glVertexAttribPointer raising impossible GL_INVALID_OPERATION? - opengl

Question

I have narrowed down the error to this OpenGL call:
glVertexAttribPointer(var.vertex, 4, GL_FLOAT, GL_FALSE, 0, 0);
there are no errors before it, and there is a GL_INVALID_OPERATION error after it.
The output to my console is:
Error in Shader.cpp : bindMesh : 294
OpenGL Error: Invalid Operation
(0x502)
Bound Buffer: 1
var.vertex = 1
According to this, the only GL_INVALID_OPERATION condition that could apply is a bound buffer ID of 0. But the bound buffer ID is 1.
I'm checking (and printing) the error with:
printErrors();
int i = 0;
glGetIntegerv(GL_ARRAY_BUFFER_BINDING, &i);
printf("\tBound Buffer: %i\n", i);
printf("\tvar.vertex = %i\n", var.vertex);
where printErrors() is defined as
void PrintErrors(const char* file, const char* func, int line);
#define printErrors() PrintErrors(__BASE_FILE__,__func__,__LINE__)
void PrintErrors(const char* file, const char* func, int line)
{
const char* errors[] = {
"OpenGL Error: Invalid Enumeration\n",
"OpenGL Error: Invalid Value\n",
"OpenGL Error: Invalid Operation\n",
"OpenGL Error: Invalid Frame-Buffer Operation\n",
"OpenGL Error: Out of Memory\n",
"OpenGL Error: Stack Underflow\n",
"OpenGL Error: Stack Overflow\n",
};
uint32 i = glGetError();
while (i != GL_NO_ERROR)
{
printf("Error in %s : %s : %i\n\t%s\t(0x%x)\n", file, func, line, errors[i - GL_INVALID_ENUM], i);
i = glGetError();
}
}
What's going wrong?
Edit:
var.vertex is found with:
var.vertex = glGetAttribLocation(programID, "Vertex");
and there is a check to ensure that var.vertex was found in the bound shader by an if-statement:
if (var.vertex != INVALID_SHADER_VARIABLE) // INVALID_SHADER_VARIABLE = 0xFFFFFFFF (-1 if it where a signed int)
{
glBindBuffer(...);
glBuffferData(...);
glEnableVertexAttribArray(var.vertex);
glVertexAttribPointer(var.vertex, 4, GL_FLOAT, GL_FALSE, 0, 0);
}

Answer 1

You must have a non-zero Vertex Array Object bound in an OpenGL 3.1 context without the extension GL_ARB_compatibility or a core profile context. This is one of the hidden conditions that will generate a GL_INVALID_OPERATION whenever you attempt to do anything related to vertex arrays (e.g. draw, setup vertex pointers, etc.)
The good news is this is a trivial issue to fix.
At minimum, all you need to do is generate (glGenVertexArrays (...)) and bind (glBindVertexArray (...)) a vertex array object when you initialize your program.