Tweaks to the wait(2) manpage
Konstantin Belousov
kostikbel at gmail.com
Tue Sep 24 21:34:59 UTC 2013
On Tue, Sep 24, 2013 at 04:31:44PM -0400, John Baldwin wrote:
> 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:
Still, this sentence highlights the difference with wait4(), but there were
no description for wait4() given before. Might be, modulo the language
rewording, say
The wait6() function is the most general function in this family and
its distinctive features are:
>
> 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:
And there,
The wait6() function takes the wrusage argument, which points to ...
I like other parts of the text.
>
> 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
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 834 bytes
Desc: not available
URL: <http://lists.freebsd.org/pipermail/freebsd-doc/attachments/20130925/f8efa286/attachment.sig>
More information about the freebsd-doc
mailing list