Checking for errors in OpenGL

OpenGL will not explicitly notify you when errors occur. Instead, you can use the glGetError() and gluErrorString() functions to ask OpenGL if errors have occured.

Example C++ code

void checkGLErrors(const char *label) {
    GLenum errCode;
    const GLubyte *errStr;
    if ((errCode = glGetError()) != GL_NO_ERROR) {
        errStr = gluErrorString(errCode);
        cerr << "OpenGL ERROR: " << (char*)errStr << "; Label: " <<
                label << endl;
    }
}
    

checkGLErrors("label") checks if any errors have occurred since the last call to checkGLErrors (or since the program started, on the first call). If so, it prints a message identifying the error (within OpenGL's somewhat limited set of error messages), along with the identifying label.

Typical usage

You can break your OpenGL code into logical chunks and surround each with a pair of labeled checkGLErrors calls, i.e.:

checkGLErrors("begin foo");
// code using OpenGL to do "foo"
checkGLErrors("end foo");
    

If you see an error labeled with end foo, then OpenGL is unhappy with some call you made within the "foo" chunk. If you see begin foo, then an error occurred somewhere else, before this code ran. If you see no error output, then your OpenGL calls were all semantically correct (of course, this doesn't mean the output is right!) You can do a binary search with this technique, bracketing smaller and smaller pieces of code until you isolate the error to a single call.

Common sources of errors

The most common source of OpenGL errors is probably misplaced GLenum constants. In C/C++, all of OpenGL's symbolic constants (e.g., GL_LIGHTING, GL_FRONT, GL_MODELVIEW) are of the same type, GLenum. Therefore, the type system will not prevent you from making a nonsensical call like

glMaterialfv(GL_MODELVIEW, GL_FRONT, 0);

This will cause a GL_INVALID_ENUM error.