forkedChildLinux01.cpp

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

#define PIPE_READ 0
#define PIPE_WRITE 1

int createChild(const char* szCommand, char* const aArguments[], char* const aEnvironment[], const char* szMessage)
{
    int aStdinPipe[2];
    int aStdoutPipe[2];
    int nChild;
    char nChar;
    int nResult;

    if(pipe(aStdinPipe) < 0)
    {
        perror("allocating pipe for child input redirect");
        return -1;
    }
    if(pipe(aStdoutPipe) < 0)
    {
        close(aStdinPipe[PIPE_READ]);
        close(aStdinPipe[PIPE_WRITE]);
        perror("allocating pipe for child output redirect");
        return -1;
    }

    nChild = fork();
    if (0 == nChild)
    {
        // child continues here

        // redirect stdin
        if(dup2(aStdinPipe[PIPE_READ], STDIN_FILENO) == -1)
        {
            exit(errno);
        }

        // redirect stdout
        if(dup2(aStdoutPipe[PIPE_WRITE], STDOUT_FILENO) == -1)
        {
            exit(errno);
        }

        // redirect stderr
        if(dup2(aStdoutPipe[PIPE_WRITE], STDERR_FILENO) == -1)
        {
            exit(errno);
        }

        // all these are for use by parent only
        close(aStdinPipe[PIPE_READ]);
        close(aStdinPipe[PIPE_WRITE]);
        close(aStdoutPipe[PIPE_READ]);
        close(aStdoutPipe[PIPE_WRITE]);

        // run child process image
        // replace this with any exec* function find easier to use ("man exec")
        nResult = execve(szCommand, aArguments, aEnvironment);

        // if we get here at all, an error occurred, but we are in the child
        // process, so just exit
        exit(nResult);
    }
    else if(nChild > 0)
    {
        // parent continues here

        // close unused file descriptors, these are for child only
        close(aStdinPipe[PIPE_READ]);
        close(aStdoutPipe[PIPE_WRITE]);

        // Include error check here
        if(NULL != szMessage)
        {
            write(aStdinPipe[PIPE_WRITE], szMessage, strlen(szMessage));
        }

        // Just a char by char read here, you can change it accordingly
        while(read(aStdoutPipe[PIPE_READ], &nChar, 1) == 1)
        {
            write(STDOUT_FILENO, &nChar, 1);
        }

        // done with these in this example program, you would normally keep these
        // open of course as long as you want to talk to the child
        close(aStdinPipe[PIPE_WRITE]);
        close(aStdoutPipe[PIPE_READ]);
    } else
    {
        // failed to create child
        close(aStdinPipe[PIPE_READ]);
        close(aStdinPipe[PIPE_WRITE]);
        close(aStdoutPipe[PIPE_READ]);
        close(aStdoutPipe[PIPE_WRITE]);
    }
    return nChild;
}