How can I read a child process's output?

Question

I have written a function that attempts to read a child process's command line output via a pipe. This should be a simple subset of the MSDN Creating a Child Process with Redirected Input and Output article, but I am clearly making an error of some sort.

The ReadFile(...) call below blocks forever no matter if I place it before or after the WaitForSingleObject(...) call that should signal the end of the child process.

I have read all the answers that suggest "Use asynchronous ReadFile" and I am open to that suggestion if someone could give me some idea how that is accomplished on a pipe. Although I don't understand why asynchronous I/O should be needed for this case.

#include "stdafx.h"
#include <string>
#include <windows.h>

unsigned int launch( const std::string & cmdline );

int _tmain(int argc, _TCHAR* argv[])
{
    launch( std::string("C:/windows/system32/help.exe") );  
    return 0;
}

void print_error( unsigned int err )
{
    char* msg = NULL;
    FormatMessageA(
        FORMAT_MESSAGE_ALLOCATE_BUFFER | 
        FORMAT_MESSAGE_FROM_SYSTEM |
        FORMAT_MESSAGE_IGNORE_INSERTS,
        NULL,
        err,
        MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
        (LPSTR)&msg,
        0, NULL );
    std::cout << "------ Begin Error Msg ------" << std::endl;
    std::cout << msg << std::endl;
    std::cout << "------  End Error Msg  ------" << std::endl;

    LocalFree( msg );
}

unsigned int launch( const std::string & cmdline )
{
    TCHAR cl[_MAX_PATH*sizeof(TCHAR)]; 
    memset( cl, 0, sizeof(cl) );
    cmdline.copy( cl, (_MAX_PATH*sizeof(TCHAR)) - 1);    

    HANDLE stdoutReadHandle = NULL;
    HANDLE stdoutWriteHandle = NULL;

    SECURITY_ATTRIBUTES saAttr; 
    memset( &saAttr, 0, sizeof(saAttr) ); 
    saAttr.nLength = sizeof(SECURITY_ATTRIBUTES); 
    saAttr.bInheritHandle = TRUE; 
    saAttr.lpSecurityDescriptor = NULL; 

    // Create a pipe for the child process's STDOUT. 
    if ( ! CreatePipe(&stdoutReadHandle, &stdoutWriteHandle, &saAttr, 5000) )      
        throw std::runtime_error( "StdoutRd CreatePipe" ); 
    // Ensure the read handle to the pipe for STDOUT is not inherited.
    if ( ! SetHandleInformation(stdoutReadHandle, HANDLE_FLAG_INHERIT, 0) )
        throw std::runtime_error( "Stdout SetHandleInformation" ); 

    STARTUPINFO startupInfo; 
    memset( &startupInfo, 0, sizeof(startupInfo) ); 
    startupInfo.cb = sizeof(startupInfo);
    startupInfo.hStdError = stdoutWriteHandle;
    startupInfo.hStdOutput = stdoutWriteHandle;
    startupInfo.hStdInput = GetStdHandle(STD_INPUT_HANDLE);
    startupInfo.dwFlags |= STARTF_USESTDHANDLES;

    char* rawEnvVars = GetEnvironmentStringsA();

    //__asm _emit 0xcc;

    PROCESS_INFORMATION processInfo;
    memset( &processInfo, 0, sizeof(processInfo) );

    std::cout << "Start [" << cmdline << "]" << std::endl;
    if ( CreateProcessA( 0, &cl[0], 0, 0, false, 
        CREATE_NO_WINDOW | CREATE_UNICODE_ENVIRONMENT, 
        rawEnvVars, 0, &startupInfo, &processInfo ) )
    {       
        //CloseHandle( stdoutWriteHandle );
        DWORD wordsRead;
        char tBuf[257] = {'\0'};
        bool success = true;
        std::string outBuf("");
        unsigned int t;
        while(success) {
            //__asm _emit 0xcc;
            std::cout << "Just before ReadFile(...)" << std::endl;
            success = ReadFile( stdoutReadHandle, tBuf, 256, &wordsRead, NULL);
            (t=GetLastError())?print_error(t):t=t;
            std::cout << "Just after ReadFile(...) | read " << wordsRead<< std::endl;
            std::cout << ".";
            if( success == false ) break;
            outBuf += tBuf;
            tBuf[0] = '\0';
        }
        std::cout << "output = [" << outBuf << "]" << std::endl;


        if ( WaitForSingleObject( processInfo.hProcess, INFINITE ) == WAIT_OBJECT_0 )
        {
            unsigned int exitcode = 0;
            GetExitCodeProcess( processInfo.hProcess, (LPDWORD)&exitcode );                
            std::cout << "exitcode = [" << exitcode << "]" << std::endl;

            //__asm _emit 0xcc;
            CloseHandle( processInfo.hProcess );
            CloseHandle( processInfo.hThread );

            return exitcode;
        }
    }
    else
    {
        DWORD procErr = GetLastError();
        std::cout << "FAILED TO CREATE PROCESS!" << std::endl;
        print_error( procErr );
    }
    return -1;
} // end launch()

Answer 1

There are a few bugs in your code, but the most important is that you've specified FALSE for the bInheritHandles argument to CreateProcess. The new process can't use the pipe if it doesn't inherit the handle to it. In order for a handle to be inherited, the bInheritHandles argument must be TRUE and the handle must have inheritance enabled.

