How do I fix the following gcc warnings?

Question

I've started learning OpenGL and managed to create a spinning cube using vertex buffer objects. However, when I compile my code, gcc issues the following warnings:

|| sdlogl.c: In function ‘initGL’:
sdlogl.c|48| warning: implicit declaration of function ‘glGenBuffers’
sdlogl.c|50| warning: implicit declaration of function ‘glBindBuffer’
sdlogl.c|51| warning: implicit declaration of function ‘glBufferData’

What should I do to fix these warnings?

This is my code:

#include <SDL/SDL.h>
#include <SDL/SDL_opengl.h>
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glext.h>

SDL_Surface *surface;
float rquad=0.0f;
GLuint vertexBuffer,colorBuffer;

float vert[]={
     1, 1, 1,   1, 1,-1,  -1, 1,-1,  -1, 1, 1,
     1,-1, 1,   1,-1,-1,  -1,-1,-1,  -1,-1, 1,
     1, 1, 1,   1,-1, 1,   1,-1,-1,   1, 1,-1,
    -1, 1, 1,  -1,-1, 1,  -1,-1,-1,  -1, 1,-1,
     1, 1, 1,  -1, 1, 1,  -1,-1, 1,   1,-1, 1,
     1, 1,-1,  -1, 1,-1,  -1,-1,-1,   1,-1,-1
};
float colors[]={
    1,0,0,  1,0,0,  1,0,0,  1,0,0,
    0,1,0,  0,1,0,  0,1,0,  0,1,0,
    0,0,1,  0,0,1,  0,0,1,  0,0,1,
    1,1,0,  1,1,0,  1,1,0,  1,1,0, 
    1,0,1,  1,0,1,  1,0,1,  1,0,1, 
    0,1,1,  0,1,1,  0,1,1,  0,1,1
};

Uint32 timerfunc(Uint32 interval,void* param){
    rquad+=0.8f;
    return interval;
}

void initGL(){

    glViewport(0,0,640,480);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(45,640.0/480.0,0.1,100);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glShadeModel(GL_SMOOTH);
    glClearColor(0,0,0,0);
    glClearDepth(1);
    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LEQUAL);

    glGenBuffers(1,&vertexBuffer);
    glGenBuffers(1,&colorBuffer);
    glBindBuffer(GL_ARRAY_BUFFER,vertexBuffer);
    glBufferData(GL_ARRAY_BUFFER,72*sizeof(float),vert,
            GL_STATIC_DRAW);
    glBindBuffer(GL_ARRAY_BUFFER,colorBuffer);
    glBufferData(GL_ARRAY_BUFFER,72*sizeof(float),colors,
            GL_STATIC_DRAW);
}

void drawGLScene(){
    glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
    glLoadIdentity();
    glTranslatef(0,0,-6);
    glRotatef(rquad,1,1,0.5);

    glBindBuffer(GL_ARRAY_BUFFER,vertexBuffer);
    glVertexPointer(3,GL_FLOAT,0,0);
    glBindBuffer(GL_ARRAY_BUFFER,colorBuffer);
    glColorPointer(3,GL_FLOAT,0,0);

    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_COLOR_ARRAY);

    glDrawArrays(GL_QUADS,0,24);

    glDisableClientState(GL_VERTEX_ARRAY);
    glDisableClientState(GL_COLOR_ARRAY);

    glBindBuffer(GL_ARRAY_BUFFER,0);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER,0);

    SDL_GL_SwapBuffers();
}

int main(int argc, char** argv){
    int videoFlags=0;
    videoFlags|=SDL_OPENGL;
    videoFlags|=SDL_GL_DOUBLEBUFFER;
    videoFlags|=SDL_HWPALETTE;
    videoFlags|=SDL_HWSURFACE;
    videoFlags|=SDL_HWACCEL;

    int done=0;
    SDL_Event event;
    const SDL_VideoInfo* videoInfo;

    SDL_Init(SDL_INIT_VIDEO|SDL_INIT_TIMER);
    videoInfo=SDL_GetVideoInfo();
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER,1);

    SDL_WM_SetCaption("Hello OpenGL!",NULL);
    surface=SDL_SetVideoMode(640,480,32,videoFlags);

    initGL();
    SDL_AddTimer(10,timerfunc,NULL);
    while(!done){
        while(SDL_PollEvent(&event)){
            if(event.type==SDL_QUIT){
                done=1;
            }
        }
        drawGLScene();
    }
    SDL_Quit();
    return 0;
}

In addition, I compile my code using the following command: gcc sdlogl.c -g -Wall -lSDL -lGL -lGLU

Answer 1

Those warnings you get tell you, that the mentioned functions' prototypes have not been properly declared and the compiler is presuming the default C function signature int ().

Including glext.h will not give you the declarations though (for various reasons), but gives you proper typedefs to introduce those identifiers yourself. It gets tedious over time though. That's why wrapper libraries have been implemented. I recomment GLEW or GLEE. Using GLEE the whole thing boils down to including GL/glee.h instead of GL/gl.h.