Initial Spring 2016 commit.
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Geoffrey Challen
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man/syscall/waitpid.html
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man/syscall/waitpid.html
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<html>
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<head>
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<title>waitpid</title>
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<link rel="stylesheet" type="text/css" media="all" href="../man.css">
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</head>
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<body bgcolor=#ffffff>
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<h2 align=center>waitpid</h2>
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<h4 align=center>OS/161 Reference Manual</h4>
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<h3>Name</h3>
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<p>
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waitpid - wait for a process to exit
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</p>
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<h3>Library</h3>
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<p>
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Standard C Library (libc, -lc)
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</p>
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<h3>Synopsis</h3>
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<p>
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<tt>#include <sys/wait.h></tt><br>
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<br>
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<tt>pid_t</tt><br>
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<tt>waitpid(pid_t </tt><em>pid</em><tt>, int *</tt><em>status</em><tt>,
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int </tt><em>options</em><tt>);</tt>
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</p>
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<h3>Description</h3>
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<p>
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Wait for the process specified by <em>pid</em> to exit, and return an
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encoded exit status in the integer pointed to by <em>status</em>. If
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that process has exited already, <tt>waitpid</tt> returns
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immediately. If that process does not exist, <tt>waitpid</tt> fails.
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</p>
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<p>
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It is explicitly allowed for <em>status</em> to be <tt>NULL</tt>, in
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which case waitpid operates normally but the status value is not
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produced.
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</p>
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<p>
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A process moves from "has exited already" to "does not exist" when
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every process that is expected to collect its exit status with
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<tt>waitpid</tt> has done so.
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</p>
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<p>
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In the standard Unix model of processes, the only process that is
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expected to collect another process's exit status is its parent.
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(If you feel this is restrictive, you might try to extend the model or
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define a new one; however, this is not recommended. The only other
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model that really makes much sense is to let any process wait for any
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other process; but then you need to check for and reject combinations
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that would cause deadlock.)
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</p>
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<p>
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There are several semi-standard and messy/ugly ways in Unix for a
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process to indicate that it doesn't want to collect the exit status of
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a child it forks and therefore shouldn't be expected to. You do not
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need to implement any of these, but you might find it convenient for
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your own purposes to provide this functionality.
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</p>
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<p>
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If a parent process exits before one or more of its children, it can
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no longer be expected collect their exit status. There are several
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ways to handle this case in practice, of which the traditional Unix
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method is only one. This is something you should design.
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</p>
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<p>
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The <em>options</em> argument should be 0. You are not required to
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implement any options. (However, your system should check to make sure
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that requests for options you do not support are rejected.)
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</p>
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<p>
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If you desire, you may implement the Unix option WNOHANG; this causes
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waitpid, when called for a process that has not yet exited, to return
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0 immediately instead of waiting.
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</p>
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<p>
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The Unix option WUNTRACED, to ask for reporting of processes that stop
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as well as exit, is also defined in the header files, but implementing
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this feature is not required or necessary unless you are implementing
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job control.
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</p>
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<p>
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You may also make up your own options if you find them helpful.
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However, please, document anything you make up.
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</p>
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<p>
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The encoding of the exit status is comparable to Unix and is defined
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by the flags found in <kern/wait.h>. (Userlevel code should
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include <sys/wait.h> to get these definitions.) A process can
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exit by calling <A HREF=_exit.html>_exit()</A> or it can exit by
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receiving a fatal signal. In the former case the
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<tt>_MKWAIT_EXIT()</tt> macro should be used with the user-supplied
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exit code value to prepare the exit status; in the latter, the
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<tt>_MKWAIT_SIG()</tt> macro (or <tt>_MKWAIT_CORE()</tt> if a core
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file was generated) should be used with the signal number. The result
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encoding also allows notification of processes that have stopped; this
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would be used in connection with job control and with
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<tt>ptrace</tt>-based debugging if you were to implement those things.
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</p>
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<p>
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The <tt>_MKWAIT</tt> flags are not standard and should be considered
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part of the implementation.
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</p>
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<p>
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To <em>read</em> the wait status, use the macros <tt>WIFEXITED()</tt>,
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<tt>WIFSIGNALED()</tt>, and/or <tt>WIFSTOPPED()</tt> to find out what
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happened, and then <tt>WEXITSTATUS()</tt>, <tt>WTERMSIG()</tt>, or
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<tt>WSTOPSIG()</tt> respectively to get the exit code or signal
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number. If <tt>WIFSIGNALED()</tt> is true, <tt>WCOREDUMP()</tt> can be
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used to check if a core file was generated. This is the same as Unix,
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although the value encoding is different from the historic Unix
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format.
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</p>
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<h3>Return Values</h3>
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<p>
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<tt>waitpid</tt> returns the process id whose exit status is reported in
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<em>status</em>. In OS/161, this is always the value of <em>pid</em>.
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<p>
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<p>
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(In Unix, but not by default OS/161, you can wait for any of several
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processes by passing magic values of <em>pid</em>, so this return
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value can actually be useful.)
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</p>
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<p>
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If you implement WNOHANG, and WNOHANG is given, and the process
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specified by <em>pid</em> has not yet exited, waitpid returns 0.
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</p>
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<p>
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On error, -1 is returned, and <A HREF=errno.html>errno</A> is set to a
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suitable error code for the error condition encountered.
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</p>
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<h3>Errors</h3>
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<p>
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The following error codes should be returned under the conditions
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given. Other error codes may be returned for other cases not
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mentioned here.
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<table width=90%>
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<tr><td width=5% rowspan=4> </td>
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<td width=10% valign=top>EINVAL</td>
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<td>The <em>options</em> argument requested invalid or
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unsupported options.</td></tr>
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<tr><td valign=top>ECHILD</td>
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<td>The <em>pid</em> argument named a process
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that was not a child of the current
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process.</td></tr>
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<tr><td valign=top>ESRCH</td>
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<td>The <em>pid</em> argument named a
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nonexistent process.</td></tr>
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<tr><td valign=top>EFAULT</td>
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<td>The <em>status</em> argument was an
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invalid pointer.</td></tr>
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</table>
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</p>
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</body>
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</html>
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