Capture Output of Spawned Process to string

Eddy Luten’s answer led me in a good direction, but the MSDN documentation (while elaborate) had some issues. Mainly, you need to ensure you close all handles you don’t use. Also it just has code it expects the user to understand.

So instead, here’s my wall of code I expect people to just understand 😀

#include <string>
#include <iostream>
#include <windows.h> 
#include <stdio.h>
#pragma warning( disable : 4800 ) // stupid warning about bool
#define BUFSIZE 4096
HANDLE g_hChildStd_OUT_Rd = NULL;
HANDLE g_hChildStd_OUT_Wr = NULL;
HANDLE g_hChildStd_ERR_Rd = NULL;
HANDLE g_hChildStd_ERR_Wr = NULL;

PROCESS_INFORMATION CreateChildProcess(void); 
void ReadFromPipe(PROCESS_INFORMATION); 

int main(int argc, char *argv[]){ 
    SECURITY_ATTRIBUTES sa; 
    printf("\n->Start of parent execution.\n");
    // Set the bInheritHandle flag so pipe handles are inherited. 
    sa.nLength = sizeof(SECURITY_ATTRIBUTES); 
    sa.bInheritHandle = TRUE; 
    sa.lpSecurityDescriptor = NULL; 
    // Create a pipe for the child process's STDERR. 
    if ( ! CreatePipe(&g_hChildStd_ERR_Rd, &g_hChildStd_ERR_Wr, &sa, 0) ) {
        exit(1); 
    }
    // Ensure the read handle to the pipe for STDERR is not inherited.
    if ( ! SetHandleInformation(g_hChildStd_ERR_Rd, HANDLE_FLAG_INHERIT, 0) ){
        exit(1);
    }
    // Create a pipe for the child process's STDOUT. 
    if ( ! CreatePipe(&g_hChildStd_OUT_Rd, &g_hChildStd_OUT_Wr, &sa, 0) ) {
        exit(1);
    }
    // Ensure the read handle to the pipe for STDOUT is not inherited
    if ( ! SetHandleInformation(g_hChildStd_OUT_Rd, HANDLE_FLAG_INHERIT, 0) ){
        exit(1); 
    }
    // Create the child process. 
    PROCESS_INFORMATION piProcInfo = CreateChildProcess();

    // Read from pipe that is the standard output for child process. 
    printf( "\n->Contents of child process STDOUT:\n\n", argv[1]);
    ReadFromPipe(piProcInfo); 

    printf("\n->End of parent execution.\n");

    // The remaining open handles are cleaned up when this process terminates. 
    // To avoid resource leaks in a larger application, 
    //   close handles explicitly.
    return 0; 
} 

// Create a child process that uses the previously created pipes
//  for STDERR and STDOUT.
PROCESS_INFORMATION CreateChildProcess(){
    // Set the text I want to run
    char szCmdline[]="test --log_level=all --report_level=detailed";
    PROCESS_INFORMATION piProcInfo; 
    STARTUPINFO siStartInfo;
    bool bSuccess = FALSE; 

    // Set up members of the PROCESS_INFORMATION structure. 
    ZeroMemory( &piProcInfo, sizeof(PROCESS_INFORMATION) );

    // Set up members of the STARTUPINFO structure. 
    // This structure specifies the STDERR and STDOUT handles for redirection.
    ZeroMemory( &siStartInfo, sizeof(STARTUPINFO) );
    siStartInfo.cb = sizeof(STARTUPINFO); 
    siStartInfo.hStdError = g_hChildStd_ERR_Wr;
    siStartInfo.hStdOutput = g_hChildStd_OUT_Wr;
    siStartInfo.dwFlags |= STARTF_USESTDHANDLES;

    // Create the child process. 
    bSuccess = CreateProcess(NULL, 
        szCmdline,     // command line 
        NULL,          // process security attributes 
        NULL,          // primary thread security attributes 
        TRUE,          // handles are inherited 
        0,             // creation flags 
        NULL,          // use parent's environment 
        NULL,          // use parent's current directory 
        &siStartInfo,  // STARTUPINFO pointer 
        &piProcInfo);  // receives PROCESS_INFORMATION
    CloseHandle(g_hChildStd_ERR_Wr);
    CloseHandle(g_hChildStd_OUT_Wr);
    // If an error occurs, exit the application. 
    if ( ! bSuccess ) {
        exit(1);
    }
    return piProcInfo;
}

// Read output from the child process's pipe for STDOUT
// and write to the parent process's pipe for STDOUT. 
// Stop when there is no more data. 
void ReadFromPipe(PROCESS_INFORMATION piProcInfo) {
    DWORD dwRead; 
    CHAR chBuf[BUFSIZE];
    bool bSuccess = FALSE;
    std::string out = "", err = "";
    for (;;) { 
        bSuccess=ReadFile( g_hChildStd_OUT_Rd, chBuf, BUFSIZE, &dwRead, NULL);
        if( ! bSuccess || dwRead == 0 ) break; 

        std::string s(chBuf, dwRead);
        out += s;
    } 
    dwRead = 0;
    for (;;) { 
        bSuccess=ReadFile( g_hChildStd_ERR_Rd, chBuf, BUFSIZE, &dwRead, NULL);
        if( ! bSuccess || dwRead == 0 ) break; 

        std::string s(chBuf, dwRead);
        err += s;

    } 
    std::cout << "stdout:" << out << std::endl;
    std::cout << "stderr:" << err << std::endl;
}