放假在家，繼續調試《家園》。目前的進度是MinGW上的編譯鏈接都已通過，遊戲程序也已經可以跑起來並進入主菜單界面，但加載關卡之後就會閃退。這讓我想起了以前上中學時玩盜版遊戲的日子。那個年代的單機遊戲估計大多是用C/C++寫的，一個不小心的內存操作就會讓進程崩掉；而且那個年代的操作系統沒現在穩定，可能破解技術也不夠先進，從電腦城裏買來的五六塊錢的盜版遊戲質量參差不齊。很多遊戲跑着跑着就閃退，有的甚至連打都打不開，讓人甚爲惱火。如今源代碼在手，並且我也是程序員了，可以對閃退的原因一探究竟，再也不用怕。
不過讓人失望的是，用MinGW構建出的程序不會像Linux程序那樣在崩潰時吐核。還好這回的閃退是可以必現的，所以就在gdb中運行程序，看看它崩在什麼地方。
結果程序如預期崩潰後，調用棧成了下面這個樣子：
Program received signal SIGSEGV, Segmentation fault.
0x0ddcf5c0 in ?? ()
(gdb) bt
#0 0x0ddcf5c0 in ?? ()
#1 0xabababab in ?? ()
#2 0x0000abab in ?? ()
#3 0x00000000 in ?? ()
(gdb)
看樣子調用棧已經壞掉了。記得以前在老東家遇到過這種損壞的調用棧，但後來很喜感地發現原來是機器的內存壞了。我相信我家電腦還沒有到如此風燭殘年的地步。
在StackOverflow上搜到一篇帖子：http://stackoverflow.com/questions/9809810/gdb-corrupted-stack-frame-how-to-debug，正好是我想問的問題：如何在gdb中調試這種已經損壞的調用棧。帖子裏的答案說這種情況99%是因爲調用了非法的函數指針。在32位環境中可以用如下方法恢復調用棧：
(gdb) set $pc = *(void **)$esp
(gdb) set $esp = $esp + 4
可是我檢查了一下esp寄存器指向的內存：
(gdb) p $esp
$1 = (void *) 0x22fa34
(gdb) x $esp
0x22fa34: 0x0000007f
(gdb) x/i 0x7f
0x7f: Cannot access memory at address 0x7f
(gdb)
0x7f顯然不可能是一個合法的指令地址。看來我落到剩下那1%的區間裏了=A=。
把這事情分享到朋友圈裏後，主程建議我讓程序鏈接tcmalloc試試，看看能否讓程序在應用代碼進行非法內存操作時就崩潰，興許那時調用棧還沒損壞。可是試過後情況並無改變。不過這倒是提醒了我，以後不妨讓自己的程序都鏈接tcmalloc，這樣可以讓很多問題都提前暴露。順便寫下，我的tcmalloc鏈接選項是-L/local/lib -ltcmalloc_minimal -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -fno-builtin-free。
無奈，最後還是通過打日誌和單步調試的方法，通過應用代碼本身的邏輯定位到了具體崩潰位置。原來程序崩在了一個OpenGL接口——glDrawElements的調用。多留日誌和熟練掌握項目代碼邏輯真是重要啊。
如果是在工作中，我的排查工作一般在這一步也就該結束了。因爲我從未接觸過OpenGL，所以此時我應該讓有OpenGL經驗的同事來幫忙處理。不過這回不是在工作而是在玩耍，所以我打算滿足下自己用牛刀殺雞、用導彈打蚊子的癖好，好好研究一番，一是看看能否在gdb中恢復出調用棧，二是研究下glDrawElements這個調用爲啥會崩潰，趁機接觸下OpenGL。
調用棧的恢復
先看看崩潰的直接原因是什麼，看看崩潰時執行的彙編指令是什麼：
(gdb) x/i $pc
=> 0xddcf5c0: mov (%esi),%edi
看來esi寄存器裏存了一個非法內存地址。
(gdb) p/x $esi
$3 = 0xfeee4
(gdb) p *(void **)$esi
Cannot access memory at address 0xf00d4
(gdb)
果然如此。
再看看這之前還執行了什麼指令。
(gdb) x/40i $pc-90
0xc54f7a6: add %al,(%eax)
0xc54f7a8: add %al,(%eax)
0xc54f7aa: add %al,(%eax)
0xc54f7ac: add %al,(%eax)
0xc54f7ae: add %al,(%eax)
0xc54f7b0: add %al,(%eax)
0xc54f7b2: add %al,(%eax)
0xc54f7b4: add %al,(%eax)
0xc54f7b6: add %al,(%eax)
0xc54f7b8: cmp $0xff,%bh
0xc54f7bb: incl 0x550000f7(%eax)
0xc54f7c1: mov %esp,%ebp
0xc54f7c3: push %ebx
0xc54f7c4: push %esi
0xc54f7c5: push %edi
