Date: Tue, 24 Sep 2013 16:31:44 -0400 From: John Baldwin <jhb@freebsd.org> To: Warren Block <wblock@wonkity.com> Cc: Konstantin Belousov <kostikbel@gmail.com>, doc@freebsd.org Subject: Re: Tweaks to the wait(2) manpage Message-ID: <201309241631.44920.jhb@freebsd.org> In-Reply-To: <alpine.BSF.2.00.1309140656570.36819@wonkity.com> References: <201309121643.46422.jhb@freebsd.org> <20130914081144.GZ41229@kib.kiev.ua> <alpine.BSF.2.00.1309140656570.36819@wonkity.com>
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On Saturday, September 14, 2013 9:11:48 am Warren Block wrote:
> On Sat, 14 Sep 2013, Konstantin Belousov wrote:
>
> > On Thu, Sep 12, 2013 at 04:43:46PM -0400, John Baldwin wrote:
> >> I have some tweaks to the wait(2) manpage, in particular to the sections on
> >> wait6() and idtypes. I did also change two other places to use uppercase for
> >> ID since that seems to be what we do in other pages. The alternate rendered
> >> text is below followed by the diff. One structural change I chose to make was
> >> using a tagged list for the non-standard idtypes. Our manpages in general
> >> prefer tagged lists to bullet lists for enumerations. I left the list of
> >> standard types as-is as it includes a fourth bullet point that would not have
> >> an associated tag (though one could perhaps move that into the paragraph
> >> introducing the list of standard types if a tagged list was desired). I kept
> >> reading this page as I was writing this e-mail and changed more bits to
> >> attempt to be more consisent with existing paragraphs, etc.:
> >>
> >> The broadest interface of all functions in this family is wait6() which
> >> is otherwise very much like wait4() but with a few very important dis-
> >> tinctions. To wait for exited processes the option flag WEXITED must be
> > I did not liked the introductory sentence above, but was not able to
> > formulate the idea better. Specifically, I do not like the narrative tone,
> > and think that 'broadest' and 'distinction' should be expressed better.
>
> Agreed about "broadest", it's hard to tell what that means. How about:
>
> wait6() is the most general function in this family, differing from
> wait4() in these important ways:
>
> To wait for exited processes, the WEXITED option flag must 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.
>
> Instead of the traditional rusage argument, the wrusage argument
> points to a structure defined as:
I've fixed the zero -> NULL case Konstantin pointed out (and another one).
I also made a stab at fixing this. I actually changed the prior paragraph
to state that both wait4 and wait6 are the general functions for this API
and then went from there. I also reworded the descriptions of the flags
in options and clarified that WTRAPPED is implicit the same as WEXITED in
a few places.
At this point it's probably easiest for me to just put the entire page here:
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 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 informa-
tion about the process that reported a status change 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 NULL.
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 dif-
fers from wait4() in these important ways:
All of the desired process statuses to be waited on must be explicitly
specified in options. The wait(), waitpid(), wait3(), and wait4() func-
tions 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 pro-
cesses which have experienced other status changes without having to also
handle the exit status from terminated processes.
Instead of the traditional rusage argument, the wrusage arguments 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 identifers in addition to process IDs and process group IDs.
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 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 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 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 break-
point. 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 termi-
nated. 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 termi-
nated 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 hang-
ing 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 chil-
dren.
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.
When the WNOHANG option is specified and no processes wish to report sta-
tus, 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 sta-
tus, wait4() and wait6() return a process id of 0.
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 spec-
ified 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 termina-
tion 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 indi-
cates normal termination.
If a parent process terminates without waiting for all of its child pro-
cesses to terminate, the remaining child processes are assigned the par-
ent 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, con-
tinued, 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.
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 avail-
able 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().
POSIX requires waitid() to return the full 32 bits passed to _exit(2);
this implementation only returns 8 bits like in the other calls.
HISTORY
The wait() function appeared in Version 6 AT&T UNIX.
--
John Baldwin
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