ch16_11.htm

By Tom Christiansen and Nathan Torkington ISBN 1-56592-243-3 First Edition, published August 1998

><PRE
CLASS="programlisting"
>#!/usr/bin/perl -w
# <A
CLASS="indexterm"
NAME="ch16-idx-1000006353-0"
></A
>pipe3 - use forking open so parent can send to child

use IO::Handle;
if ($pid = open(CHILD, &quot;|-&quot;)) {
    CHILD-&gt;autoflush(1);
    print CHILD &quot;Parent Pid $$ is sending this\n&quot;;
    close(CHILD);
} else {
    die &quot;cannot fork: $!&quot; unless defined $pid;
    chomp($line = &lt;STDIN&gt;);
    print &quot;Child Pid $$ just read this: `$line'\n&quot;;
    exit;
}</PRE
></DIV
><P
CLASS="para"
>Since the child already has STDIN set to the parent, the child could <CODE
CLASS="literal"
>exec</CODE
> some other program that expects to read from standard input, such as <EM
CLASS="emphasis"
>lpr</EM
>. In fact, this is useful and commonly done.</P
><P
CLASS="para"
>If the child wants to write to the parent, it does something like what's shown in <A
CLASS="xref"
HREF="ch16_11.htm#ch16-42618"
TITLE="pipe4"
>Example 16.6</A
>.</P
><DIV
CLASS="example"
><H4
CLASS="example"
><A
CLASS="title"
NAME="ch16-42618"
>Example 16.6: pipe4</A
></H4
><PRE
CLASS="programlisting"
>#!/usr/bin/perl -w
# <A
CLASS="indexterm"
NAME="ch16-idx-1000006354-0"
></A
>pipe4 - use forking open so child can send to parent

use IO::Handle;
if ($pid = open(CHILD, &quot;-|&quot;)) {
    chomp($line = &lt;CHILD&gt;);
    print &quot;Parent Pid $$ just read this: `$line'\n&quot;;
    close(CHILD);
} else {
    die &quot;cannot fork: $!&quot; unless defined $pid;
    STDOUT-&gt;autoflush(1);
    print STDOUT &quot;Child Pid $$ is sending this\n&quot;;
    exit;
}</PRE
></DIV
><P
CLASS="para"
>Again, since the child already has its STDOUT connected to the parent, this child could <CODE
CLASS="literal"
>exec</CODE
> some other program to produce something interesting on its standard output. That output would be available to the parent as input from &lt;<CODE
CLASS="literal"
>CHILD&gt;</CODE
>.</P
><P
CLASS="para"
>When using <CODE
CLASS="literal"
>open</CODE
> this way, we don't have to manually call <CODE
CLASS="literal"
>waitpid</CODE
> since we didn't do a manual fork. We do have to call <CODE
CLASS="literal"
>close</CODE
>, though. In both cases, the <CODE
CLASS="literal"
>$?</CODE
> variable will have the child's wait status in it (see <A
CLASS="xref"
HREF="ch16_20.htm"
TITLE="Avoiding Zombie Processes"
>Recipe 16.19</A
> to see how to interpret this status value).</P
><P
CLASS="para"
>The preceding examples were unidirectional. What if you want both processes talking to each other? Just make two calls to <CODE
CLASS="literal"
>pipe</CODE
> before forking. You must be careful about who tells whom what and when, though, or you're apt to deadlock. (See <A
CLASS="xref"
HREF="ch16_11.htm#ch16-42071"
TITLE="pipe5"
>Example 16.7</A
>.)</P
><DIV
CLASS="example"
><H4
CLASS="example"
><A
CLASS="title"
NAME="ch16-42071"
>Example 16.7: pipe5</A
></H4
><PRE
CLASS="programlisting"
>#!/usr/bin/perl -w
# <A
CLASS="indexterm"
NAME="ch16-idx-1000006355-0"
></A
>pipe5 - bidirectional communication using two pipe pairs
#         designed for the socketpair-challenged
use IO::Handle;
pipe(PARENT_RDR, CHILD_WTR);
pipe(CHILD_RDR,  PARENT_WTR);
CHILD_WTR-&gt;autoflush(1);
PARENT_WTR-&gt;autoflush(1);

