qtmain.lib 分析

本文所使用的qt版本为5.7.0，编译器为vs2013

背景

在vc下使用qt的库时候，会发现有一个qtmain.lib的静态库，其他的qt库都是以动态库的形式提供的，只有这个是以静态库提供的，并且发现只有windows下的库会有这个，linux的库里并没有相应的库。
那么这个库是必须使用的吗。在使用时会发现，有时候不链接这个库就会造成链接失败，但是有时候不链接这个库也没什么问题。
我就在这篇文章讨论一下他的作用和原理。

作用

要想知道他的作用，首先看官方文档：

qtmain is a helper library that enables the developer to write a cross-platform main() function on Windows. If you do not use qmake or other build tools such as CMake, then you need to link against the qtmain library.
以上是官方文档对qtmain这个库介绍的原文，大意如下：
qtmain这个库是一个用来使得开发者可以在windows平台上写出跨平台的main函数的辅助库。

什么意思呢？众所周知，windows窗口程序的入口函数是WinMain函数，但是linux程序的入口函数是main。那么如果要写出一个跨平台的窗口程序程序，那么就必须对入口函数分别做处理，通常的方法是在windows平台的上写一个WinMain函数，并在里面调用main函数。注意只有窗口程序才会有这个问题，windows的控制台程序就没有这个问题了。

而qtmain库就是帮助开发者自动处理这种问题的一个库，qtmain库会包含一个WinMain函数，在里面自动调用main函数并转发命令参数。而其实现原理正是上文所说的通用做法。

源码分析

qtmain库的源码非常少，就在以qtmain开头的cpp文件里。后缀是对应不同的windows的的平台。这里以qtmain_win.cpp为例分析。

/****************************************************************************
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#include "qt_windows.h"
#include "qbytearray.h"
#include "qstring.h"
#include "qvector.h"

#ifndef Q_OS_WINCE
#  include <shlobj.h>
#endif

/*
  This file contains the code in the qtmain library for Windows.
  qtmain contains the Windows startup code and is required for
  linking to the Qt DLL.

  When a Windows application starts, the WinMain function is
  invoked. WinMain calls qWinMain in the Qt DLL/library, which
  initializes Qt.
*/

QT_BEGIN_NAMESPACE

#if defined(Q_OS_WINCE)
extern void __cdecl qWinMain(HINSTANCE, HINSTANCE, LPSTR, int, int &, QVector<char *> &);
#else
extern void qWinMain(HINSTANCE, HINSTANCE, LPSTR, int, int &, QVector<char *> &);
#endif

QT_END_NAMESPACE

QT_USE_NAMESPACE


#if defined(QT_NEEDS_QMAIN)
int qMain(int, char **);
#define main qMain
#else
#ifdef Q_OS_WINCE
extern "C" int __cdecl main(int, char **);
#else
extern "C" int main(int, char **);
#endif
#endif

/*
  WinMain() - Initializes Windows and calls user's startup function main().
  NOTE: WinMain() won't be called if the application was linked as a "console"
  application.
*/

#ifndef Q_OS_WINCE

// Convert a wchar_t to char string, equivalent to QString::toLocal8Bit()
// when passed CP_ACP.
static inline char *wideToMulti(int codePage, const wchar_t *aw)
{
    const int required = WideCharToMultiByte(codePage, 0, aw, -1, NULL, 0, NULL, NULL);
    char *result = new char[required];
    WideCharToMultiByte(codePage, 0, aw, -1, result, required, NULL, NULL);
    return result;
}

extern "C" int APIENTRY WinMain(HINSTANCE, HINSTANCE, LPSTR /*cmdParamarg*/, int /* cmdShow */)
{
    int argc;
    wchar_t **argvW = CommandLineToArgvW(GetCommandLineW(), &argc);
    if (!argvW)
        return -1;
    char **argv = new char *[argc + 1];
    for (int i = 0; i < argc; ++i)
        argv[i] = wideToMulti(CP_ACP, argvW[i]);
    argv[argc] = Q_NULLPTR;
    LocalFree(argvW);
    const int exitCode = main(argc, argv);
    for (int i = 0; i < argc && argv[i]; ++i)
        delete [] argv[i];
    delete [] argv;
    return exitCode;
}

#else // !Q_OS_WINCE

int WINAPI WinMain(HINSTANCE instance, HINSTANCE prevInstance, LPWSTR /*wCmdParam*/, int cmdShow)
{
    QByteArray cmdParam = QString::fromWCharArray(GetCommandLine()).toLocal8Bit();

    wchar_t appName[MAX_PATH];
    GetModuleFileName(0, appName, MAX_PATH);
    cmdParam.prepend(QString(QLatin1String("\"%1\" ")).arg(QString::fromWCharArray(appName)).toLocal8Bit());

    int argc = 0;
    QVector<char *> argv(8);
    qWinMain(instance, prevInstance, cmdParam.data(), cmdShow, argc, argv);

    wchar_t uniqueAppID[MAX_PATH];
    GetModuleFileName(0, uniqueAppID, MAX_PATH);
    QString uid = QString::fromWCharArray(uniqueAppID).toLower().replace(QLatin1String("\\"), QLatin1String("_"));

    // If there exists an other instance of this application
    // it will be the owner of a mutex with the unique ID.
    HANDLE mutex = CreateMutex(NULL, TRUE, (LPCWSTR)uid.utf16());
    if (mutex && ERROR_ALREADY_EXISTS == GetLastError()) {
        CloseHandle(mutex);

        // The app is already running, so we use the unique
        // ID to create a unique messageNo, which is used
        // as the registered class name for the windows
        // created. Set the first instance's window to the
        // foreground, else just terminate.
        // Use bitwise 0x01 OR to reactivate window state if
        // it was hidden
        UINT msgNo = RegisterWindowMessage((LPCWSTR)uid.utf16());
        HWND aHwnd = FindWindow((LPCWSTR)QString::number(msgNo).utf16(), 0);
        if (aHwnd)
            SetForegroundWindow((HWND)(((ULONG)aHwnd) | 0x01));
        return 0;
    }

    int result = main(argc, argv.data());
    CloseHandle(mutex);
    return result;
}

#endif // Q_OS_WINCE

首先可以看到根据Q_OS_WINCE宏的定义有两种实现，从名字上可以看出是关于winCE平台上的。我们只讨论没有定义Q_OS_WINCE宏的实现。

主要需要关注的内容有以下几点：

extern "C" int main(int, char **);
这是一个main函数的外部声明，因为真正的main函数肯定是写在别的模块中的，所以这里必须先声明。

wideToMulti函数是一个宽字符转换工具函数，作用是把宽字符转为窄字符。

WinMain函数内容就是关键了，可以看到首先获取命令行参数，然后转换字符串编码，接着用转换后的参数调用main函数。
得到返回值后清理内存，继续将得到的返回值返回。

这就是qtmain的工作原理，并没有什么复杂的东西。

结论

看过了他的原理和实现后我们可以总结出以下几点：
- 当程序类型为windows窗口程序的时候，自己实现了入口函数为main函数，那么直接链接qtmain库即可。
- 当程序类型为控制台程序的时候，不需要链接链接qtmain库。但是链接了也不会出错。
- 当程序类型为windows窗口程序的时候，但是自己实现了入口函数WinMain，不要链接链接qtmain库。链接了可能会造成重定义错误。
- 当程序类型为windows窗口程序的时候，自己实现了入口函数为main函数，又不想链接qtmain库，将qtmain库中的WinMain函数内容照抄一遍和main函数写在一起即可。