0xc54f7c6: mov 0x8(%ebp),%ebx
0xc54f7c9: mov 0xc(%ebp),%eax
0xc54f7cc: mov 0x14(%ebp),%ebp
0xc54f7cf: mov %ebp,%edi
0xc54f7d1: shl $0x14,%edi
0xc54f7d4: lea 0x40003640(%edi),%esi
0xc54f7da: mov %esi,(%eax)
0xc54f7dc: add $0x4,%eax
0xc54f7df: mov 0x1c(%esp),%edx
0xc54f7e3: lea (%edx,%ebp,2),%ebp
0xc54f7e6: mov %ebp,0x20(%esp)
0xc54f7ea: movzwl (%edx),%ecx
0xc54f7ed: add $0x2,%edx
0xc54f7f0: mov 0xc509890,%esi
0xc54f7f6: mov 0x4(%esi),%esi
0xc54f7f9: mov %ecx,%edi
0xc54f7fb: shl $0x4,%edi
0xc54f7fe: add %edi,%esi
=> 0xc54f800: mov (%esi),%edi
0xc54f802: mov 0x4(%esi),%ebp
0xc54f805: mov %edi,(%eax)
0xc54f807: mov %ebp,0x4(%eax)
0xc54f80a: mov 0x8(%esi),%edi
0xc54f80d: mov %edi,0x8(%eax)
0xc54f810: mov 0xc509890,%esi
(gdb)
看樣子在0xc54f7bb附近很可能有一個函數頭。函數開頭通常由兩條彙編指令組成——第一條指令保存當前棧幀的幀底地址，第二條指令將當前的棧頂指爲棧幀底，開啓新棧幀：
push %ebp
move %esp %ebp
於是從0xc54f7bc開始，一路用x命令檢查：
(gdb) x/40i 0xc54f7bc
...
(gdb) x/40i 0xc54f7bd
...
(gdb) x/40i 0xc54f7be
0xc54f7be: add %al,(%eax)
0xc54f7c0: push %ebp
0xc54f7c1: mov %esp,%ebp
0xc54f7c3: push %ebx
0xc54f7c4: push %esi
0xc54f7c5: push %edi
0xc54f7c6: mov 0x8(%ebp),%ebx
0xc54f7c9: mov 0xc(%ebp),%eax
0xc54f7cc: mov 0x14(%ebp),%ebp
0xc54f7cf: mov %ebp,%edi
0xc54f7d1: shl $0x14,%edi
0xc54f7d4: lea 0x40003640(%edi),%esi
0xc54f7da: mov %esi,(%eax)
0xc54f7dc: add $0x4,%eax
0xc54f7df: mov 0x1c(%esp),%edx
0xc54f7e3: lea (%edx,%ebp,2),%ebp
0xc54f7e6: mov %ebp,0x20(%esp)
0xc54f7ea: movzwl (%edx),%ecx
0xc54f7ed: add $0x2,%edx
0xc54f7f0: mov 0xc509890,%esi
0xc54f7f6: mov 0x4(%esi),%esi
0xc54f7f9: mov %ecx,%edi
0xc54f7fb: shl $0x4,%edi
0xc54f7fe: add %edi,%esi
=> 0xc54f800: mov (%esi),%edi
0xc54f802: mov 0x4(%esi),%ebp
0xc54f805: mov %edi,(%eax)
0xc54f807: mov %ebp,0x4(%eax)
0xc54f80a: mov 0x8(%esi),%edi
0xc54f80d: mov %edi,0x8(%eax)
0xc54f810: mov 0xc509890,%esi
0xc54f816: mov 0x54(%esi),%esi
0xc54f819: mov %ecx,%edi
0xc54f81b: shl $0x4,%edi
0xc54f81e: add %edi,%esi
0xc54f820: mov (%esi),%edi
0xc54f822: mov %edi,0xc(%eax)
0xc54f825: add $0x10,%eax
0xc54f828: cmp 0x20(%esp),%edx
0xc54f82c: jne 0xc54f7ea
(gdb)
果真如此。地址0xc54f7c0和0xc54f7c1這兩條指令就是典型的函數開頭：
0xc54f7c0: push %ebp
0xc54f7c1: mov %esp,%ebp
從這之後到崩潰處0xc54f800，有兩處修改ebp的指令：
0xc54f7cc: mov 0x14(%ebp),%ebp
...
