如何读取子进程的输出？

Question

我编写了一个函数,试图通过管道读取子进程的命令行输出.这应该是MSDN创建具有重定向输入和输出的子进程的一个简单子集,但我显然是在做某种错误.

无论是在WaitForSingleObject(...)调用之前还是之后将ReadFile(...)调用放在子进程结束时,下面的ReadFile(...)调用都会永久阻塞.

我已经阅读了所有建议"使用异步ReadFile"的答案,如果有人能让我知道如何在管道上完成,我对这个建议持开放态度.虽然我不明白为什么在这种情况下需要异步I/O.

#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

您的代码中存在一些错误,但最重要的是您已FALSE为bInheritHandles参数指定了错误CreateProcess.如果新进程没有继承它的句柄,则它不能使用该管道.为了继承句柄,bInheritHandles参数必须是TRUE ,并且句柄必须启用继承.

其他问题:

• 您正在指定CREATE_UNICODE_ENVIRONMENT但是传递ANSI环境块.需要注意的是容易通过NULL的lpEnvironment,并让系统复制环境块给你.即使您在Unicode模式下进行编译,在这种情况下也不需要指定CREATE_UNICODE_ENVIRONMENT.

• 同样,如果您正在调用CreateProcessA,那么您应该使用STARTUPINFOA.

• tBuf每次循环都没有零终止,所以你会在输出缓冲区中得到虚假的额外字符.

• 您需要stdoutWriteHandle在进入读取循环之前关闭,否则您将不知道子进程何时退出.(或者您可以使用异步IO并明确检查进程退出.)

• GetLastError()如果API函数成功,则未定义,因此只应在ReadFile返回时调用它FALSE.(当然,在这种情况下,这纯粹是装饰性的,因为您没有对错误代码采取行动.)

作为参考,这是我的代码的更正版本.我把它变成了普通的C(对不起!),因为那是我熟悉的.我在Unicode模式下编译和测试,但我认为它也可以在ANSI模式下无需修改.

#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, 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;
}