C++ - Problems while taking over STDIN and STDOUT of child process!

MaxistXXL · 09-30-2010, 02:02 AM

Hello,
I want to write a program A, that starts another program B and takes over its STDIN and STDOUT, so that everything B reads A has to put in to the STDIN and everything B writes, A gets.
I am aware of all the other ways to do IPC, but I want to manipulate B randomly on bit-level. So I want to keep the part of the source code for IPC as small and simple as possible.
I managed to take over the STDOUT of B, but I cant take over the STDIN. The source code is at the end. If I put an integer into the pipe, cin >> buff does not block, but seems to be skipped. If I put a char-Array into the pipe, cin blocks forever. To take read() or scanf() does not help neither.
Does anyone has a working code example or knows what I'm doing wrong?

thank you
Max

The main parts of the source code in A are:

Code:

int pipefd_stdin[2];
...
if(vfork() == 0){
...
dup2(pipefd_stdin[READ_END], STDIN_FILENO);
...
}else{
 if(write(pipefd_stdin[WRITE_END], &buff, sizeof(buff)) < 0){
   std::cout << "ERROR!\n";	
 }
}

Programm B can be given completly and is:

Code:

#include <unistd.h>
#include <sys/wait.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <strings.h>
#include <iostream>

int main()
{
    char z[4];
    int i = 1000;
    //std::cout << "Program B started\n";
    std::cerr << "TEST " << i << "\n";
    i = 90;
    std::cin >> i;
    std::cerr << "TEST2 " << i << "\n";
    //90, if an int was given, else unreachable
}

neonsignal · 09-30-2010, 07:47 PM

You need to show some more of the code from your program A.

Have you created the pipe?

Have you got the read and write end around the correct way?

Where is the exec that calls program B?

Is the buffer a character string? If so, what is in it? Are you ending the integer with a new line or an end of file?

Code:

#include <unistd.h>
#include <iostream>
int main()
{
	int pipefd_stdin[2];
	pipe(pipefd_stdin);
	if (fork() == 0)
	{
		dup2(pipefd_stdin[0], STDIN_FILENO);
		execve("./b", 0, 0);
	}
	else
		write(pipefd_stdin[1], "42\n", 3);
}