if ($pid = fork) {
    close PARENT_RDR; close PARENT_WTR;
    print CHILD_WTR &quot;Parent Pid $$ is sending this\n&quot;;
    chomp($line = &lt;CHILD_RDR&gt;);
    print &quot;Parent Pid $$ just read this: `$line'\n&quot;;
    close CHILD_RDR; close CHILD_WTR;
    waitpid($pid,0);
} else {
    die &quot;cannot fork: $!&quot; unless defined $pid;
    close CHILD_RDR; close CHILD_WTR;
    chomp($line = &lt;PARENT_RDR&gt;);
    print &quot;Child Pid $$ just read this: `$line'\n&quot;;
    print PARENT_WTR &quot;Child Pid $$ is sending this\n&quot;;
    close PARENT_RDR; close PARENT_WTR;
    exit;
}</PRE
></DIV
><P
CLASS="para"
>That's getting complicated. It just so happens that there's a special system call, shown in <A
CLASS="xref"
HREF="ch16_11.htm#ch16-38392"
TITLE="pipe6"
>Example 16.8</A
>, that makes the last example simpler. It's called <CODE
CLASS="literal"
>socketpair</CODE
>, and it works like <CODE
CLASS="literal"
>pipe</CODE
>, except that both handles can be used for reading and for writing.</P
><DIV
CLASS="example"
><H4
CLASS="example"
><A
CLASS="title"
NAME="ch16-38392"
>Example 16.8: pipe6</A
></H4
><PRE
CLASS="programlisting"
>#!/usr/bin/perl -w
# <A
CLASS="indexterm"
NAME="ch16-idx-1000006356-0"
></A
>pipe6 - bidirectional communication using socketpair
#   &quot;the best ones always go both ways&quot;

use Socket;
use IO::Handle;
# We say AF_UNIX because although *_LOCAL is the
# POSIX 1003.1g form of the constant, many machines
# still don't have it.
socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
    or  die &quot;socketpair: $!&quot;;

CHILD-&gt;autoflush(1);
PARENT-&gt;autoflush(1);

if ($pid = fork) {
    close PARENT;
    print CHILD &quot;Parent Pid $$ is sending this\n&quot;;
    chomp($line = &lt;CHILD&gt;);
    print &quot;Parent Pid $$ just read this: `$line'\n&quot;;
    close CHILD;
    waitpid($pid,0);
} else {
    die &quot;cannot fork: $!&quot; unless defined $pid;
    close CHILD;
    chomp($line = &lt;PARENT&gt;);
    print &quot;Child Pid $$ just read this: `$line'\n&quot;;
    print PARENT &quot;Child Pid $$ is sending this\n&quot;;
    close PARENT;
    exit;
}</PRE
></DIV
><P
CLASS="para"
>In fact, some systems have historically implemented pipes as two half-closed ends of a socketpair. They essentially define <CODE
CLASS="literal"
>pipe(READER,</CODE
> <CODE
CLASS="literal"
>WRITER)</CODE
> this way:</P
><PRE
CLASS="programlisting"
>socketpair(READER, WRITER, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(READER, 1);        # no more writing for reader
shutdown(WRITER, 0);        # no more reading for writer</PRE
><P
CLASS="para"
>On Linux kernels before 2.0.34, the <I
CLASS="filename"
>shutdown (2)</I
> system call was broken. Instead of telling the reader not to write and the writer not to read, you had to tell the reader not to read and the writer not to write.<A
CLASS="indexterm"
NAME="ch16-idx-1000006347-0"
></A
><A
CLASS="indexterm"
NAME="ch16-idx-1000006347-1"
></A
><A
CLASS="indexterm"
NAME="ch16-idx-1000006347-2"
></A
></P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="ch16-pgfId-1950"
>See Also</A
></H3
><P
CLASS="para"
><A
CLASS="olink"
HREF="../prog/ch03_01.htm"
>Chapter 3</A
> of <A
CLASS="citetitle"
HREF="../prog/index.htm"
TITLE="Programming Perl"
><CITE
CLASS="citetitle"
>Programming Perl</CITE
></A
> or <I
CLASS="filename"
>perlfunc </I
>(1) for all functions used here; the documentation for the standard IPC::Open2 module; <EM
CLASS="emphasis"
>Advanced Programming in the Unix Environment</EM
>; <A
CLASS="xref"
HREF="ch16_09.htm"
TITLE="Controlling Input and Output of Another Program"
>Recipe 16.8</A
>; <A
CLASS="xref"
HREF="ch19_07.htm"
TITLE="Executing Commands Without Shell Escapes"
>Recipe 19.6</A
></P
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