qtmain.lib 分析
本文所使用的qt版本為5.7.0，編譯器為vs2013
背景
在vc下使用qt的庫時候，會發現有一個qtmain.lib的靜態庫，其他的qt庫都是以動態庫的形式提供的，只有這個是以靜態庫提供的，并且發現只有windows下的庫會有這個，linux的庫里并沒有相應的庫。 那么這個庫是必須使用的嗎。在使用時會發現，有時候不鏈接這個庫就會造成鏈接失敗，但是有時候不鏈接這個庫也沒什么問題。 我就在這篇文章討論一下他的作用和原理。
作用
要想知道他的作用，首先看官方文檔：
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. 以上是官方文檔對qtmain這個庫介紹的原文，大意如下： qtmain這個庫是一個用來使得開發者可以在windows平臺上寫出跨平臺的main函數的輔助庫。
什么意思呢？眾所周知，windows窗口程序的入口函數是WinMain函數，但是linux程序的入口函數是main。那么如果要寫出一個跨平臺的窗口程序程序，那么就必須對入口函數分別做處理，通常的方法是在windows平臺的上寫一個WinMain函數，并在里面調用main函數。注意只有窗口程序才會有這個問題，windows的控制臺程序就沒有這個問題了。
而qtmain庫就是幫助開發者自動處理這種問題的一個庫，qtmain庫會包含一個WinMain函數，在里面自動調用main函數并轉發命令參數。而其實現原理正是上文所說的通用做法。
qtmain庫的源碼非常少，就在以qtmain開頭的cpp文件里。后綴是對應不同的windows的的平臺。這里以qtmain_win.cpp為例分析。
/******************************************************************************** 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("http://"), 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首先可以看到根據Q_OS_WINCE宏的定義有兩種實現，從名字上可以看出是關于winCE平臺上的。我們只討論沒有定義Q_OS_WINCE宏的實現。
主要需要關注的內容有以下幾點：
extern "C" int main(int, char **); 這是一個main函數的外部聲明，因為真正的main函數肯定是寫在別的模塊中的，所以這里必須先聲明。
wideToMulti函數是一個寬字符轉換工具函數，作用是把寬字符轉為窄字符。
WinMain函數內容就是關鍵了，可以看到首先獲取命令行參數，然后轉換字符串編碼，接著用轉換后的參數調用main函數。 得到返回值后清理內存，繼續將得到的返回值返回。
這就是qtmain的工作原理，并沒有什么復雜的東西。
結論
看過了他的原理和實現后我們可以總結出以下幾點： - 當程序類型為windows窗口程序的時候，自己實現了入口函數為main函數，那么直接鏈接qtmain庫即可。 - 當程序類型為控制臺程序的時候，不需要鏈接鏈接qtmain庫。但是鏈接了也不會出錯。 - 當程序類型為windows窗口程序的時候，但是自己實現了入口函數WinMain，不要鏈接鏈接qtmain庫。鏈接了可能會造成重定義錯誤。 - 當程序類型為windows窗口程序的時候，自己實現了入口函數為main函數，又不想鏈接qtmain庫，將qtmain庫中的WinMain函數內容照抄一遍和main函數寫在一起即可。
