C version of pyopengl problem

#include <GL/gl.h>
#include <GL/glut.h>
#include <stdlib.h>
#include <stddef.h>
#include <math.h>
#include <stdio.h>

GLchar *file_contents(const char *filename, GLint *length)
{
    FILE *f = fopen(filename, "r");
    GLchar *buffer;

    if (!f) {
        fprintf(stderr, "Unable to open %s for reading\n", filename);
        return NULL;
    }

    fseek(f, 0, SEEK_END);
    *length = ftell(f);
    fseek(f, 0, SEEK_SET);

    buffer = (GLchar*) malloc(*length+1);
    *length = fread(buffer, 1, *length, f);
    fclose(f);
    ((char*)buffer)[*length] = '\0';

    return buffer;
}

GLuint make_shader(GLenum type, const char *filename)
{
    GLint length;
    GLchar *source = file_contents(filename, &length);
    GLuint shader;
    GLint shader_ok;

    if (!source)
        return 0;

    shader = glCreateShader(type);
    glShaderSource(shader, 1, (const GLchar**)&source, &length);
    free(source);
    glCompileShader(shader);

    glGetShaderiv(shader, GL_COMPILE_STATUS, &shader_ok);
    if (!shader_ok) {
        fprintf(stderr, "Failed to compile %s:\n", filename);
        glDeleteShader(shader);
        return 0;
    }
    return shader;
}

GLuint make_program(GLuint vertex_shader, GLuint fragment_shader)
{
    GLint program_ok;

    GLuint program = glCreateProgram();

    glAttachShader(program, vertex_shader);
    glAttachShader(program, fragment_shader);
    glLinkProgram(program);

    glGetProgramiv(program, GL_LINK_STATUS, &program_ok);
    if (!program_ok) {
        fprintf(stderr, "Failed to link shader program:\n");
        glDeleteProgram(program);
        return 0;
    }
    return program;
}

static const GLfloat verts[] = {
    -1.0f, -1.0f,
     1.0f, -1.0f,
    -1.0f,  1.0f,
     1.0f,  1.0f
};
static const GLushort polys[] = { 0, 1, 2, 2,1,3 };
GLuint program, posidx, vbuf, ebuf;

void init_gl() {
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glClearColor(0.5,0.5,0.5,1);
    glClearDepth(1.0);
    glDepthFunc(GL_LESS);
    glEnable(GL_DEPTH_TEST);
    glEnable(GL_TEXTURE_2D);
    glViewport(0,0,800,600);

    program = make_program(make_shader(GL_VERTEX_SHADER, "stupid.v.glsl"), make_shader(GL_FRAGMENT_SHADER, "stupid.f.glsl"));
    posidx = glGetAttribLocation(program, "position");

    glGenBuffers(1, &vbuf);
    glGenBuffers(1, &ebuf);
    glBindBuffer(GL_ARRAY_BUFFER, vbuf);
    glBufferData(GL_ARRAY_BUFFER, 4*8, verts, GL_STATIC_DRAW);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebuf);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, 2*6, polys, GL_STATIC_DRAW);
}


void render() {
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glUseProgram(program);

    glEnableVertexAttribArray(posidx);
    glBindBuffer(GL_ARRAY_BUFFER, vbuf);
    glVertexAttribPointer( posidx, 2, GL_FLOAT, GL_FALSE, 4*2, 0);

    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebuf);
    glDrawElements(
        GL_TRIANGLES,
        6,
        GL_UNSIGNED_SHORT,
        0
    );

    glDisableVertexAttribArray(posidx);
    glutSwapBuffers();
}

int main(int argc, char* argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGB | GLUT_DEPTH | GLUT_DOUBLE);
    glutInitWindowSize(800, 600);
    glutCreateWindow("Flag");
    glutDisplayFunc(&render);

    init_gl();

    glutMainLoop();
    return 0;
}