Minimal C++ OpenGL Segmentation Fault - c++

Question

I am following the first instalment in Tom Dalling's Modern OpenGL tutorial and when I downloaded his code it worked perfectly. So then I wanted to create my own minimal version to help me understand how it all worked.
#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include <GLM/glm.hpp>
#include <stdexcept>
GLFWwindow* gWindow = NULL;
GLint gProgram = 0;
GLuint gVertexShader = 0;
GLuint gFragmentShader = 0;
GLuint gVAO = 0;
GLuint gVBO = 0;
int main() {
/** Initialise GLFW **/
if (!glfwInit()){
throw std::runtime_error("GLFW not initialised.");
}
glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_COMPAT_PROFILE);
glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
glfwWindowHint(GLFW_RESIZABLE, GL_FALSE);
gWindow = glfwCreateWindow(800, 600, "OpenGL 3.2", NULL, NULL);
if(!gWindow) {
throw std::runtime_error("glfwCreateWindow failed.");
}
glfwMakeContextCurrent(gWindow);
/** Initialise GLEW **/
glewExperimental = GL_TRUE;
if (glewInit() != GLEW_OK) {
throw std::runtime_error("Glew not initialised.");
}
if (!GLEW_VERSION_3_2) {
throw std::runtime_error("OpenGL 3.2 not supported.");
}
/** Set up shaders **/
gVertexShader = glCreateShader(GL_VERTEX_SHADER); //Segmentation fault here
gFragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(gVertexShader, 1, (const GLchar* const*)R"(
#version 150
in vec3 vertexPos
out vec colour;
void main() {
colour = vertexPos;
gl_Position = vec4(vertexPos, 1);
}
)", NULL);
glShaderSource(gFragmentShader, 1, (const GLchar* const*)R"(
#version 150
in vec3 colour;
out vec4 fragColour;
void main() {
fragColour = colour;
}
)", NULL);
glCompileShader(gVertexShader);
glCompileShader(gFragmentShader);
/** Set up program **/
gProgram = glCreateProgram();
glAttachShader(gProgram, gVertexShader);
glAttachShader(gProgram, gFragmentShader);
glLinkProgram(gProgram);
glDetachShader(gProgram, gVertexShader);
glDetachShader(gProgram, gFragmentShader);
glDeleteShader(gVertexShader);
glDeleteShader(gFragmentShader);
/** Set up VAO and VBO **/
float vertexData[] = {
// X, Y, Z
0.2f, 0.2f, 0.0f,
0.8f, 0.2f, 0.0f,
0.5f, 0.8f, 0.0f,
};
glGenVertexArrays(1, &gVAO);
glBindVertexArray(gVAO);
glGenBuffers(1, &gVBO);
glBindBuffer(GL_ARRAY_BUFFER, gVBO);
glBufferData(GL_ARRAY_BUFFER, sizeof(GL_FLOAT)*9, vertexData, GL_STATIC_DRAW);
glVertexAttribPointer(glGetAttribLocation(gProgram, "vertexPos"), 3, GL_FLOAT, GL_FALSE, 0, NULL);
/** Main Loop **/
while (!glfwWindowShouldClose(gWindow)) {
glfwPollEvents();
glBindBuffer(GL_ARRAY_BUFFER, gVBO);
glDrawArrays(GL_TRIANGLES, 0, 1);
glfwSwapBuffers(gWindow);
}
return 0;
}
This is what I created and it builds correctly. However when running it I get a segmentation fault upon calling glCreateShader.
I have done a lot of searching to find a solution to this and so far I've found two main reasons for this happening:
One is that glewExperimental needs to be set to GL_TRUE, however nothing changed after adding this line.
The second reason was that the OpenGL context was not being set, but I believe the call to glfwMakeContextCurrent covers this point.
From here I have no clue as of what to do, so any tips would be a great help!
EDIT: Thank you all for your help! I eventually managed to get it working and have uploaded the code to pastebin for future reference.

Answer 1

Problem solution
You program crashes not at the line you pointed, but on the first glShaderSource call. You actually pass a pointer to a string as the third argument, casting it to (const GLchar* const*). That's not what glShaderSource expects. It rather expects a pointer to an array of pointers to strings, whose (array of pointer's) length should be passed as the second argument (see documentation).
As a quick-n-dirty fix for your code, I declared the following:
const char *vs_source[] = {
"#version 150\n",
"in vec3 vertexPos\n",
"out vec colour;\n",
"void main() {\n",
" colour = vertexPos;\n",
" gl_Position = vec4(vertexPos, 1);\n",
"}\n"};
const char *fs_source[] = {
"#version 150\n",
"in vec3 colour;\n",
"out vec4 fragColour;\n",
"void main() {\n",
" fragColour = colour;\n",
"}\n"};
and replaced the calls to glShaderSource to
glShaderSource(gVertexShader, 7, (const char**)vs_source, NULL);
glShaderSource(gFragmentShader, 6, (const char**)fs_source, NULL);
Now the program works fine and shows a window as expected.
Hints
Your code did not even compile on my machine (I use gcc 4.8.3 # Linux) because of cast of a string literal to (const GLchar* const*). So try to set more restrictive flags in your compiler options and pay attention at compiler warnings. Moreover, treat warnings as errors.
Use some kind of memory profiler to determine the exact location of crashes. For example, I used Valgrind to localize the problem.