0xc54f7e3: lea (%edx,%ebp,2),%ebp
因此在0xc54f800: mov (%esi),%edi 崩潰的時候，寄存器中記錄的就是錯誤的棧幀。也就是說，在gdb中查看的調用棧不正常是因爲ebp被篡改了。從這兩條指令還可以看出，崩潰時ebp的值取決於傳入函數的參數。
在指令0xc54f7c1: mov %esp,%ebp剛執行之後，esp的值和ebp的值是相等的。在這之後直到崩潰前，只有三條壓棧指令（0xc54f7c6至0xc54f7cc）會修改esp。它們會使esp自減3個word，即3*4=12字節。分析到這裏，就有辦法恢復ebp的值了：
(gdb) set $ebp = $esp + 12
通過檢查esp指向的內存段可以進一步確認：
(gdb) x/8x $esp
0x22fa34: 0x0000007f 0x0c4f0000 0x0cbb6660 0x0022fa70
0x22fa44: 0x69a84ce2 0x0c4f0000 0x0cd3e900 0x0cbb6660
(gdb)
可見棧在內存段0x22fa??附近，0x002fa70想必就是上一個棧幀的幀底，0x69a84ce2就是函數調用前的指令地址，也就是函數的返回地址。
現在可以看到正確的調用棧了：
(gdb) bt
#0 0x0c54f800 in ?? ()
#1 0x69a84ce2 in nvoglv32!DrvPresentBuffers () from C:\Windows\system32\nvoglv32.dll
#2 0x69a85ed6 in nvoglv32!DrvPresentBuffers () from C:\Windows\system32\nvoglv32.dll
#3 0x69a86214 in nvoglv32!DrvPresentBuffers () from C:\Windows\system32\nvoglv32.dll
#4 0x695f8988 in ?? () from C:\Windows\system32\nvoglv32.dll
#5 0x0049dd33 in btgRender () at ../../../src/Game/BTG.c:1307
#6 0x00417d0c in rndBackgroundRender (radius=100000, camera=0x9259c0 <universe+32>, bDrawStars=1) at ../../../src/SDL/render.c:1287
#7 0x00419c37 in rndMainViewRenderFunction (camera=0x9259c0 <universe+32>) at ../../../src/SDL/render.c:2440
#8 0x0040d730 in mrRegionDraw (reg=0xa66a0a0) at ../../../src/SDL/mainrgn.c:5509
#9 0x0056508f in regFunctionsDraw () at ../../../src/Game/Region.c:1094
#10 0x0041c47a in rndRenderTask (taskContextPtr=0xa666710) at ../../../src/SDL/render.c:3869
#11 0x005b0bc7 in taskExecuteAllPending (ticks=4) at ../../../src/Game/Task.c:370
#12 0x0042bb26 in utyTasksDispatch () at ../../../src/SDL/utility.c:4721
#13 0x00402df6 in HWSDL_main (argc=4, argv=0x59315d8) at ../../../src/SDL/main.c:2252
#14 0x004013e0 in main (argc=4, argv=0x59315d8) at ../../src/homeworld.c:32
(gdb)
崩潰的應用程序代碼（BTG.c:1307）與通過日誌和單步調試分析出的結果完全一致。
看樣子崩在了OpenGL內部。既然是崩在了glDrawElements裏面，那就要研究下glDrawElements的使用，想必是API使用不當。
glDrawElements的使用
從glDrawElements的官方文檔來看，這個接口的作用是批量繪製多個基本圖元（如點、線、三角形和多邊形）。不過這個接口並沒有參數可以直接傳入頂點數據，那個indices參數只是頂點數據的索引而已。這一點讓我花了很長時間琢磨。用glDrawElements作關鍵詞搜了很多文章，基本都能看懂，但還是不知道該如何從零開始用起來。
還是先把這函數放一放，從基本的OpenGL程序開始吧，先寫個Helloworld。從網上的文章得知現代OpenGL和過去的OpenGL 1.x在用法上似乎有很大不同；而《家園》是很老的遊戲了，早在1999年就已經發行，即便是HomeworldSDL的代碼也非常老，更新很緩慢。所以我恐怕還得學習老式OpenGL的用法。幸運的是從這裏搜到了Tutorial：http://en.wikibooks.org/wiki/OpenGL_Programming#Legacy_OpenGL_1.x。根據以下兩節教程寫出了一個Windows上運行的OpenGL小程序window.c：
http://en.wikibooks.org/wiki/OpenGL_Programming/GLStart/Tut1
http://en.wikibooks.org/wiki/OpenGL_Programming/GLStart/Tut2
