NSError *error;
NSString *vertPath = [[NSBundle mainBundle] pathForResource:@"Simple" ofType:@"vert"];
NSString *vertShaderString = [NSString stringWithContentsOfFile:vertPath encoding:NSUTF8StringEncoding error:&error];
const GLchar *vertShaderText = [vertShaderString UTF8String];

if (!vertShaderString) {
    NSLog(@"%@", [error localizedDescription]);
}

vertShader = glCreateShader(GL_VERTEX_SHADER);
glShaderSource(vertShader, 1, &vertShaderText, NULL);

glCompileShader(vertShader);
glGetShaderiv(vertShader, GL_COMPILE_STATUS, &vertCompiled);
if (!vertCompiled) {

    NSLog(@"Vert shader not compiled.");
}

glAttachShader(prog, vertShader);

GLfloat vertices[] = { 0.0, 0.0, 0.0, 0.5, 0.5, 0.5 };
GLubyte indices[3] = { 0, 1, 2 };

// Position vertex attribute data
glEnableVertexAttribArray(0);
glBindAttribLocation(prog, 0, "a_position");
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, vertices);


// Color vertex attribute data
GLfloat vColors[] = {
    1.0, 0.0, 0.0, 1.0,
    0.0, 1.0, 0.0, 1.0,
    0.0, 0.0, 1.0, 1.0 };

glEnableVertexAttribArray(1);
glBindAttribLocation(prog, 1, "a_color");
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, vColors);


glDrawElements(GL_TRIANGLES, sizeof(indices)/sizeof(GLubyte), GL_UNSIGNED_BYTE, indices);

attribute vec4 a_position;
attribute vec4 a_color;

varying vec4 varColor;

void main() {

gl_Position = a_position;
varColor = a_color;
}