait6 , are:

• The Fa idtype value P_UID waits for processes which effective UID is equal to (uid_t)id
• The Fa idtype value P_GID waits for processes which effective GID is equal to (gid_t)id
• The Fa 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.
• The Fa idtype value P_JAILID waits for processes within a jail which jail identifier is equal to id

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

• If Fa wpid is -1, the call waits for any child process.
• If Fa wpid is 0, the call waits for any child process in the process group of the caller.
• If Fa wpid is greater than zero, the call waits for the process with process id Fa wpid .
• If Fa wpid is less than -1, the call waits for any process whose process group id equals the absolute value of Fa wpid .

The Fa status argument is defined below.

The Fa 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 Fn wait , Fn waitpid , Fn wait3 , and Fn wait4 .
For the Fn waitid and Fn wait6 functions, the flag has to be explicitly included in the Fa 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 Fn waitid and Fn 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 Fa rusage is non-NULL, a summary of the resources used by the terminated process and all its children is returned.

If Fa 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 Fa 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 Fn 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 Fn waitid is be zero as it is for any successful return from Fn waitid .

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

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

Fn 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).

Fn WIFEXITED status

True if the process terminated normally by a call to _exit2 or exit(3).

Fn WIFSIGNALED status

True if the process terminated due to receipt of a signal.

Fn 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:

Fn WEXITSTATUS status

If Fn WIFEXITED status is true, evaluates to the low-order 8 bits of the argument passed to _exit2 or exit(3) by the child.

Fn WTERMSIG status

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

Fn WCOREDUMP status

If Fn 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.

Fn WSTOPSIG status

If Fn 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 Fn 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 Fn 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, Fn 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 Fn 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 Fn wait6 , Fn wait4 , Fn wait3 , or Fn 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 Er 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 Fn 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 Fn wait function will fail and return immediately if:

Bq Er ECHILD

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

Bq Er 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.

Bq Er EFAULT

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

Bq Er EINTR

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

Bq Er EINVAL

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

SEE ALSO

_exit2, ptrace(2), sigaction(2), exit(3), siginfo(3)

STANDARDS

The Fn wait , Fn waitpid , and Fn waitid functions are defined by POSIX; Fn wait6 , Fn wait4 , and Fn wait3 are not specified by POSIX. The Fn WCOREDUMP macro is an extension to the POSIX interface.

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

POSIX requires Fn waitid to return the full 32 bits passed to _exit2; this implementation only returns 8 bits like in the other calls.

HISTORY

The Fn wait function appeared in AT&T System v6 .