<?xml version="1.0" encoding="utf-8"?>+
−
<!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. -->+
−
<!-- This component and the accompanying materials are made available under the terms of the License +
−
"Eclipse Public License v1.0" which accompanies this distribution, +
−
and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". -->+
−
<!-- Initial Contributors:+
−
    Nokia Corporation - initial contribution.+
−
Contributors: +
−
-->+
−
<!DOCTYPE concept+
−
  PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">+
−
<concept id="GUID-AA49FB68-22A1-417F-AB57-9C22CE016B21" xml:lang="en"><title>Parent+
−
and Child IPC Using a Single Pipe</title><prolog><metadata><keywords/></metadata></prolog><conbody>+
−
<p>In this scenario, the parent and child processes communicate using a single+
−
pipe. The following pseudo code details the mechanism used by code in the+
−
parent process in a Unix-like system. <codeblock xml:space="preserve">//Call pipe() to create the pipe.+
−
//Call Fork().+
−
if (child process)+
−
{+
−
   //Child Process.+
−
   //duplicate (via a dup2() call) the read/write end of the pipe using  +
−
   //prior agreed file descriptor numbers which will subsequently be used by the   //child process following the exec() call.+
−
   //Close the original file descriptors.+
−
   //Call exec() to replace the code image in the child process.+
−
}+
−
else+
−
{+
−
   //Parent process+
−
   //Do Whatever+
−
}+
−
</codeblock></p>+
−
<p>Instead of using the <codeph>fork()</codeph>/<codeph>exec()</codeph> as+
−
described above, POSIX libraries including P.I.P.S. provide the <xref href="GUID-A9DB6E7C-B8D6-377A-BBE6-39E0A7A09E5D.dita"><apiname>popen()</apiname></xref> function+
−
as defined in <filepath>stdio.h</filepath>. Rather than using a pipe for the+
−
IPC mechanism the parent process communicates with the child using the stream+
−
returned by the <xref href="GUID-A9DB6E7C-B8D6-377A-BBE6-39E0A7A09E5D.dita"><apiname>popen()</apiname></xref> call, while the child process input+
−
or output is using the <xref href="GUID-C0C1D22B-298F-3E8D-A1E9-6F5EFA81F9E8.dita"><apiname>stdin()</apiname></xref>/<xref href="GUID-85797574-E2A2-3C0C-9670-C178078DE199.dita"><apiname>stdout()</apiname></xref> streams. </p>+
−
<p>However if the parent process requires a file descriptor it can be obtained+
−
by using the <xref href="GUID-7A399B7D-553D-345E-92BD-43A8A8224D39.dita"><apiname>fileno()</apiname></xref> API call. For more information about+
−
the <xref href="GUID-A9DB6E7C-B8D6-377A-BBE6-39E0A7A09E5D.dita"><apiname>popen()</apiname></xref> function, see <xref href="http://www.opengroup.org" scope="external">http:\\www.opengroup.org</xref>. </p>+
−
<p><b>Parent process <codeph>fork()</codeph> and <codeph>exec()</codeph> functions</b></p>+
−
<p>The following code shows the use of a pipe and subsequent <codeph>fork()</codeph>/<codeph>exec()</codeph> calls+
−
to create a Pipe for IPC. The example shown is the child process writing data+
−
to the pipe, and the parent process receiving the data.</p>+
−
<codeblock xml:space="preserve">//Child Process File Descriptors for the pipe. These should be defined in a common header +
−
//file used to compile both the parent and child process's executable.+
−
//However #define is here for clarity.+
−
#define WRITE_FD  104+
−
int main(int argc, char *argv[])+
−
{+
−
  int pipeEnds[2]; //Pipe file descriptors [0] is for read [1] is for write+
−
  +
−
  //create the pipe+
−
  if(pipe(pipeEnds) != 0)+
−
  {+
−
     //Pipe creation error+
−
     return EXIT_FAILURE;+
−
  }+
−
  else+
−
  {+
−
     pid_t Childpid = fork();+
−
     +
−
     if(Childpid == 0)+
−
     {+
−
         //close the redundant read FD obtained from the parent process+
−
         (void)close(pipeEnds[0]);+
−
    +
−
         //duplicate the file descriptor for use following exec().+
−
         if(dup2(pipeEnds[1], WRITE_FD) == -1)+
−
         {+
−
            printf("dup2 error\n");+
−
            return EXIT_FAILURE;+
−
         }+
−
         //close the redundant write FD obtained from the parent process+
−
         (void)close(pipeEnds[1]);+
−
    +
−
         //exec() to replace the child process executable+
−
         char execFileName[] = "/root/PortDoc/Example1_c/Posix/Child/ChildProg";+
