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 re-assigned to
the reaper of the exiting process as the parent, see procctl(2)
PROC_REAP_ACQUIRE. If no specific reaper was assigned, the process with
ID 1, the init process, becomes the parent of the orphaned children by
default.
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.
The wait() family of functions will not return a child process created
with pdfork(2) unless specifically directed to do so by specifying its
process ID.
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), procctl(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 1 AT&T UNIX.
FreeBSD 13.1-RELEASE-p6 August 11, 2019 FreeBSD 13.1-RELEASE-p6
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