Re: Problem in Interprocess Communication using anonymous pipes,
when
Vinayak wrote:
Hi all,
I am getting some problems with Interprocess communication.
I am providing the codes for Parent and Child processes. Although the code
looks lengthy, but it's very simple. I will describe the problem later.
Parent Process Code:
// Creates two anonymous pipes to the STDIN and STDOUT of the child process
and then creates the Child process
// After that, sends data from a file "1.txt" to the child process' stdin
and then tries to retrieve the output of the child process from it's stdout
which is connected to parent process using an anonymous pipe.
Ok, I've added some comments inline...
// 1.txt - will contain 2 lines
// My name is Vinayak. <Return>
//
// liveLink.cpp : Defines the entry point for the console application.
#include "stdafx.h"
#include <stdio.h>
#include <windows.h>
#define BUFSIZE 4096
HANDLE hChildStdinRd, hChildStdinWr, hChildStdinWrDup,
hChildStdoutRd, hChildStdoutWr, hChildStdoutRdDup,
hInputFile, hSaveStdin, hSaveStdout;
That is an excessive number of handles. Firstly, why are you redirecting
the stdin and stdout of the parent process? That doesn't seem to be a
requirement.
BOOL CreateChildProcess(VOID);
VOID WriteToPipe(VOID);
VOID ReadFromPipe(VOID);
VOID ErrorExit(LPTSTR);
VOID ErrMsg(LPTSTR, BOOL);
DWORD main(int argc, char *argv[])
main usually returns int, not DWORD.
// The steps for redirecting child process's STDOUT:
// 1. Save current STDOUT, to be restored later.
// 2. Create anonymous pipe to be STDOUT for child process.
// 3. Set STDOUT of the parent process to be write handle to
// the pipe, so it is inherited by the child process.
// 4. Create a noninheritable duplicate of the read handle and
// close the inheritable read handle.
Ahh, this isn't necessary. You can set the input and output standard
handles directly in your call to CreateProcess. You can remove all this
code for redirecting the parent processes handles.
BOOL CreateChildProcess()
{
PROCESS_INFORMATION piProcInfo;
STARTUPINFO siStartInfo;
BOOL bFuncRetn = FALSE;
// Set up members of the PROCESS_INFORMATION structure.
ZeroMemory( &piProcInfo, sizeof(PROCESS_INFORMATION) );
// Set up members of the STARTUPINFO structure.
ZeroMemory( &siStartInfo, sizeof(STARTUPINFO) );
siStartInfo.cb = sizeof(STARTUPINFO);
//the crucial bit!
siStartInfo.hStdInput = hChildStdinRd;
siStartInfo.hStdOutput = hChildStdoutWr;
siStartInfo.hStdError = hChildStdoutWr;
//note, you should not need to call DuplicateHandle anywhere in
//your code AFAICT.
// Create the child process.
bFuncRetn = CreateProcess(NULL,
[snip]
#include <windows.h>
#include <string.h>
#define BUFSIZE 4096
VOID main(VOID)
{
CHAR chBuf[BUFSIZE];
DWORD dwRead, dwWritten;
HANDLE hStdin, hStdout;
BOOL fSuccess;
hStdout = GetStdHandle(STD_OUTPUT_HANDLE);
hStdin = GetStdHandle(STD_INPUT_HANDLE);
if ((hStdout == INVALID_HANDLE_VALUE) ||
(hStdin == INVALID_HANDLE_VALUE))
ExitProcess(1);
scanf("%s", chBuf);
printf("%s", chBuf);
You might want to do:
fflush(stdout);
here to be safe.
Problem:
When executed, WriteToPipe() works nicely, but in ReadFromPipe() the
processes get stuck, when the code reaches ReadFile() function. I guess it
happens due to scanf(). But I don't know how to solve it. Would anyone please
suggest me anything? I will be really grateful.
My changes greatly shorten your code, and hopefully fix it too.
Tom