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(kù)時(shí)候，會(huì)發(fā)現(xiàn)有一個(gè)qtmain.lib的靜態(tài)庫(kù)，其他的qt庫(kù)都是以動(dòng)態(tài)庫(kù)的形式提供的，只有這個(gè)是以靜態(tài)庫(kù)提供的，并且發(fā)現(xiàn)只有windows下的庫(kù)會(huì)有這個(gè)，linux的庫(kù)里并沒有相應(yīng)的庫(kù)。 那么這個(gè)庫(kù)是必須使用的嗎。在使用時(shí)會(huì)發(fā)現(xiàn)，有時(shí)候不鏈接這個(gè)庫(kù)就會(huì)造成鏈接失敗，但是有時(shí)候不鏈接這個(gè)庫(kù)也沒什么問(wèn)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(duì)qtmain這個(gè)庫(kù)介紹的原文，大意如下： qtmain這個(gè)庫(kù)是一個(gè)用來(lái)使得開發(fā)者可以在windows平臺(tái)上寫出跨平臺(tái)的main函數(shù)的輔助庫(kù)。
什么意思呢？眾所周知，windows窗口程序的入口函數(shù)是WinMain函數(shù)，但是linux程序的入口函數(shù)是main。那么如果要寫出一個(gè)跨平臺(tái)的窗口程序程序，那么就必須對(duì)入口函數(shù)分別做處理，通常的方法是在windows平臺(tái)的上寫一個(gè)WinMain函數(shù)，并在里面調(diào)用main函數(shù)。注意只有窗口程序才會(huì)有這個(gè)問(wèn)題，windows的控制臺(tái)程序就沒有這個(gè)問(wèn)題了。
而qtmain庫(kù)就是幫助開發(fā)者自動(dòng)處理這種問(wèn)題的一個(gè)庫(kù)，qtmain庫(kù)會(huì)包含一個(gè)WinMain函數(shù)，在里面自動(dòng)調(diào)用main函數(shù)并轉(zhuǎn)發(fā)命令參數(shù)。而其實(shí)現(xiàn)原理正是上文所說(shuō)的通用做法。
qtmain庫(kù)的源碼非常少，就在以qtmain開頭的cpp文件里。后綴是對(duì)應(yīng)不同的windows的的平臺(tái)。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(jù)Q_OS_WINCE宏的定義有兩種實(shí)現(xiàn)，從名字上可以看出是關(guān)于winCE平臺(tái)上的。我們只討論沒有定義Q_OS_WINCE宏的實(shí)現(xiàn)。
主要需要關(guān)注的內(nèi)容有以下幾點(diǎn)：
extern "C" int main(int, char **); 這是一個(gè)main函數(shù)的外部聲明，因?yàn)檎嬲?code>main函數(shù)肯定是寫在別的模塊中的，所以這里必須先聲明。
wideToMulti函數(shù)是一個(gè)寬字符轉(zhuǎn)換工具函數(shù)，作用是把寬字符轉(zhuǎn)為窄字符。
WinMain函數(shù)內(nèi)容就是關(guān)鍵了，可以看到首先獲取命令行參數(shù)，然后轉(zhuǎn)換字符串編碼，接著用轉(zhuǎn)換后的參數(shù)調(diào)用main函數(shù)。 得到返回值后清理內(nèi)存，繼續(xù)將得到的返回值返回。
這就是qtmain的工作原理，并沒有什么復(fù)雜的東西。
結(jié)論
看過(guò)了他的原理和實(shí)現(xiàn)后我們可以總結(jié)出以下幾點(diǎn)： - 當(dāng)程序類型為windows窗口程序的時(shí)候，自己實(shí)現(xiàn)了入口函數(shù)為main函數(shù)，那么直接鏈接qtmain庫(kù)即可。 - 當(dāng)程序類型為控制臺(tái)程序的時(shí)候，不需要鏈接鏈接qtmain庫(kù)。但是鏈接了也不會(huì)出錯(cuò)。 - 當(dāng)程序類型為windows窗口程序的時(shí)候，但是自己實(shí)現(xiàn)了入口函數(shù)WinMain，不要鏈接鏈接qtmain庫(kù)。鏈接了可能會(huì)造成重定義錯(cuò)誤。 - 當(dāng)程序類型為windows窗口程序的時(shí)候，自己實(shí)現(xiàn)了入口函數(shù)為main函數(shù)，又不想鏈接qtmain庫(kù)，將qtmain庫(kù)中的WinMain函數(shù)內(nèi)容照抄一遍和main函數(shù)寫在一起即可。
