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<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>