glewInit() fails with "Missing GL version", SDL2 OpenGL context, cygwin compiler - c++

Question

The program following, is one that creates a window which does nothing except close when you press esc. When I compile it with cygwin, there are no errors. The GLEW I use is from Cygwin Ports, and the SDL2 is version 2.0.3, from their website's SDL2-devel-2.0.3-mingw.tar.gz download. I have SDL2.dll in the directory of the compiled executable.
Links with: -lSDL2 -lSDL2main -lGLEW -lGLU -lGL -lSDL2 -lSDL2main -lGLEW -lGLU -lGL, twice to ensure everything is linked.
Also compiled with: -std=c++11
On my computer, the following program prints out:
OpenGL Vendor: (null)
OpenGL Renderer: (null)
OpenGL Shading Language Version: (null)
OpenGL Extensions: (null)
Error initializing GLEW! Missing GL version
The program appears to work otherwise. The main problem is that if I try to call, for example glGenVertexArrays, the program will crash with STATUS_ACCESS_VIOLATION. (See the crashing code here. I think this has something to do with GLEW's error Missing GL version.
#include <cstdio>
#include <chrono>
#include <thread>
#include <SDL2/SDL.h>
#include <GL/glew.h>
#include <SDL2/SDL_opengl.h>
#include <GL/glu.h>
const int width = 1000;
const int height = 500;
bool Running = true;
#undef main
int main (int argc, char *argv[]) {
FILE* cdebug = fopen("cdebug.txt", "w");
if (SDL_Init(SDL_INIT_VIDEO) < 0) {
fprintf(cdebug, "SDL could not initialize! SDL Error: %s\n", SDL_GetError()); fflush(cdebug);
}
#define setAttr(attr, value) \
if (SDL_GL_SetAttribute(attr, value) < 0) { \
fprintf(cdebug, "SDL failed to set %s to %s, SDL Error: %s\n", #attr, #value, SDL_GetError()); fflush(cdebug);\
}
setAttr(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
setAttr(SDL_GL_CONTEXT_MINOR_VERSION, 3);
setAttr(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
setAttr(SDL_GL_RED_SIZE, 8);
setAttr(SDL_GL_GREEN_SIZE, 8);
setAttr(SDL_GL_BLUE_SIZE, 8);
setAttr(SDL_GL_DEPTH_SIZE, 24);
setAttr(SDL_GL_DOUBLEBUFFER, 1);
#undef setAttr
/*
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
SDL_GL_SetAttribute(SDL_GL_RED_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_GREEN_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_BLUE_SIZE, 8);
SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
*/
SDL_Window *window = SDL_CreateWindow(
"test",
SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
640, 480,
SDL_WINDOW_OPENGL | SDL_WINDOW_RESIZABLE
);
if (window == NULL) {
fprintf(cdebug, "Window could not be created! SDL Error: %s\n", SDL_GetError()); fflush(cdebug);
}
SDL_GLContext GLContext = SDL_GL_CreateContext(window);
if (GLContext == NULL) {
fprintf(cdebug, "OpenGL context could not be created! SDL Error: %s\n", SDL_GetError()); fflush(cdebug);
}
if (SDL_GL_MakeCurrent(window, GLContext) < 0) {
fprintf(cdebug, "OpenGL context could not be made current! SDL Error: %s\n", SDL_GetError()); fflush(cdebug);
}
fprintf(cdebug, "OpenGL Vendor: %s\n", glGetString(GL_VENDOR));
fprintf(cdebug, "OpenGL Renderer: %s\n", glGetString(GL_RENDERER));
fprintf(cdebug, "OpenGL Shading Language Version: %s\n", glGetString(GL_SHADING_LANGUAGE_VERSION));
fprintf(cdebug, "OpenGL Extensions: %s\n", glGetString(GL_EXTENSIONS));
fflush(cdebug);
glewExperimental = GL_TRUE;
{
GLenum glewError = glewInit();
if (glewError != GLEW_OK) {
fprintf(cdebug, "Error initializing GLEW! %s\n", glewGetErrorString(glewError)); fflush(cdebug);
}
}
SDL_Event event;
while (Running) {
while (SDL_PollEvent(&event)) {
switch (event.type) {
case SDL_KEYUP: {
switch (event.key.keysym.scancode) {
case SDL_SCANCODE_ESCAPE:
Running = false;
break;
}
break;
}
}
}
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
SDL_GL_SwapWindow(window);
std::this_thread::sleep_for(std::chrono::milliseconds(50));
}
SDL_GL_DeleteContext(GLContext);
SDL_DestroyWindow(window);
window = NULL;
SDL_Quit();
return 0;
}

Answer 1

You are mixing cygwin and mingw in ways in which you shouldn't.
If you use cygwin's toolchain and -lGL and so on, you link against cygwin's OpenGL - which is not the native OpenGL lib on windows, but the one provided by cygwin's X server, implementing the GLX protocol.
The mingw version of SDL will use the native GL lib (opengl32.dll) on windows, using the wgl API. So SDL might even create a context for you, but the GL functions your programm are calling belong to a completely different GL implementation - for which your program never created a GL context.
The solution is to stick with one or the other: Completely use cygwin, and a cygwin version of SDL, and a cygwin X server. However, that is not the path I would recommend. I don't know if that would even get you some HW acceleration at all.
The more useful solution would be to not use cygwin, but mingw, for the whole project, with a mingw version of GLEW. That will result in a completely native windows binrary which will use the native OpenGL library with all features provided by the driver and not require cygwin's dlls and especially not cygwin's X server.

Answer 2

I managed to get things working in a weird way.
I am using a self compiled version of SDL2 but the SDL2-devel-2.0.3-mingw.tar.gz provided by the SDL website seems to work as well and using a combination of them (such as mingw version's libs and self-compiled .dll) seem to work as well.
For GLEW, I am using my own compiled version. To compile this, I used their website's source glew-1.11.0.zip and extracted this. Then I edited glew-1.11.0/Makefile and edited line 24 to SYSTEM = cygming. Then in glew-1.11.0/config/Makefile.cygming on line's 7 and 8, I removed the -mno-cygwin flag (so the line's are CC := gcc and LD := gcc) and added -D_WIN32 to line 10 (so the line becomes CFLAGS.SO = -DGLEW_BUILD -D_WIN32). Then in glew-1.11.0, I ran make all and let it compile. After that, I copied glew-1.11.0/include/GL to my includes directory. Next, I copied glew-1.11.0/lib/libglew32.dll.a to my libs folder. I also copied glew-1.11.0/lib/glew32.dll to my .exe's folder. Then to get it to not produce a linker error, I had to place a #define _WIN32 before my #include <GL/glew.h>.
To link everything, I managed to compile it with a minimum of -lSDL2 -lSDL2main -lglew32.dll -lopengl32.