−
         execl(execFileName,NULL);+
−
      }+
−
      else+
−
      {+
−
         //Parent process. This reads from the pipe. Therefore close the write end of the+
−
         //pipe.+
−
         close(pipeEnds[1]);+
−
    +
−
         //declare receive buffer, and clear its contents+
−
         char RxBuffer[100];+
−
         memset(RxBuffer,0,sizeof(RxBuffer));+
−
    +
−
         //Wait for data from the child process. Child sends a string.+
−
         read(pipeEnds[0],RxBuffer,sizeof(RxBuffer));+
−
    +
−
         printf(RxBuffer);+
−
+
−
         //close the Read end of the pipe+
−
         close(pipeEnds[0]);+
−
    +
−
         //Wait for the child process to terminate+
−
         waitpid(Childpid,NULL,0);+
−
      }+
−
   }+
−
   return EXIT_SUCCESS;+
−
}+
−
</codeblock>+
−
<p><b>Child process created using <codeph>fork()</codeph> or <codeph>exec()</codeph> functions</b></p>+
−
<p>The following code shows an example of the child process source which will+
−
be executed following the <codeph>exec()</codeph>. </p>+
−
<codeblock xml:space="preserve">//Child Process File Descriptors for the pipe. These should be defined in a common header +
−
//file used to compile both the parent and child process's executable. Shown here for+
−
//clarity.+
−
#define WRITE_FD  104+
−
+
−
int main(void)+
−
{+
−
   char TxMsg[] = "Hello Parent\n";+
−
    +
−
   //Send the message to the parent+
−
   write(WRITE_FD,TxMsg,sizeof(TxMsg));+
−
    +
−
   //close the File Descriptor+
−
   close(WRITE_FD);+
−
   return EXIT_SUCCESS;+
−
}+
−
</codeblock>+
−
<p><b>Parent process P.I.P.S. example for <xref href="GUID-A9DB6E7C-B8D6-377A-BBE6-39E0A7A09E5D.dita"><apiname>popen()</apiname></xref> function</b></p>+
−
<p>The following code shows how the above code can be modified to use <codeph>popen()</codeph>,+
−
rather than the <codeph>fork()</codeph>/<codeph>exec()</codeph> combination. </p>+
−
<codeblock xml:space="preserve">int main(int argc, char *argv[])+
−
{+
−
   //Create child process using popen(). Child process writes to the Parent therefore "r" +
−
   //parameter.+
−
   FILE* ChildProcessStream = popen("/root/PortDoc/Example1_c/Symbian/Child/ChildProg","r");+
−
          +
−
   if(ChildProcessStream == NULL)+
−
   {+
−
      printf("\n Failure to create child process with popen()\n");+
−
      return EXIT_FAILURE;+
−
   }+
−
   else+
−
   {+
−
      //Use a file descriptor rather than a stream+
−
      int ChildProcessFD = fileno(ChildProcessStream);+
−
+
−
      //Create a receive buffer, and zero contents before receiving.+
−
      char RxBuffer[100];+
−
      memset(RxBuffer,0,sizeof(RxBuffer));+
−
      +
−
      //Wait for data from the child process. Child sends a string.+
−
      int nbytes = read(ChildProcessFD,RxBuffer,sizeof(RxBuffer));+
−
+
−
      //printf is slightly different from the POSIX example to verify that the data output to the +
−
      //screen is done by the parent process.+
−
      printf("\n Message Received by Parent=%s",RxBuffer);+
−
      +
−
      //Wait for Child Process to complete+
−
      pclose(ChildProcessStream);+
−
   }+
−
   return EXIT_SUCCESS;+
−
}+
−
</codeblock>+
−
<p><b>Child process P.I.P.S. example for <xref href="GUID-A9DB6E7C-B8D6-377A-BBE6-39E0A7A09E5D.dita"><apiname>popen()</apiname></xref> function</b></p>+
−
<p>The following code shows how the child process source can be modified when+
−
it is executed using <xref href="GUID-A9DB6E7C-B8D6-377A-BBE6-39E0A7A09E5D.dita"><apiname>popen()</apiname></xref>. Note that rather+
−
than using a file descriptor for the communication via the pipe the <xref href="GUID-C0C1D22B-298F-3E8D-A1E9-6F5EFA81F9E8.dita"><apiname>stdin()</apiname></xref> or+
−
the <codeph>stdout</codeph> streams are directed to the parent process, as+
−
defined in the usage of the <xref href="GUID-A9DB6E7C-B8D6-377A-BBE6-39E0A7A09E5D.dita"><apiname>popen()</apiname></xref> call. </p>+
−
<codeblock xml:space="preserve">int main(void)+
−
{+
−
    //Child process created by popen() so that its stdout is streamed to the parent process+
−
    char TxMsg[] = "Hello Parent\n";+
−
    +
−
    //Send the message to the parent+
−
    printf(TxMsg);+
−
    +
−
    return EXIT_SUCCESS;+
−
}+
−
</codeblock>+
−
</conbody></concept>+
−