有没有办法将 stdout/stderr 重定向到字符串？

问题描述 投票：0回答：6

之前有很多关于将 stdout/stderr 重定向到文件的问题。有没有办法将 stdout/stderr 重定向到字符串？

c++ c stdout stderr
6个回答
74
投票

是的，您可以将其重定向到

std::stringstream
：

std::stringstream buffer;
std::streambuf * old = std::cout.rdbuf(buffer.rdbuf());

std::cout << "Bla" << std::endl;

std::string text = buffer.str(); // text will now contain "Bla\n"

您可以使用一个简单的保护类来确保缓冲区始终重置：

struct cout_redirect {
    cout_redirect( std::streambuf * new_buffer ) 
        : old( std::cout.rdbuf( new_buffer ) )
    { }

    ~cout_redirect( ) {
        std::cout.rdbuf( old );
    }

private:
    std::streambuf * old;
};
45
投票

你可以使用这个类：

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string>

class StdCapture
{
public:
    StdCapture(): m_capturing(false), m_init(false), m_oldStdOut(0), m_oldStdErr(0)
    {
        m_pipe[READ] = 0;
        m_pipe[WRITE] = 0;
        if (_pipe(m_pipe, 65536, O_BINARY) == -1)
            return;
        m_oldStdOut = dup(fileno(stdout));
        m_oldStdErr = dup(fileno(stderr));
        if (m_oldStdOut == -1 || m_oldStdErr == -1)
            return;

        m_init = true;
    }

    ~StdCapture()
    {
        if (m_capturing)
        {
            EndCapture();
        }
        if (m_oldStdOut > 0)
            close(m_oldStdOut);
        if (m_oldStdErr > 0)
            close(m_oldStdErr);
        if (m_pipe[READ] > 0)
            close(m_pipe[READ]);
        if (m_pipe[WRITE] > 0)
            close(m_pipe[WRITE]);
    }


    void BeginCapture()
    {
        if (!m_init)
            return;
        if (m_capturing)
            EndCapture();
        fflush(stdout);
        fflush(stderr);
        dup2(m_pipe[WRITE], fileno(stdout));
        dup2(m_pipe[WRITE], fileno(stderr));
        m_capturing = true;
    }

    bool EndCapture()
    {
        if (!m_init)
            return false;
        if (!m_capturing)
            return false;
        fflush(stdout);
        fflush(stderr);
        dup2(m_oldStdOut, fileno(stdout));
        dup2(m_oldStdErr, fileno(stderr));
        m_captured.clear();

        std::string buf;
        const int bufSize = 1024;
        buf.resize(bufSize);
        int bytesRead = 0;
        if (!eof(m_pipe[READ]))
        {
            bytesRead = read(m_pipe[READ], &(*buf.begin()), bufSize);
        }
        while(bytesRead == bufSize)
        {
            m_captured += buf;
            bytesRead = 0;
            if (!eof(m_pipe[READ]))
            {
                bytesRead = read(m_pipe[READ], &(*buf.begin()), bufSize);
            }
        }
        if (bytesRead > 0)
        {
            buf.resize(bytesRead);
            m_captured += buf;
        }
        m_capturing = false;
        return true;
    }

    std::string GetCapture() const
    {
        std::string::size_type idx = m_captured.find_last_not_of("\r\n");
        if (idx == std::string::npos)
        {
            return m_captured;
        }
        else
        {
            return m_captured.substr(0, idx+1);
        }
    }

private:
    enum PIPES { READ, WRITE };
    int m_pipe[2];
    int m_oldStdOut;
    int m_oldStdErr;
    bool m_capturing;
    bool m_init;
    std::string m_captured;
};

需要开始捕捉时拨打

BeginCapture()

当您需要停止拍摄时拨打
EndCapture()

调用 
GetCapture()
检索捕获的输出

20
投票

为了提供线程安全和跨平台的解决方案，我将 rmflow 的方法改编为类似的接口。由于此类修改全局文件描述符，我将其改编为互斥锁保护的静态类，以防止多个实例对全局文件描述符造成影响。此外，rmflow 的答案不会清除所有已使用的文件描述符，如果在一个应用程序中使用许多 BeginCapture() 和 EndCapture() 调用，这可能会导致打开新文件描述符（对于输出流或文件）时出现问题。此代码已在 Windows 7/8、Linux、OSX、Android 和 iOS 上进行了测试。

