Opengl 3 and sdl2 project has random results - c++

Question

Ok Im trying to learn OpenGL 3.x, I used to use the old interface, and its not really going to well. Im using SDL2 for windowing and input.
What I would think the code should do is draw a red triangle on a blue background. The Problem is that it only does this sometimes, actually it seems to be completely random, sometimes it does that sometimes it just displays a black window.
I have no ideas as to why.
#include "SDL2/SDL.h"
#include "GL/glew.h"
GLuint makeShader(const char * shader, GLint t){
//---- Make Shader ----
GLuint s = glCreateShader(t);
glShaderSource(s, 1, &shader, NULL);
glCompileShader(s);
//---- Get Error Msg ----
int l=0;
glGetShaderiv(s, GL_INFO_LOG_LENGTH, &l);
char *msg = new char [l+1];
glGetShaderInfoLog(s, l, NULL, msg);
printf("Shader MSG: %s", msg);
delete [] msg;
return s;
}
int main(){
//----Init SDL & GL Context----
SDL_Init(SDL_INIT_EVERYTHING);
SDL_Window *screen = SDL_CreateWindow("My Game Window",
SDL_WINDOWPOS_UNDEFINED,
SDL_WINDOWPOS_UNDEFINED,
//1024, 768,
640, 480,
SDL_WINDOW_OPENGL);
SDL_GLContext glContext = SDL_GL_CreateContext(screen);
SDL_Event e;
//----Init GLEW---
glewExperimental=true;
GLenum err = glewInit();
if(GLEW_OK != err)
fprintf(stderr, "Error: %s\n", glewGetErrorString(err));
//----VAO----
GLuint VertexArrayID;
glGenVertexArrays(1, &VertexArrayID);
glBindVertexArray(VertexArrayID);
//----Make Shaders----
const char *vscr = "#version 330 core\nlayout(location = 0) in vec4 position;\nvoid main(){gl_Position = position * 0.5; gl_Position.w = 1.0;}";
GLuint vshader = makeShader(vscr, GL_VERTEX_SHADER);
const char *fscr = "#version 330 core\nout vec3 color;\nvoid main(){ color = vec3(1,0,0);}";
GLuint fshader = makeShader(fscr, GL_FRAGMENT_SHADER);
GLuint ProgramID = glCreateProgram();
glAttachShader(ProgramID, vshader);
glAttachShader(ProgramID, fshader);
glLinkProgram(ProgramID);
// ----make geometry----
static const GLfloat g_vertex_buffer_data[] = {
-1.0f, -1.0f, 0.0f,
1.0f, -1.0f, 0.0f,
0.0f, 1.0f, 0.0f,
};
GLuint vertexbuffer;
glGenBuffers(1, &vertexbuffer);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(g_vertex_buffer_data), g_vertex_buffer_data, GL_STATIC_DRAW);
glViewport(0,0,640,480);
glClearColor(0.0f, 0.0f, 1.0f, 0.0f);
//----MAIN LOOP----
while(1){
glClear(GL_COLOR_BUFFER_BIT);
SDL_PollEvent(&e);
glUseProgram(ProgramID);
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);
glDrawArrays(GL_TRIANGLES, 0, 3);
glDisableVertexAttribArray(0);
glFlush();
SDL_GL_SwapWindow(screen);
if(e.type == SDL_QUIT)
break;
}
SDL_GL_DeleteContext(screen);
return 0;
}