Why can't I create a headless OpenGl context with EGL?

Question

I am currently working on a program for which I need a headless opengl context. Ideally, it will be possible to run without an X server on Linux. My research tells me that EGL is the only way to do this (please tell me if there is another way, as I strongly prefer C++ APIs to C apis like EGL). Looking at this tutorial and the EGL reference, I came up with this code, in the form of a minimum and complete example:

#include <EGL/egl.h>
#include <glad/glad.h>
#include <iostream>
#include <exception>

class GlException : public std::exception{
public:
    GlException(const std::string &message) : m_message(message) {}
    const char *what() const noexcept override  { return m_message.c_str(); }
private:
    std::string m_message;
};

void createOpenGlContext() {
    constexpr EGLint configAttribs[] = {
        EGL_SURFACE_TYPE, EGL_PBUFFER_BIT,
        EGL_BLUE_SIZE, 8,
        EGL_GREEN_SIZE, 8,
        EGL_RED_SIZE, 8,
        EGL_RENDERABLE_TYPE, EGL_OPENGL_BIT,
        EGL_NONE
    };

    constexpr EGLint pbufferAttribs[] = {
        EGL_WIDTH, 1600,
        EGL_HEIGHT, 900,
        EGL_NONE,
    };

    EGLDisplay m_display = eglGetDisplay(EGL_DEFAULT_DISPLAY);

    EGLBoolean result = eglInitialize(m_display, nullptr, nullptr);
    if(result != EGL_TRUE){
        switch(eglGetError()){
            case EGL_BAD_DISPLAY:
                throw GlException("Failed to initialize EGL Display: EGL_BAD_DISPLAY");
            case EGL_NOT_INITIALIZED:
                throw GlException("Failed to initialize EGL Display: EGL_NOT_INITIALIZED");
            default:
                throw GlException("Failed to initialize EGL Display: unknown error");
        }
    }

    EGLint numConfigs;
    EGLConfig eglConfig;
    result = eglChooseConfig(m_display, configAttribs, &eglConfig, 1, &numConfigs);
    if(result != EGL_TRUE){
        switch(eglGetError()){
            case EGL_BAD_DISPLAY:
                throw GlException("Failed to configure EGL Display: EGL_BAD_DISPLAY");
            case EGL_BAD_ATTRIBUTE:
                throw GlException("Failed to configure EGL Display: EGL_BAD_ATTRIBUTE");
            case EGL_NOT_INITIALIZED:
                throw GlException("Failed to configure EGL Display: EGL_NOT_INITIALIZED");
            case EGL_BAD_PARAMETER:
                throw GlException("Failed to configure EGL Display: EGL_BAD_PARAMETER");
            default:
                throw GlException("Failed to configure EGL Display: unknown error");
        }
    }

    EGLSurface surface = eglCreatePbufferSurface(m_display, eglConfig, pbufferAttribs);

    eglBindAPI(EGL_OPENGL_API);

    EGLContext context = eglCreateContext(m_display, eglConfig, EGL_NO_CONTEXT, nullptr);
}

int main(){
    createOpenGlContext();
    if(!gladLoadGL()){
        std::cerr << "Failed to load OpenGl function pointers" << std::endl;
        return -1;
    }
    return 0;
}

When I call gladLoadGl in that code, it fails as can be determined via its return value, suggesting that a context has not been created, despite almost exactly using the code from the Nvidia blog (presumably Nvidia knows what they are doing with OpenGl.). I attempted to find someone else with the same error with my preferred search engine, but nothing useful came up. What is wrong with my code?

I am using this glad configuration. I am using Arch Linux, fully updated. I have a Gtx 1070 with the latest drivers from Nvidia. The above code fails when run with an available X server. I have not tested it in a headless environment.

Answer 1

@GM的建议起作用了，在现有的createOpenGlContext函数解决了问题之前，添加了对eglMakeCurrent的调用。