注意： 为了使用 std::mutex，你必须针对 c++ 11 进行编译。如果你不/不能使用 c++11，你可以完全删除互斥调用（牺牲线程安全），或者你可以找到遗留的同步机制来完成工作。 

#ifdef _MSC_VER
#include <io.h>
#define popen _popen 
#define pclose _pclose
#define stat _stat 
#define dup _dup
#define dup2 _dup2
#define fileno _fileno
#define close _close
#define pipe _pipe
#define read _read
#define eof _eof
#else
#include <unistd.h>
#endif
#include <fcntl.h>
#include <stdio.h>
#include <mutex>

class StdCapture
{
public:
    static void Init()
    {
        // make stdout & stderr streams unbuffered
        // so that we don't need to flush the streams
        // before capture and after capture 
        // (fflush can cause a deadlock if the stream is currently being 
        std::lock_guard<std::mutex> lock(m_mutex);
        setvbuf(stdout,NULL,_IONBF,0);
        setvbuf(stderr,NULL,_IONBF,0);
    }

    static void BeginCapture()
    {
        std::lock_guard<std::mutex> lock(m_mutex);
        if (m_capturing)
            return;

        secure_pipe(m_pipe);
        m_oldStdOut = secure_dup(STD_OUT_FD);
        m_oldStdErr = secure_dup(STD_ERR_FD);
        secure_dup2(m_pipe[WRITE],STD_OUT_FD);
        secure_dup2(m_pipe[WRITE],STD_ERR_FD);
        m_capturing = true;
#ifndef _MSC_VER
        secure_close(m_pipe[WRITE]);
#endif
    }
    static bool IsCapturing()
    {
        std::lock_guard<std::mutex> lock(m_mutex);
        return m_capturing;
    }
    static bool EndCapture()
    {
        std::lock_guard<std::mutex> lock(m_mutex);
        if (!m_capturing)
            return;

        m_captured.clear();
        secure_dup2(m_oldStdOut, STD_OUT_FD);
        secure_dup2(m_oldStdErr, STD_ERR_FD);

        const int bufSize = 1025;
        char buf[bufSize];
        int bytesRead = 0;
        bool fd_blocked(false);
        do
        {
            bytesRead = 0;
            fd_blocked = false;
#ifdef _MSC_VER
            if (!eof(m_pipe[READ]))
                bytesRead = read(m_pipe[READ], buf, bufSize-1);
#else
            bytesRead = read(m_pipe[READ], buf, bufSize-1);
#endif
            if (bytesRead > 0)
            {
                buf[bytesRead] = 0;
                m_captured += buf;
            }
            else if (bytesRead < 0)
            {
                fd_blocked = (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR);
                if (fd_blocked)
                    std::this_thread::sleep_for(std::chrono::milliseconds(10));
            }
        }
        while(fd_blocked || bytesRead == (bufSize-1));

        secure_close(m_oldStdOut);
        secure_close(m_oldStdErr);
        secure_close(m_pipe[READ]);
#ifdef _MSC_VER
        secure_close(m_pipe[WRITE]);
#endif
        m_capturing = false;
    }
    static std::string GetCapture()
    {
        std::lock_guard<std::mutex> lock(m_mutex);
        return m_captured;
    }
private:
    enum PIPES { READ, WRITE };

    int StdCapture::secure_dup(int src)
    {
        int ret = -1;
        bool fd_blocked = false;
        do
        {
             ret = dup(src);
             fd_blocked = (errno == EINTR ||  errno == EBUSY);
             if (fd_blocked)
                std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
        while (ret < 0);
        return ret;
    }
    void StdCapture::secure_pipe(int * pipes)
    {
        int ret = -1;
        bool fd_blocked = false;
        do
        {
#ifdef _MSC_VER
            ret = pipe(pipes, 65536, O_BINARY);
#else
            ret = pipe(pipes) == -1;
#endif
            fd_blocked = (errno == EINTR ||  errno == EBUSY);
            if (fd_blocked)
                std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
        while (ret < 0);
    }
    void StdCapture::secure_dup2(int src, int dest)
    {
        int ret = -1;
        bool fd_blocked = false;
        do
        {
             ret = dup2(src,dest);
             fd_blocked = (errno == EINTR ||  errno == EBUSY);
             if (fd_blocked)
                std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
        while (ret < 0);
    }

