Loading thread using a second shared OpenGL context - multithreading

Question

My plan was to create a loading thread inside of which I load resources for a game; such as 3D models, shaders, textures, etc. On the main thread I perform all the game logic and rendering. Then, on my loading thread, I create a sf::Context (SFML shared OpenGL context) which is used only for loading.
This is working for loading shaders. However, xserver sometimes crashes when attempting to load models. I have narrowed the crash down to the glBufferData() call. I have checked that there is nothing wrong with the data that I am sending.
Is it possible call glBufferData() from a second thread using a second OpenGL context? If not, why is it possible to load shaders in the second context? If it is possible, what could be going wrong?
#include <iostream>
#include <thread>
#include <GL/glew.h>
#include <SFML/OpenGL.hpp>
#include <SFML/Graphics.hpp>
#include <X11/Xlib.h>
class ResourceLoader
{
public:
void Run()
{
sf::Context loadingContext;
loadingContext.setActive(true);
// Some test data.
float* testData = new float[3000];
for (unsigned int i = 0; i < 3000; ++i)
{
testData[i] = 0.0f;
}
// Create lots of VBOs containing our
// test data.
for (unsigned int i = 0; i < 1000; ++i)
{
// Create VBO.
GLuint testVBO = 0;
glGenBuffers(1, &testVBO);
std::cout << "Buffer ID: " << testVBO << std::endl;
// Bind VBO.
glBindBuffer(GL_ARRAY_BUFFER, testVBO);
// Crashes on this call!
glBufferData(
GL_ARRAY_BUFFER,
sizeof(float) * 3000,
&testData[0],
GL_STATIC_DRAW
);
// Unbind VBO.
glBindBuffer(GL_ARRAY_BUFFER, 0);
// Sleep for a bit.
std::this_thread::sleep_for(std::chrono::milliseconds(10));
}
delete[] testData;
}
};
int main()
{
XInitThreads();
// Create the main window.
sf::RenderWindow window(sf::VideoMode(800, 600), "SFML window", sf::Style::Default, sf::ContextSettings(32));
window.setVerticalSyncEnabled(true);
// Make it the active window for OpenGL calls.
window.setActive();
// Configure the OpenGL viewport to be the same size as the window.
glViewport(0, 0, window.getSize().x, window.getSize().y);
// Initialize GLEW.
glewExperimental = GL_TRUE; // OSX fix.
if (glewInit() != GLEW_OK)
{
window.close();
exit(1); // failure
}
// Enable Z-buffer reading and writing.
glEnable(GL_DEPTH_TEST);
glDepthMask(GL_TRUE);
// Create the resource loader.
ResourceLoader loader;
// Run the resource loader in a separate thread.
std::thread loaderThread(&ResourceLoader::Run, &loader);
// Detach the loading thread, allowing it to run
// in the background.
loaderThread.detach();
// Game loop.
while (window.isOpen())
{
// Event loop.
sf::Event event;
while (window.pollEvent(event))
{
if (event.type == sf::Event::Closed)
{
window.close();
}
}
// Clear scren.
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// Switch to SFML's OpenGL state.
window.pushGLStates();
{
// Perform SFML drawing here...
sf::RectangleShape rect(sf::Vector2f(100.0f, 100.0f));
rect.setPosition(100.0f, 100.0f);
rect.setFillColor(sf::Color(255, 255, 255));
window.draw(rect);
}
// Switch back to our game rendering OpenGL state.
window.popGLStates();
// Perform OpenGL drawing here...
// Display the rendered frame.
window.display();
}
return 0;
}

Answer 1

I think the problem is that you call glBufferData before setting up GLEW, so the function pointer to glBufferData is not initialized. Please try this ordering for initializing your program:
Initialize the RenderWindow
Initialize GLEW
Start threads, and create additional contexts as required!