c++ - OpenGL Index Buffering To Make A Square -

i'm new opengl , have been following youtube tutorial jeffrey chastine. personal exercise, wanted make simple square using index buffers. sadly attempt, see 1 triangle drawn (when expect two). i'm not sure i'm missing. here code, tried explaining logic/reasoning in comments. feel free correct me wherever i'm wrong:

#include <gl/glew.h> #include <gl/freeglut.h> #include <iostream>  #define buffer_offset(i) ((char *)null + (i)) gluint shaderprogramid; gluint vao = 0; gluint vbo; gluint positionid, colorid; gluint indexbufferid;   #pragma region shader_functions  static char* readfile(const char* filename) {     //open file     file* fp = fopen(filename, "r");      //move file pointer end of file , determining length     fseek(fp, 0, seek_end);     long file_length = ftell(fp);     fseek(fp, 0, seek_set);     char* contents = new char[file_length + 1];      //zero out memory     for(int = 0; < file_length+1; i++)     {         contents[i] = 0;     }      //here's actual read     fread(contents, 1, file_length, fp);     contents[file_length + 1] = '\0';     fclose(fp);     return contents; }  bool compiledstatus(glint shaderid){     glint compiled = 0;     glgetshaderiv(shaderid, gl_compile_status, &compiled);     if (compiled) {         return true;     }     else {         glint loglength;         glgetshaderiv(shaderid, gl_info_log_length, &loglength);         char* msgbuffer = new char[loglength];         glgetshaderinfolog(shaderid, loglength, null, msgbuffer);         printf ("%s\n", msgbuffer);         delete (msgbuffer);         return false;     } }  //takes in source code string gluint makevertexshader(const char* shadersource) {     //call gl make vertex shader , id     gluint vertexshaderid = glcreateshader(gl_vertex_shader);      //bind provided source code shader id     glshadersource(vertexshaderid, 1, (const glchar**)&shadersource, null);      //compile vertex shader     glcompileshader(vertexshaderid);      bool compiledcorrect = compiledstatus(vertexshaderid);     if(compiledcorrect)     {         return vertexshaderid;     }     return -1;      return vertexshaderid; }  gluint makefragmentshader(const char* shadersource) {     //call gl make fragment shader , id     gluint fragmentshaderid = glcreateshader(gl_fragment_shader);     //bind provided source code shader id     glshadersource(fragmentshaderid, 1, (const glchar**)&shadersource, null);     //compile fragment shader     glcompileshader(fragmentshaderid);      bool compiledcorrect = compiledstatus(fragmentshaderid);     if(compiledcorrect)     {         return fragmentshaderid;     }     return -1;      return fragmentshaderid; }  #pragma endregion shader_functions   void changeviewport(int w, int h) {     glviewport(0, 0, w, h); }  //this function using each time window needs redrawn void render() {     glclear(gl_color_buffer_bit | gl_depth_buffer_bit);     gldrawelements(gl_triangles, 4, gl_unsigned_int, null);     glutswapbuffers(); }  gluint makeshaderprogram(gluint vertextshaderid, gluint fragmentshaderid) {     gluint shaderid = glcreateprogram();     //attach vertex shader shader program     glattachshader(shaderid, vertextshaderid);      //attatch fragment shader shader program     glattachshader(shaderid, fragmentshaderid);      //link shaders     gllinkprogram(shaderid);     return shaderid; }  int main (int argc, char** argv) {     //standards     glutinit(&argc, argv);     glutinitdisplaymode(glut_rgba);     glutinitwindowsize(800, 600);     glutcreatewindow("test");     glutreshapefunc(changeviewport);     glutdisplayfunc(render);     glewinit();      //verticies     glfloat vertices_0[] = {-0.5f, -0.5f, 0.0f, //0 -> bottom left                             -0.5f,  0.5f, 0.0f, //1 -> top left                              0.5f, -0.5f, 0.0f, //2 -> bottom right                              0.05f, 0.5f, 0.0f};//3 -> top right     //colors     glfloat colors_0[] = {1.0f, 0.0, 0.0f, 1.0f,//0                           0.0f, 1.0f, 0.0f, 1.0f,//1                           0.0f, 0.0f, 1.0f, 1.0f,//2                           1.0f, 0.0f, 1.0f, 1.0f};//3      //indicies -> triangle 1 -> 0 1 2, traingle 2 -> 1 3 2     gluint indicies_0[] = {0, 1, 2, 1, 3, 2};       //read vertex shader     char* vertexshadersourcecode = readfile("vertexshader.vsh");      //read fragment shader     char* fragmentshadersourcecode = readfile("fragmentshader.fsh");      //make vertex shader     gluint vertexshaderid = makevertexshader(vertexshadersourcecode);      //make fragment shader     gluint fragmentshaderid = makefragmentshader(fragmentshadersourcecode);      //make shader program     shaderprogramid = makeshaderprogram(vertexshaderid, fragmentshaderid);       printf("vertex shader id %d\n", vertexshaderid);     printf("fragment shader id %d\n", fragmentshaderid);     printf("shader program id %d\n", shaderprogramid);     printf("s_vposition's id %d\n", positionid);      //create vertex array object     glgenvertexarrays(1, &vao);      //bind vertex array object     glbindvertexarray(vao);      //create vertex buffer object     glgenbuffers(1, &vbo);      //bind vertex buffer object     glbindbuffer(gl_array_buffer, vbo);      //create buffer ->7 values 4 vertices     glbufferdata(gl_array_buffer, 7*4*sizeof(glfloat), null, gl_static_draw);      //starting @ beggining of buffer, place position data (3 values 4 verticies)     glbuffersubdata(gl_array_buffer, 0, 3*4*sizeof(glfloat), vertices_0);      //starting after placement of position data, place color data (4 values 4 verticies)     glbuffersubdata(gl_array_buffer, 3*4*sizeof(glfloat), 4*4*sizeof(glfloat), colors_0);      //generate index buffer     glgenbuffers(1, &indexbufferid);      //bind buffer     glbindbuffer(gl_element_array_buffer, indexbufferid);      //place index buffer data 6 indicies     glbufferdata(gl_element_array_buffer, 6*sizeof(gluint), indicies_0, gl_static_draw);       //get position attribute shader     positionid = glgetattriblocation(shaderprogramid, "s_vposition");      //get color attribute shader     colorid = glgetattriblocation(shaderprogramid, "s_vcolor");      //tell variables can find info in buffer     glvertexattribpointer(positionid, 3, gl_float, gl_false, 0, 0);      glvertexattribpointer(colorid, 4, gl_float, gl_false, 0, buffer_offset(sizeof(vertices_0)));      //tell program use shader program     gluseprogram(shaderprogramid);     //turn on position variable in shader     glenablevertexattribarray(positionid);     //turn on color variable in shader     glenablevertexattribarray(colorid);      glutmainloop();     return 0; } 

as mentioned in comments, there error in vertices definition (0.05f should 0.5f).

but, reason seeing 1 triangle instead of 2 line:

gldrawelements(gl_triangles, 4, gl_unsigned_int, null); 

the second parameter described in opengl documentation as

count - specifies number of elements rendered.

which index count (each element referenced entry in index array), , should number of entries in index array, not number of unique indexes. change 4 6.

note if used gl_triangle_strip or gl_triangle_fan instead of gl_triangles need 4 indexes {0, 1, 2, 3} , pass 4 doing, because each triangle after first 1 shares 2 of indexes previous triangle, total index count of (triangle count + 2). when using gl_triangles, each triangle specified separately 3 indexes each (triangle count * 3).