    void StdCapture::secure_close(int & fd)
    {
        int ret = -1;
        bool fd_blocked = false;
        do
        {
             ret = close(fd);
             fd_blocked = (errno == EINTR);
             if (fd_blocked)
                std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
        while (ret < 0);

        fd = -1;
    }

    static int m_pipe[2];
    static int m_oldStdOut;
    static int m_oldStdErr;
    static bool m_capturing;
    static std::mutex m_mutex;
    static std::string m_captured;
};

// actually define vars.
int StdCapture::m_pipe[2];
int StdCapture::m_oldStdOut;
int StdCapture::m_oldStdErr;
bool StdCapture::m_capturing;
std::mutex StdCapture::m_mutex;
std::string StdCapture::m_captured;

调用 

Init()
一次（在捕获之前）以删除对 stdout / stderr 的缓冲

需要开始捕捉时拨打

BeginCapture()

需要停止拍摄时拨打

EndCapture()

调用 

GetCapture()
检索捕获的输出

调用 

IsCapturing()
查看 stdout/stderr 当前是否已重定向

6
投票

我已经提供了 Björn Pollex 代码的 qt osx 就绪变体

#include <stdio.h>
#include <iostream>
#include <streambuf>
#include <stdlib.h>
#include <string>
#include <sstream>

class CoutRedirect {

public:
    CoutRedirect() {
        old = std::cout.rdbuf( buffer.rdbuf() ); // redirect cout to buffer stream
    }

    std::string getString() {
        return buffer.str(); // get string
    }

    ~CoutRedirect( ) {
        std::cout.rdbuf( old ); // reverse redirect
    }

private:
    std::stringstream buffer;
    std::streambuf * old;
};
6
投票

我修改了

Sir Digby Chicken Caesar
中的类，使其不是静态的并且可以在单元测试中轻松使用。它在 Windows 上适用于 gcc (g++) 编译，但我不能保证它是 100% 正确的，如果不是请留下评论。

创建 StdCapture 类的对象，只需调用 

BeginCapture()
即可开始捕获，并在结束时调用 
EndCapture()
。 
Init()
中的代码已移至构造函数。一次只能有一个这样的对象在工作。

StdCapture.h：

#ifdef _MSC_VER
#include <io.h>
#define popen _popen 
#define pclose _pclose
#define stat _stat 
#define dup _dup
#define dup2 _dup2
#define fileno _fileno
#define close _close
#define pipe _pipe
#define read _read
#define eof _eof
#else
#include <unistd.h>
#endif
#include <fcntl.h>
#include <stdio.h>
#include <mutex>
#include <chrono>
#include <thread>

#ifndef STD_OUT_FD 
#define STD_OUT_FD (fileno(stdout)) 
#endif 

#ifndef STD_ERR_FD 
#define STD_ERR_FD (fileno(stderr)) 
#endif

class StdCapture
{
public:

    StdCapture();

    void BeginCapture();
    bool IsCapturing();
    bool EndCapture();
    std::string GetCapture();

private:
    enum PIPES { READ, WRITE };
    
    int secure_dup(int src);
    void secure_pipe(int * pipes);
    void secure_dup2(int src, int dest);
    void secure_close(int & fd);

    int m_pipe[2];
    int m_oldStdOut;
    int m_oldStdErr;
    bool m_capturing;
    std::mutex m_mutex;
    std::string m_captured;
};

StdCapture.cpp：

#include "StdCapture.h"

