c++ - nvoglv32.dll Access violation reading location 0x00000000 -

i've been having problem lately opengl. in program has worked fine before, try draw group of polygons screen program throws error , life of me can't find cause of it. i've literally changed nothing code, started throwing errors 1 day after coming after month. error is:

unhandled exception @ 0x69f2883e (nvoglv32.dll) in opengltest.exe:  0xc0000005: access violation reading location 0x00000000. 

i have reproduced error code below. more strange behavior i've noticed if change call gldrawarrays like:

gldrawarrays(gl_triangles, 3, numv); 

or

gldrawarrays(gl_triangles, 0, 258); 

then program doesn't throw error though doesn't render properly, rendering half of rectangle, or rendering rectangle plus garbage input.

it's strange, if renders more 255 points works fine less , throws error.

#include <sdl.h> #include <gl/glew.h> #include <math.h> #include <vector>  struct vector2f{ public:     float x, y;     vector2f() {}     vector2f(float _x, float _y) { x = _x; y = _y; } }; struct vector3f { public:     float x, y, z;     vector3f() { x = 0.0, y = 0.0, z = 0.0; }     vector3f(float _x, float _y, float _z) { x = _x; y = _y; z = _z; } }; struct vertex {     vector3f m_pos;     vector2f m_tex;     vector3f m_normal;     vertex() {}     vertex(vector3f pos, vector2f tex) {         m_pos = pos;         m_tex = tex;         m_normal = vector3f(0.0f, 0.0f, 0.0f);     }     vertex(vector3f pos, vector2f tex, vector3f norm) {         m_pos = pos;         m_tex = tex;         m_normal = norm;     } };  sdl_window*         window_display  = null; sdl_glcontext       glcontext; gluint              vbo; int                 width           = 800; int                 height          = 600; float               ratio           = (float)width / (float)height; int                 numv; std::vector<vertex> _vbo;  bool initwindow() {     //draw square     _vbo.push_back(vertex(vector3f(-0.8f, -0.8f, 0.0f), vector2f(0.0f, 0.0f), vector3f(0.0f, 0.0f, 0.0f)));     _vbo.push_back(vertex(vector3f(0.8f, -0.8f, 0.0f),  vector2f(0.0f, 0.0f), vector3f(0.0f, 0.0f, 0.0f)));     _vbo.push_back(vertex(vector3f(0.8f, 0.8f, 0.0f),   vector2f(0.0f, 0.0f), vector3f(0.0f, 0.0f, 0.0f)));     _vbo.push_back(vertex(vector3f(-0.8f, -0.8f, 0.0f), vector2f(0.0f, 0.0f), vector3f(0.0f, 0.0f, 0.0f)));     _vbo.push_back(vertex(vector3f(-0.8f, 0.8f, 0.0f),  vector2f(0.0f, 0.0f), vector3f(0.0f, 0.0f, 0.0f)));     _vbo.push_back(vertex(vector3f(0.8f, 0.8f, 0.0f),   vector2f(0.0f, 0.0f), vector3f(0.0f, 0.0f, 0.0f)));     numv = 6;      if (sdl_init(sdl_init_video) < 0) return false;     if ((window_display = sdl_createwindow("opengl drawing test", sdl_windowpos_undefined, sdl_windowpos_undefined, width, height, sdl_window_opengl)) == null) return false;      glcontext = sdl_gl_createcontext(window_display);      glewinit();      return true; }  bool drawscene() {     glenableclientstate(gl_vertex_array);      glclear(gl_color_buffer_bit | gl_depth_buffer_bit);     glmatrixmode(gl_modelview); //switch drawing perspective     glloadidentity(); //reset drawing perspective      glenablevertexattribarray(0);     glenablevertexattribarray(1);     glenablevertexattribarray(2);      glgenbuffers(1, &vbo);     glbindbuffer(gl_array_buffer, vbo);     glbufferdata(gl_array_buffer, numv * sizeof(vertex), &_vbo[0], gl_static_draw);      glbindbuffer(gl_array_buffer, vbo);     glvertexattribpointer(0, 3, gl_float, gl_false, sizeof(vertex), 0);     glvertexattribpointer(1, 2, gl_float, gl_false, sizeof(vertex), (const glvoid*)12);     glvertexattribpointer(2, 3, gl_float, gl_false, sizeof(vertex), (const glvoid*)20);     gldrawarrays(gl_triangles, 0, numv);    //unhandled exception @ 0x69f2883e (nvoglv32.dll) in opengltest.exe: 0xc0000005: access violation reading location 0x00000000.      gldisablevertexattribarray(0);     gldisablevertexattribarray(1);     gldisablevertexattribarray(2);      sdl_gl_swapwindow(window_display); //send 3d scene screen      gldisableclientstate(gl_vertex_array);      return true; }  int main(int argc, char* args[]) {     if (initwindow()) {         while (drawscene()) {}     }     return 0; }