#include <windows.h> #include <GL/gl.h> #include <GL/glu.h> HDC hDC; //device context HGLRC hglrc; //rendering context void SetupPixels(HDC hDC) { int pixelFormat; PIXELFORMATDESCRIPTOR pfd; pfd.nSize = sizeof(PIXELFORMATDESCRIPTOR); pfd.dwFlags = PFD_SUPPORT_OPENGL | PFD_DRAW_TO_WINDOW | PFD_DOUBLEBUFFER; pfd.nVersion = 1; pfd.iPixelType = PFD_TYPE_RGBA; pfd.cColorBits = 32; pfd.cDepthBits = 24; pixelFormat = ChoosePixelFormat(hDC, &pfd); if(!SetPixelFormat(hDC, pixelFormat, &pfd)) { MessageBox(NULL,"Error setting up Pixel Format","ERROR",MB_OK); PostQuitMessage(0); } } void Resize(int width, int height) { glViewport(0,0,(GLsizei)width,(GLsizei)height); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(45.0f,(GLfloat)width/(GLfloat)height,1.0f,1000.0f); glMatrixMode(GL_MODELVIEW); glLoadIdentity(); } void Render() { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glLoadIdentity(); glTranslatef(0.0f,0.0f,-4.0f); glColor3f(0.0f,0.0f,1.0f); glBegin(GL_POLYGON); glVertex3f(1.0f,1.0f,0.0f); glVertex3f(-1.0f,1.0f,0.0f); glVertex3f(-1.0f,-1.0f,0.0f); glVertex3f(1.0f,-1.0f,0.0f); glEnd(); } LRESULT CALLBACK WinProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam) { int w,h; switch(msg) { case WM_CREATE: hDC = GetDC(hWnd); SetupPixels(hDC); hglrc = wglCreateContext(hDC); wglMakeCurrent(hDC, hglrc); break; case WM_DESTROY: wglMakeCurrent(hDC,NULL); wglDeleteContext(hglrc); PostQuitMessage(0); break; case WM_SIZE: w = LOWORD(lParam); h = HIWORD(lParam); Resize(w,h); break; default: break; } return DefWindowProc(hWnd,msg,wParam,lParam); } int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd) { HWND hWnd; WNDCLASSEX wcex; MSG msg; wcex.cbSize = sizeof(WNDCLASSEX); wcex.style = CS_HREDRAW | CS_VREDRAW; wcex.lpfnWndProc = WinProc; wcex.cbClsExtra = 0; wcex.cbWndExtra = 0; wcex.hInstance = hInstance; wcex.hIcon = LoadIcon(NULL,IDI_APPLICATION); wcex.hCursor = LoadCursor(NULL,IDC_ARROW); wcex.hbrBackground = (HBRUSH) GetStockObject(GRAY_BRUSH); wcex.lpszMenuName = NULL; wcex.lpszClassName = "WinClass"; wcex.hIconSm = NULL; RegisterClassEx(&wcex); hWnd = CreateWindow("WinClass","My Window", WS_OVERLAPPEDWINDOW,0,0,400,400,NULL,NULL, hInstance,NULL); if(hWnd == NULL) { MessageBox(NULL,"Error: Unable to create Window","ERROR",MB_OK); return -1; } ShowWindow(hWnd,nShowCmd); UpdateWindow(hWnd); hDC = GetDC(hWnd); glClearColor(0.0f,0.0f,0.0f,0.0f); while(1) { Render(); SwapBuffers(hDC); if(PeekMessage(&msg,NULL,0,0,PM_REMOVE)) { if(msg.message == WM_QUIT) break; TranslateMessage(&msg); DispatchMessage(&msg); } } return 0; }
MinGW上的編譯命令：gcc -o window.exe window.c -mwindows -lopengl32 -lglu32
運行結果如下：
程序中關鍵的繪製代碼就是Render函數中從glBegin到glEnd的部分。
glBegin(GL_POLYGON); glVertex3f(1.0f,1.0f,0.0f); glVertex3f(-1.0f,1.0f,0.0f); glVertex3f(-1.0f,-1.0f,0.0f); glVertex3f(1.0f,-1.0f,0.0f); glEnd();
這段代碼繪製了四個頂點，從而繪製出一個正方形。這種用法是Tutorial 3中提到的Immediate Mode。看來如果要用glDrawElements，關鍵就是將這段代碼替換爲glDrawElements。
現在關鍵是要知道如何將頂點數據傳給glDrawElements。從HomeworldSDL的代碼和glDrawElements相關的資料中，我注意到兩個概念：Vertex Array Object（VAO）和Vertex Buffer Object（VBO）。Tutorial 3的末尾就簡略地提及了這兩個概念。簡要地說，VAO就是我們要傳給glDrawElements的頂點數據，這些數據是以數組形式存放的。glDrawElements被調用時可以從內存裏拿這個數據，也可以從顯存裏拿，後一種方式的性能更好。如果是從顯存裏拿數據，那glDrawElements拿的數據就是VBO。HomeworldSDL崩潰時用的就是VBO。具體說明可以參考OpenGL官方文檔中的Vertex Specification。
於是歸納了一下，採用VBO方式使用glDrawElements的步驟大致如下：
1、初始化，準備好頂點數據
