OpenGL Programming Guide First example - opengl

Question

I am following the 8Th edition and below is the code I modified from book. it does not give me what I want, but a blank window instead. I wonder what mistake(s) did I make during modifications?
below is the code I modified, hope anyone can point out the mistake.
#include <iostream>
#include <OpenGL/glu.h>
#include <OpenGL/gl3.h>
#include <GLUT/GLUT.h>
GLuint vaoNames[1];
GLuint bufferNames[1];
int vPosition = 0;
GLuint createProgram();
GLuint createShader(GLenum type, const char* src);
GLuint program;
void init()
{
glGenVertexArrays(1, vaoNames);
glBindVertexArray(vaoNames[0]);
GLfloat vertices[6][2] = {
{ -0.90, -0.90 }, // Triangle 1
{ 0.85, -0.90 },
{ -0.90, 0.85 },
{ 0.90, -0.85 }, // Triangle 2
{ 0.90, 0.90 },
{ -0.85, 0.90 }
};
glGenBuffers(1, bufferNames);
glBindBuffer(GL_ARRAY_BUFFER, bufferNames[0]);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
glUseProgram(createProgram());
glVertexAttribPointer(vPosition, 2, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(vPosition);
}
void render()
{
glClear(GL_COLOR_BUFFER_BIT);
glBindVertexArray(vaoNames[0]);
glDrawArrays(GL_TRIANGLES, 0, 6);
glFlush();
}
GLuint createProgram()
{
GLuint program = glCreateProgram();
const char* vertexShaderSrc =
"#version 400 core \n"
"layout( location = 0 ) in vec4 vPosition; \n"
"void main() \n"
"{ \n"
"gl_Position = vPosition; \n"
"}";
GLuint vertexShader = createShader(GL_VERTEX_SHADER, vertexShaderSrc);
const char* fragmentShaderSrc =
"#version 400 core \n"
"out vec4 fColor; \n"
"void main() \n"
"{ \n"
"fColor = vec4( 0.0, 0.0, 1.0, 1.0 ); \n"
"}";
GLuint fragmentShader = createShader(GL_FRAGMENT_SHADER, fragmentShaderSrc);
glAttachShader(program, vertexShader);
glAttachShader(program, fragmentShader);
glLinkProgram(program);
return program;
}
GLuint createShader(GLenum type, const char* src){
GLuint shaderID = glCreateShader(type);
glShaderSource(shaderID, 1, &src, 0);
glCompileShader(shaderID);
return shaderID;
}
int main(int argc, char ** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_RGBA);
glutInitWindowSize(512, 512);
glutCreateWindow("8877");
init();
glutDisplayFunc(render);
glutMainLoop();
}

Answer 1

When you say 'there is no error during shader compilation and program linking', do you mean that the the program doesn't stop, or do you mean that you actually ran a different program and checked the results of glGetError() at various points, or fetched the compilation or linking log? Your program as is does no actual error checking, and OpenGL doesn't throw exceptions or produce error output as a matter of course. You have to actually request these things.
Simply because glCompileShader() doesn't generate an error doesn't mean your shader actually compiled. You need to be checking the compilation status of shaders and the link status of programs with code similar to this:
GLint compiled;
glGetShaderiv(newShader, GL_COMPILE_STATUS, &compiled);
if (!compiled) {
std::string log = getLog(newShader);
SAY(log.c_str());
throw std::runtime_error("Failed to compile shader " + log);
}
where getLog is
static std::string getLog(GLuint shader) {
std::string log;
GLint infoLen = 0;
glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &infoLen);
if (infoLen > 1) {
char* infoLog = new char[infoLen];
glGetShaderInfoLog(shader, infoLen, NULL, infoLog);
log = std::string(infoLog);
delete[] infoLog;
}
return log;
}
This is for checking shaders, and there are equivalent functions for checking the results and getting the logs for program linking. Unless you're calling them you have no guarantee that the shaders are compiling. Assuming you haven't made some simple mistake in the shader syntax, the most likely problem you're facing is a lack of support for OpenGL 4.x, which would cause the shaders to fail right off.