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。
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。
/******************************************************************************** Copyright (C) 2016 The Qt Company Ltd.** Contact: https://www.qt.io/licensing/**** This file is part of the Windows main function of the Qt Toolkit.**** $QT_BEGIN_LICENSE:BSD$** Commercial License Usage** Licensees holding valid commercial Qt licenses may use this file in** accordance with the commercial license agreement PRovided with the** Software or, alternatively, in accordance with the terms contained in** a written agreement between you and The Qt Company. For licensing terms** and conditions see https://www.qt.io/terms-conditions. For further** information use the contact form at https://www.qt.io/contact-us.**** BSD License Usage** Alternatively, you may use this file under the terms of the BSD license** as follows:**** "Redistribution and use in source and binary forms, with or without** modification, are permitted provided that the following conditions are** met:** * Redistributions of source code must retain the above copyright** notice, this list of conditions and the following disclaimer.** * Redistributions in binary form must reproduce the above copyright** notice, this list of conditions and the following disclaimer in** the documentation and/or other materials provided with the** distribution.** * Neither the name of The Qt Company Ltd nor the names of its** contributors may be used to endorse or promote products derived** from this software without specific prior written permission.****** THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS** "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT** LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR** A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT** OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,** SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT** LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,** DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY** THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT** (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE** OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE."**** $QT_END_LICENSE$******************************************************************************/#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 *> &);#elseextern void qWinMain(HINSTANCE, HINSTANCE, LPSTR, int, int &, QVector<char *> &);#endifQT_END_NAMESPACEQT_USE_NAMESPACE#if defined(QT_NEEDS_QMAIN)int qMain(int, char **);#define main qMain#else#ifdef Q_OS_WINCEextern "C" int __cdecl main(int, char **);#elseextern "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_WINCEint 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。