1）用glGenBuffers申請Buffer Object的名字，也就是爲即將分配的顯存申請ID。
2）用glBindBuffer綁定Buffer Object，這樣在再次調用glBindBuffer之前，接下來的Buffer Object相關的操作都是針對當前綁定的Buffer Object。target參數需是GL_ARRAY_BUFFER。
3）用glBufferData分配並初始化一段顯存，將頂點數據傳進顯存。
4）再次調用glBindBuffer解綁Buffer Object。
2、準備好索引數組，步驟和1類似，只是調用glBindBuffer時target需是GL_ELEMENT_ARRAY_BUFFER。
3、調用glVertexPointer，指定頂點數據。
4、調用glDrawElements進行繪製。
5、如果所有繪製工作完成，之前的顯存不再需使用，就要調用glDeleteBuffers釋放顯存。
需要註明的是，以上只是一種簡單的使用VBO的方式，並不是說glDrawElements一定要嚴格按照這個流程。在頂點和索引數據都不會變化的情況下，3~4兩步可以反覆執行。索引數組也不一定要放在顯存裏，可以在調用glDrawElements的時候直接通過參數將內存中的數組傳進去，這時就不需要第2步。同理如果頂點數據也不用VBO的話，那第1步也省去了，glGenBuffers、glBindBuffer、glBufferData和glDeleteBuffers這幾個函數都不用調用。
很快，我的glDrawElements版本的Helloworld出爐了：
#include <windows.h> #include <GL/gl.h> #include <GL/glu.h> HDC hDC; //device context HGLRC hglrc; //rendering context #ifdef USE_VBO GLfloat transVerts[] = { 1.0f, 1.0f, 0.0f, -1.0f, 1.0f, 0.0f, -1.0f,-1.0f, 0.0f, 1.0f, -1.0f, 0.0f }; GLuint vboTransVerts; GLushort indices[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 }; GLuint vboIndices; #endif void SetupPixels(HDC hDC) { int pixelFormat; PIXELFORMATDESCRIPTOR pfd; pfd.nSize = sizeof(PIXELFORMATDESCRIPTOR); pfd.dwFlags = PFD_SUPPORT_OPENGL | PFD_DRAW_TO_WINDOW | PFD_DOUBLEBUFFER; pfd.nVersion = 1; pfd.iPixelType = PFD_TYPE_RGBA; pfd.cColorBits = 32; pfd.cDepthBits = 24; pixelFormat = ChoosePixelFormat(hDC, &pfd); if(!SetPixelFormat(hDC, pixelFormat, &pfd)) { MessageBox(NULL,"Error setting up Pixel Format","ERROR",MB_OK); PostQuitMessage(0); } } void Resize(int width, int height) { glViewport(0,0,(GLsizei)width,(GLsizei)height); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(45.0f,(GLfloat)width/(GLfloat)height,1.0f,1000.0f); glMatrixMode(GL_MODELVIEW); glLoadIdentity(); } #ifdef USE_VBO void InitVBO() { glGenBuffers(1, &vboTransVerts); glBindBuffer(GL_ARRAY_BUFFER, vboTransVerts); glBufferData(GL_ARRAY_BUFFER, sizeof(transVerts), transVerts, GL_STATIC_DRAW); glBindBuffer(GL_ARRAY_BUFFER, 0); glGenBuffers(1, &vboIndices); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vboIndices); glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0); } #endif void Render() { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glLoadIdentity(); glTranslatef(0.0f,0.0f,-4.0f); glColor3f(0.0f,0.0f,1.0f); #ifdef USE_VBO glEnableClientState(GL_VERTEX_ARRAY); glBindBuffer(GL_ARRAY_BUFFER, vboTransVerts); glVertexPointer(3, GL_FLOAT, 0, 0); glBindBuffer(GL_ARRAY_BUFFER, 0); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vboIndices); glDrawElements(GL_POLYGON, sizeof(transVerts) / sizeof(transVerts[0]), GL_UNSIGNED_SHORT, 0); #else glBegin(GL_POLYGON); glVertex3f(1.0f,1.0f,0.0f); glVertex3f(-1.0f,1.0f,0.0f); glVertex3f(-1.0f,-1.0f,0.0f); glVertex3f(1.0f,-1.0f,0.0f); glEnd(); #endif // USE_VBO } LRESULT CALLBACK WinProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam) { int w,h; switch(msg) { case WM_CREATE: hDC = GetDC(hWnd); SetupPixels(hDC); hglrc = wglCreateContext(hDC); wglMakeCurrent(hDC, hglrc); #ifdef USE_VBO InitVBO(); #endif break; case WM_DESTROY: #ifdef USE_VBO glDeleteBuffers(1, &vboTransVerts); glDeleteBuffers(1, &vboIndices); #endif wglMakeCurrent(hDC,NULL); wglDeleteContext(hglrc); PostQuitMessage(0); break; case WM_SIZE: w = LOWORD(lParam); h = HIWORD(lParam); Resize(w,h); break; default: break; } return DefWindowProc(hWnd,msg,wParam,lParam); } int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd) { HWND hWnd; WNDCLASSEX wcex; MSG msg; wcex.cbSize = sizeof(WNDCLASSEX); wcex.style = CS_HREDRAW | CS_VREDRAW; wcex.lpfnWndProc = WinProc; wcex.cbClsExtra = 0; wcex.cbWndExtra = 0; wcex.hInstance = hInstance; wcex.hIcon = LoadIcon(NULL,IDI_APPLICATION); wcex.hCursor = LoadCursor(NULL,IDC_ARROW); wcex.hbrBackground = (HBRUSH) GetStockObject(GRAY_BRUSH); wcex.lpszMenuName = NULL; wcex.lpszClassName = "WinClass"; wcex.hIconSm = NULL; RegisterClassEx(&wcex); hWnd = CreateWindow("WinClass","My Window", WS_OVERLAPPEDWINDOW,0,0,400,400,NULL,NULL, hInstance,NULL); if(hWnd == NULL) { MessageBox(NULL,"Error: Unable to create Window","ERROR",MB_OK); return -1; } ShowWindow(hWnd,nShowCmd); UpdateWindow(hWnd); hDC = GetDC(hWnd); glClearColor(0.0f,0.0f,0.0f,0.0f); while(1) { Render(); SwapBuffers(hDC); if(PeekMessage(&msg,NULL,0,0,PM_REMOVE)) { if(msg.message == WM_QUIT) break; TranslateMessage(&msg); DispatchMessage(&msg); } } return 0; }
MinGW上的編譯命令：gcc -o window.exe window.c -DUSE_VBO -mwindows -lopengl32 -lglu32
不過先別高興太早，報錯了：
$ gcc -o window.exe window.c -DUSE_VBO -mwindows -lopengl32 -lglu32
window.c: In function 'InitVBO':
window.c:60:18: error: 'GL_ARRAY_BUFFER' undeclared (first use in this function)
glBindBuffer(GL_ARRAY_BUFFER, vboTransVerts);
^
window.c:60:18: note: each undeclared identifier is reported only once for each function it appears in
window.c:61:67: error: 'GL_STATIC_DRAW' undeclared (first use in this function)
glBufferData(GL_ARRAY_BUFFER, sizeof(transVerts), transVerts, GL_STATIC_DRAW);
^
window.c:64:18: error: 'GL_ELEMENT_ARRAY_BUFFER' undeclared (first use in this function)
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vboIndices);
^
window.c: In function 'Render':
window.c:78:18: error: 'GL_ARRAY_BUFFER' undeclared (first use in this function)
glBindBuffer(GL_ARRAY_BUFFER, vboTransVerts);
^
window.c:81:18: error: 'GL_ELEMENT_ARRAY_BUFFER' undeclared (first use in this function)
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vboIndices);
原來GL/gl.h和GL/glu.h裏都沒有定義GL_ARRAY_BUFFER、GL_ELEMENT_ARRAY_BUFFER和GL_STATIC_DRAW。從兩篇StackOverflow的帖子裏得知，GL/gl.h（包括MinGW提供的）只提供了OpenGL 1.1（還是1.2？）的接口聲明。不僅如此，通過nm工具，發現libopengl32.a和opengl32.dll並未導出glGenBuffers等VBO接口。
http://stackoverflow.com/questions/11951380/lots-of-undeclared-identifiers-related-to-opengl-using-glfw
http://stackoverflow.com/questions/679113/trouble-porting-opengl-app-to-windows