StdCapture::StdCapture():
    m_capturing(false)
{
    // make stdout & stderr streams unbuffered
    // so that we don't need to flush the streams
    // before capture and after capture 
    // (fflush can cause a deadlock if the stream is currently being 
    std::lock_guard<std::mutex> lock(m_mutex);
    setvbuf(stdout,NULL,_IONBF,0);
    setvbuf(stderr,NULL,_IONBF,0);
}

void StdCapture::BeginCapture()
{
    std::lock_guard<std::mutex> lock(m_mutex);
    if (m_capturing)
        return;

    secure_pipe(m_pipe);
    m_oldStdOut = secure_dup(STD_OUT_FD);
    m_oldStdErr = secure_dup(STD_ERR_FD);
    secure_dup2(m_pipe[WRITE],STD_OUT_FD);
    secure_dup2(m_pipe[WRITE],STD_ERR_FD);
    m_capturing = true;
#ifndef _MSC_VER
    secure_close(m_pipe[WRITE]);
#endif
}
bool StdCapture::IsCapturing()
{
    std::lock_guard<std::mutex> lock(m_mutex);
    return m_capturing;
}
bool StdCapture::EndCapture()
{
    std::lock_guard<std::mutex> lock(m_mutex);
    if (!m_capturing)
        return true;

    m_captured.clear();
    secure_dup2(m_oldStdOut, STD_OUT_FD);
    secure_dup2(m_oldStdErr, STD_ERR_FD);

    const int bufSize = 1025;
    char buf[bufSize];
    int bytesRead = 0;
    bool fd_blocked(false);
    do
    {
        bytesRead = 0;
        fd_blocked = false;
#ifdef _MSC_VER
        if (!eof(m_pipe[READ]))
            bytesRead = read(m_pipe[READ], buf, bufSize-1);
#else
        bytesRead = read(m_pipe[READ], buf, bufSize-1);
#endif
        if (bytesRead > 0)
        {
            buf[bytesRead] = 0;
            m_captured += buf;
        }
        else if (bytesRead < 0)
        {
            fd_blocked = (errno == EAGAIN || errno == EWOULDBLOCK || errno == EINTR);
            if (fd_blocked)
                std::this_thread::sleep_for(std::chrono::milliseconds(10));
        }
    }
    while(fd_blocked || bytesRead == (bufSize-1));

    secure_close(m_oldStdOut);
    secure_close(m_oldStdErr);
    secure_close(m_pipe[READ]);
#ifdef _MSC_VER
    secure_close(m_pipe[WRITE]);
#endif
    m_capturing = false;
    return true;
}
std::string StdCapture::GetCapture()
{
    std::lock_guard<std::mutex> lock(m_mutex);
    return m_captured;
}

int StdCapture::secure_dup(int src)
{
    int ret = -1;
    bool fd_blocked = false;
    do
    {
         ret = dup(src);
         fd_blocked = (errno == EINTR ||  errno == EBUSY);
         if (fd_blocked)
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
    while (ret < 0);
    return ret;
}
void StdCapture::secure_pipe(int * pipes)
{
    int ret = -1;
    bool fd_blocked = false;
    do
    {
#ifdef _MSC_VER
        ret = pipe(pipes, 65536, O_BINARY);
#else
        ret = pipe(pipes) == -1;
#endif
        fd_blocked = (errno == EINTR ||  errno == EBUSY);
        if (fd_blocked)
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
    while (ret < 0);
}
void StdCapture::secure_dup2(int src, int dest)
{
    int ret = -1;
    bool fd_blocked = false;
    do
    {
         ret = dup2(src,dest);
         fd_blocked = (errno == EINTR ||  errno == EBUSY);
         if (fd_blocked)
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
    while (ret < 0);
}

void StdCapture::secure_close(int & fd)
{
    int ret = -1;
    bool fd_blocked = false;
    do
    {
         ret = close(fd);
         fd_blocked = (errno == EINTR);
         if (fd_blocked)
            std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
    while (ret < 0);

    fd = -1;
}
4
投票

由于您的问题被标记为 C 和 C++，因此似乎应该提到，尽管您无法将字符串与标准 C 中的 FILE * 关联起来，但有几个非标准库允许这样做。 glibc 几乎是标准的，因此您可能会非常高兴使用 fmemopen() 请参阅 http://www.gnu.org/s/libc/manual/html_mono/libc.html#String-Streams

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