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Asked: 2026-06-08T08:52:32+00:00

My code works fine when using glDrawArrays, but whenever I make an index buffer

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My code works fine when using glDrawArrays, but whenever I make an index buffer and change it to use glDrawElements, it segfaults.

(Strangely, not before or after the drawing…but when the main loop exits !)

Here is the simple code..where I declare 4 vertices, and 3 indices to them for drawing a triangle.

int main()
{
    sf::Window win(sf::VideoMode(400,400,32),"Manasij");
    GlewInit();
    mm::Program program
    (
        {
            mm::Shader(GL_VERTEX_SHADER,"vshader.vert"),
            mm::Shader(GL_FRAGMENT_SHADER,"fshader.frag")
        }
    );
    float vdata[]=
    {
        0.0f,0.0f,
        0.5f,0.0f,
        0.5f,0.5f,
        0.0f,0.5f
    };
    GLubyte indices[]={0,1,3};

    GLuint vao,vbo,ebo;
    glGenVertexArrays(1,&vao);
    glBindVertexArray(vao);

    glGenBuffers(1,&vbo);
    glBindBuffer(GL_ARRAY_BUFFER,vbo);
    glBufferData(GL_ARRAY_BUFFER,8*sizeof(float),vdata,GL_STATIC_DRAW);

    glGenBuffers(1,&ebo);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,ebo);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER,3*sizeof(GLubyte),indices,GL_STATIC_DRAW);

    glVertexAttribPointer(0,2,GL_FLOAT,GL_FALSE,0,0);
    glEnableVertexAttribArray(0);
    glBindAttribLocation(program.getHandle(),0,"pos");
    //glBindVertexArray(0);

    glUseProgram(program.getHandle());
    //glBindVertexArray(vao);
    glClearColor(1.0f,1.0f,1.0f,1.0f);

    while(win.isOpen())
    {
        sf::Event eve;
        while(win.pollEvent(eve))
            if(eve.type==sf::Event::Closed)
                win.close();
        glClear(GL_COLOR_BUFFER_BIT);

//      glDrawArrays(GL_TRIANGLES,0,3);
        glDrawElements(GL_TRIANGLES,3,GL_UNSIGNED_BYTE,(GLvoid*)0);
        win.display();  
    }
    return 0;
}

And the shaders are as simple as they get:

#version 330
in vec2 pos;
void main()
{
    gl_Position = vec4(pos,0.0f,1.0f);
}

and 

#version 330
void main()
{
    gl_FragColor = vec4(1.0f,0.3f,0.8f,1.0f);
}

And gdb shows :

Breakpoint 1, main () at test.cpp:57
57                      win.display();
(gdb) continue 
Continuing. //<- At this point I try to close the window, thus triggeing the main loop to break

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff557d164 in ?? () from /lib/libnvidia-glcore.so.302.17

Any idea what I’m botching up ?

P.S: If anyone wants to test, here an archive..complete with a makefile!

https://docs.google.com/file/d/0B0oubbp-MVYOeGFuX0owXzhRTEU/edit

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1 Answer

  1. Editorial Team

    The problem was that in the last iteration of the loop, when the ‘close’ event was detected, the window was signaled the close, but even after that I was calling glDrawElements for one last time.
    Since everything was already gone, that resulted in a segfault.

    The solution is to put a break statement after win.close() is called or to put the event handling code to the end of the loop.

    (Thanks to cmtptr from the archlinux forums for catching this, I was almost convinced that this was a driver problem)

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