於是我只好參考SDL的源代碼，寫出了可以工作的glDrawElements版Helloworld：
#include <windows.h> #include <GL/gl.h> #include <GL/glu.h> #ifdef USE_VBO // copied from SDL_opengl.h #define GL_ARRAY_BUFFER 0x8892 #define GL_ELEMENT_ARRAY_BUFFER 0x8893 #define GL_STATIC_DRAW 0x88E4 #endif HDC hDC; //device context HGLRC hglrc; //rendering context #ifdef USE_VBO typedef ptrdiff_t GLsizeiptr; //void * (WINAPI *wglGetProcAddress)(const char *proc); // May need this for VC++ GLAPI void (APIENTRY *glGenBuffers)(GLsizei, GLuint *); GLAPI void (APIENTRY *glBindBuffer)(GLenum, GLuint); GLAPI void (APIENTRY *glBufferData)(GLenum, GLsizeiptr, const GLvoid *, GLenum); GLAPI void (APIENTRY *glDeleteBuffers)(GLsizei, const GLuint *); GLfloat transVerts[] = { 1.0f, 1.0f, 0.0f, -1.0f, 1.0f, 0.0f, -1.0f,-1.0f, 0.0f, 1.0f, -1.0f, 0.0f }; GLuint vboTransVerts; GLushort indices[] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 }; GLuint vboIndices; #endif void SetupPixels(HDC hDC) { int pixelFormat; PIXELFORMATDESCRIPTOR pfd; pfd.nSize = sizeof(PIXELFORMATDESCRIPTOR); pfd.dwFlags = PFD_SUPPORT_OPENGL | PFD_DRAW_TO_WINDOW | PFD_DOUBLEBUFFER; pfd.nVersion = 1; pfd.iPixelType = PFD_TYPE_RGBA; pfd.cColorBits = 32; pfd.cDepthBits = 24; pixelFormat = ChoosePixelFormat(hDC, &pfd); if(!SetPixelFormat(hDC, pixelFormat, &pfd)) { MessageBox(NULL,"Error setting up Pixel Format","ERROR",MB_OK); PostQuitMessage(0); } } void Resize(int width, int height) { glViewport(0,0,(GLsizei)width,(GLsizei)height); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(45.0f,(GLfloat)width/(GLfloat)height,1.0f,1000.0f); glMatrixMode(GL_MODELVIEW); glLoadIdentity(); } #ifdef USE_VBO void InitVBO() { #if 0 // May need the following code for VC++ HMODULE hModule = LoadLibrary("opengl32.dll"); if (hModule == NULL) { MessageBox(NULL, "Error: Unable to load opengl32.dll", "ERROR", MB_OK); exit(-1); } wglGetProcAddress = (void * (WINAPI *)(const char *)) GetProcAddress(hModule, "wglGetProcAddress"); #endif glGenBuffers = (GLAPI void (APIENTRY *)(GLsizei, GLuint *)) wglGetProcAddress("glGenBuffers"); glBindBuffer = (GLAPI void (APIENTRY *)(GLenum, GLuint)) wglGetProcAddress("glBindBuffer"); glBufferData = (GLAPI void (APIENTRY *)(GLenum, GLsizeiptr, const GLvoid *, GLenum)) wglGetProcAddress("glBufferData"); glDeleteBuffers = (GLAPI void (APIENTRY *)(GLsizei, const GLuint *)) wglGetProcAddress("glDeleteBuffers"); glGenBuffers(1, &vboTransVerts); glBindBuffer(GL_ARRAY_BUFFER, vboTransVerts); glBufferData(GL_ARRAY_BUFFER, sizeof(transVerts), transVerts, GL_STATIC_DRAW); glBindBuffer(GL_ARRAY_BUFFER, 0); glGenBuffers(1, &vboIndices); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vboIndices); glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0); } #endif void Render() { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); glLoadIdentity(); glTranslatef(0.0f,0.0f,-4.0f); glColor3f(0.0f,0.0f,1.0f); #ifdef USE_VBO