Victor Polevoy Victor Polevoy - 1 month ago 14
C++ Question

Opengl 4 debug output does not work

I am writing a game. I use ArchLinux most of time but I have tried to run my game on the Ubuntu 16.04 recently. On Ubuntu 16.04 there is a strange error: 1280. It is too difficult to find what causes the error so I wanted to see opengl's debug output but I don't see it too. I noticed one thing during shader validation - validation seems to be unsuccessful but the log is empty:

GLint status;
glValidateProgram(program_);
glGetProgramiv(program_, GL_VALIDATE_STATUS, &status);

if (status == GL_TRUE) {
return;
}

// Store log and return false
int length = 0;

glGetProgramiv(program_, GL_INFO_LOG_LENGTH, &length);

if (length > 0) {
GLchar infoLog[512];
glGetProgramInfoLog(program_, 512, nullptr, infoLog);

throw std::runtime_error(std::string("Program failed to validate:") + infoLog);
} else {
throw std::runtime_error(std::string("Program failed to validate. Unknown error"));
}


This gives me
Unknown error
. Also the opengl's debug output can't be seen, however, user messages are written there successfully. Here is the code:

SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 4);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 4);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);

int contextFlags = 0;
SDL_GL_GetAttribute(SDL_GL_CONTEXT_FLAGS, &contextFlags);
contextFlags |= SDL_GL_CONTEXT_DEBUG_FLAG;
SDL_GL_SetAttribute(SDL_GL_CONTEXT_FLAGS, contextFlags);

sdlWindow_ = SDL_CreateWindow(title.c_str(),
SDL_WINDOWPOS_UNDEFINED,
SDL_WINDOWPOS_UNDEFINED,
0,
0,
SDL_WINDOW_OPENGL
| SDL_WINDOW_SHOWN
| SDL_WINDOW_FULLSCREEN_DESKTOP
| SDL_WINDOW_INPUT_GRABBED);

if (!sdlWindow_) {
throw std::runtime_error("Unable to create window");
}

SDL_Log("Window created");

glContext_ = SDL_GL_CreateContext(sdlWindow_);
if (!glContext_) {
throw std::runtime_error("Failed to init OpenGL");
}
SDL_Log("GL context created");

{
glewExperimental = GL_TRUE;
GLenum err = glewInit();
if (err != GLEW_OK) {
throw std::runtime_error(std::string("GLEW Error: ") + reinterpret_cast<const char*>(glewGetErrorString(err)));
}
}

if (glDebugMessageCallbackARB != nullptr) {
SDL_Log("GL debug is available.\n");
// Enable the debug callback
glEnable(GL_DEBUG_OUTPUT);
glEnable(GL_DEBUG_OUTPUT_SYNCHRONOUS);
glDebugMessageCallback(_openglDebugCallbackFunction, nullptr);
glDebugMessageControl(GL_DONT_CARE, GL_DONT_CARE, GL_DONT_CARE, 0, nullptr, GL_TRUE);
glDebugMessageInsert(GL_DEBUG_SOURCE_APPLICATION, GL_DEBUG_TYPE_MARKER, 0,
GL_DEBUG_SEVERITY_NOTIFICATION, -1 , "Started debugging");
} else {
SDL_Log("GL debug is not available.\n");
}


So the main question here is why I can't see the opengl's debug output. And, if it is possible, as an additional question, why does the shader validation fail without a log?

BDL BDL
Answer Source

GLEW 1.x has some problems when beeing used in a core context (that's also why glewExperimental=true is needed). glewInit always generates an OpenGL error while loading the extensions. You don't get this error through the debug callback because the initialization of the callback happens after the point where the error happend.

You have kind of a chicken-egg problem here: You cannot setup the debug callback before initializing GLEW, but that's where you want to get the debug output from. I recommend calling glGetError() right after glewInit to get rid of the one error you know where it is coming from.