Other issues:

• You're specifying CREATE_UNICODE_ENVIRONMENT but passing an ANSI environment block. Note that it is easier to pass NULL for lpEnvironment and let the system copy the environment block for you. You should still specify CREATE_UNICODE_ENVIRONMENT in this case, as described in the documentation, because your environment block might contain Unicode characters.

• Similarly, if you're calling CreateProcessA you should be using STARTUPINFOA.

• You don't zero-terminate tBuf each time around the loop, so you'll get spurious extra characters in your output buffer.

• You need to close stdoutWriteHandle before you enter your read loop, or you won't know when the subprocess exits. (Or you could use asynchronous IO and check for process exit explicitly.)

• GetLastError() is undefined if an API function succeeds, so you should only be calling it if ReadFile returns FALSE. (Of course, in this case this is purely cosmetic since you aren't acting on the error code.)

For reference, here is my corrected version of your code. I've turned it into plain C (sorry!) because that's what I'm familiar with. I compiled and tested in Unicode mode, but I think it should work without modification in ANSI mode too.

#define _WIN32_WINNT _WIN32_WINNT_WIN7
#include <windows.h>
#include <stdio.h>

void launch(const char * cmdline_in)
{
    PROCESS_INFORMATION processInfo;
    STARTUPINFOA startupInfo; 
    SECURITY_ATTRIBUTES saAttr; 

    HANDLE stdoutReadHandle = NULL;
    HANDLE stdoutWriteHandle = NULL;

    char cmdline[256];
    char outbuf[32768];
    DWORD bytes_read;
    char tBuf[257];

    DWORD exitcode;

    strcpy_s(cmdline, sizeof(cmdline), cmdline_in);

    memset(&saAttr, 0, sizeof(saAttr));
    saAttr.nLength = sizeof(SECURITY_ATTRIBUTES); 
    saAttr.bInheritHandle = TRUE; 
    saAttr.lpSecurityDescriptor = NULL; 

    // Create a pipe for the child process's STDOUT. 
    if (!CreatePipe(&stdoutReadHandle, &stdoutWriteHandle, &saAttr, 5000))
    {
        printf("CreatePipe: %u\n", GetLastError());
        return;
    }

    // Ensure the read handle to the pipe for STDOUT is not inherited.
    if (!SetHandleInformation(stdoutReadHandle, HANDLE_FLAG_INHERIT, 0))
    {
        printf("SetHandleInformation: %u\n", GetLastError());
        return;
    }

    memset(&startupInfo, 0, sizeof(startupInfo));
    startupInfo.cb = sizeof(startupInfo);
    startupInfo.hStdError = stdoutWriteHandle;
    startupInfo.hStdOutput = stdoutWriteHandle;
    startupInfo.hStdInput = GetStdHandle(STD_INPUT_HANDLE);
    startupInfo.dwFlags |= STARTF_USESTDHANDLES;

    // memset(&processInfo, 0, sizeof(processInfo));  // Not actually necessary

    printf("Starting.\n");

    if (!CreateProcessA(NULL, cmdline, NULL, NULL, TRUE,
        CREATE_NO_WINDOW | CREATE_UNICODE_ENVIRONMENT, NULL, 0, &startupInfo, &processInfo))
    {
        printf("CreateProcessA: %u\n", GetLastError());
        return;
    }

    CloseHandle(stdoutWriteHandle);

    strcpy_s(outbuf, sizeof(outbuf), "");

    for (;;) {
        printf("Just before ReadFile(...)\n");
        if (!ReadFile(stdoutReadHandle, tBuf, 256, &bytes_read, NULL))
        {
            printf("ReadFile: %u\n", GetLastError());
            break;
        }
        printf("Just after ReadFile, read %u byte(s)\n", bytes_read);
        if (bytes_read > 0)
        {
            tBuf[bytes_read] = '\0';
            strcat_s(outbuf, sizeof(outbuf), tBuf);
        }
    }

    printf("Output: %s\n", outbuf);

    if (WaitForSingleObject(processInfo.hProcess, INFINITE) != WAIT_OBJECT_0)
    {
        printf("WaitForSingleObject: %u\n", GetLastError());
        return;
    }

    if (!GetExitCodeProcess(processInfo.hProcess, &exitcode))
    {
        printf("GetExitCodeProcess: %u\n", GetLastError());
        return;
    }

    printf("Exit code: %u\n", exitcode);

    CloseHandle( processInfo.hProcess );
    CloseHandle( processInfo.hThread );

    return;
}

int main(int argc, char** argv)
{
    launch("C:\\windows\\system32\\help.exe");
    return 0;
}

Answer 2

There is an "LPOVERLAPPED lpOverlapped" parameter to ReadFile() which you have set to NULL. Looks like the only way to go is to allow overlapped I/O on your pipe and then use the WaitForSingleObject() for the "overlapped.hEvent".

Another way is to use the ConnectNamedPipe function and create the OVERLAPPED struct for the pipe.