glEnableClientState(GL_VERTEX_ARRAY); glBindBuffer(GL_ARRAY_BUFFER, vboTransVerts); glVertexPointer(3, GL_FLOAT, 0, 0); glBindBuffer(GL_ARRAY_BUFFER, 0); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vboIndices); glDrawElements(GL_POLYGON, sizeof(transVerts) / sizeof(transVerts[0]), GL_UNSIGNED_SHORT, 0); //glDrawElements(GL_POLYGON, sizeof(transVerts) / sizeof(transVerts[0]), GL_UNSIGNED_SHORT, transIndices); // equivalent as above //glDrawArrays(GL_POLYGON, 0, sizeof(transVerts) / sizeof(transVerts[0])); // equivalent as above #else glBegin(GL_POLYGON); glVertex3f(1.0f,1.0f,0.0f); glVertex3f(-1.0f,1.0f,0.0f); glVertex3f(-1.0f,-1.0f,0.0f); glVertex3f(1.0f,-1.0f,0.0f); glEnd(); #endif // USE_VBO } LRESULT CALLBACK WinProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam) { int w,h; switch(msg) { case WM_CREATE: hDC = GetDC(hWnd); SetupPixels(hDC); hglrc = wglCreateContext(hDC); wglMakeCurrent(hDC, hglrc); #ifdef USE_VBO InitVBO(); #endif break; case WM_DESTROY: #ifdef USE_VBO glDeleteBuffers(1, &vboTransVerts); glDeleteBuffers(1, &vboIndices); #endif wglMakeCurrent(hDC,NULL); wglDeleteContext(hglrc); PostQuitMessage(0); break; case WM_SIZE: w = LOWORD(lParam); h = HIWORD(lParam); Resize(w,h); break; default: break; } return DefWindowProc(hWnd,msg,wParam,lParam); } int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd) { HWND hWnd; WNDCLASSEX wcex; MSG msg; wcex.cbSize = sizeof(WNDCLASSEX); wcex.style = CS_HREDRAW | CS_VREDRAW; wcex.lpfnWndProc = WinProc; wcex.cbClsExtra = 0; wcex.cbWndExtra = 0; wcex.hInstance = hInstance; wcex.hIcon = LoadIcon(NULL,IDI_APPLICATION); wcex.hCursor = LoadCursor(NULL,IDC_ARROW); wcex.hbrBackground = (HBRUSH) GetStockObject(GRAY_BRUSH); wcex.lpszMenuName = NULL; wcex.lpszClassName = "WinClass"; wcex.hIconSm = NULL; RegisterClassEx(&wcex); hWnd = CreateWindow("WinClass","My Window", WS_OVERLAPPEDWINDOW,0,0,400,400,NULL,NULL, hInstance,NULL); if(hWnd == NULL) { MessageBox(NULL,"Error: Unable to create Window","ERROR",MB_OK); return -1; } ShowWindow(hWnd,nShowCmd); UpdateWindow(hWnd); hDC = GetDC(hWnd); glClearColor(0.0f,0.0f,0.0f,0.0f); while(1) { Render(); SwapBuffers(hDC); if(PeekMessage(&msg,NULL,0,0,PM_REMOVE)) { if(msg.message == WM_QUIT) break; TranslateMessage(&msg); DispatchMessage(&msg); } } return 0; }
MinGW上的編譯命令還是不變：gcc -o window.exe window.c -DUSE_VBO -mwindows -lopengl32 -lglu32
這下知道glDrawElements怎麼用了。再回到HomeworldSDL的代碼中，我發現程序沒有調用glBufferData開闢顯存空間就調用了glDrawElements。想必這就是崩潰的原因。於是我將我的window.c中的glBufferData調用刪除，再運行程序，果然就崩潰了，並且崩潰處的彙編代碼和之前查看的基本一模一樣。
這蟲子算是確確實實地捉到了。
說實話，作爲完全沒接觸過OpenGL的新手，我到處搜資料，花了好些時間才掌握了glDrawElements的用法。雖然Unity和CUDA的經驗讓我在相關概念的理解上沒什麼大礙，但我花了一天左右時間才真正能用這函數寫一個OpenGL小程序。這是因爲還有好些其它OpenGL的函數要了解；而且到最後一步還發現，爲了像崩潰的代碼一樣，在這函數中使用OpenGL的頂點緩存對象（vertex buffer object，簡稱vbo），OpenGL的初始化會麻煩很多。正如《關於遊戲開發，學校沒有教給我的十件事》一文寫道：“瞭解和理解是不一樣的……當你做一個項目時，你可能會想‘我知道怎麼做’。然而，除非你之前做過，否則你僅僅是有怎麼做的想法。‘我知道’和‘我有一個如何做的想法’是不同的，它們的區別可能會讓你頭疼好幾個小時。”想必這是每個做過實際項目的程序員的感受。