Question

The following code compiles on VS Express Edition 2008, yet I get the following errors when trying to debug/release:

1>Linking...
1>MSVCRT.lib(crtexew.obj) : error LNK2001: unresolved external symbol _WinMain@16
1>E:\Documents and Settings\Administrator\Desktop\Proyecto 2 gràficas\testing\Release\testing.exe : fatal error LNK1120: 1 unresolved externals

is it because it’s unable to link a library? How can I solve this?

#include <windows.h>   // use as needed for your system

#include <gl/Gl.h>
#include <gl/glut.h>
#include <iostream>
#include <fstream>

using namespace std;

//**************Global Data
char ifileName[30];
char oFileName[30];
fstream inFile;
fstream outFile;

//************ Data structure 
struct GLfloatPoint
{ GLfloat x,y;
};

const int MAX = 100;
class GLfloatPointArray
{
public:
  int num;
  GLfloatPoint pt[MAX];
};

//***************** subprograms
typedef GLfloat colorType[3];
// subprogram used to draw the control points separately
void drawDot (GLfloat x, GLfloat y, GLfloat r, GLfloat g, GLfloat b)
{ glColor3f(r,g,b);
  glBegin (GL_POINTS);
      glVertex2f (x,y);
  glEnd();
}
// Drawing subprogram - this will draw the curve from a set of points
void drawFloatPolyLine (GLfloatPointArray P, colorType c)
{ glColor3fv (c);
  glBegin(GL_LINE_STRIP);
   for (int i=0; i < P.num; i++)
     glVertex2f (P.pt[i].x,P.pt[i].y);
  glEnd();
}

//******************** myInit 
 void myInit(void)
 {
    glClearColor(1.0,1.0,1.0,0.0);  // set white background color
    glColor3f (0.0f,0.0f,0.0f);    //default color
    glMatrixMode(GL_PROJECTION); 
    glLoadIdentity();
    gluOrtho2D(0.0, 640.0, 0.0, 480.0);
    // get data files
    cout << "Enter the input file name: ";
    cin >> ifileName;
    inFile.open (ifileName,ios::in);
    if (inFile.fail())
      return;
    // if want to see points for debugging add output
    // cout << "Enter the output file name: ";
    // cin >> oFileName;
    // outFile.open (oFileName,ios::out);
    // if (outFile.fail())
    //   return;
}

//***************** BEZIER Curve subprograms
// Read the control points 
void readControlPoints (GLfloatPointArray &P)
{
  inFile >> P.num;
  for (int j = 0; j < P.num; j++)
    inFile >> P.pt[j].x >> P.pt[j].y;
}
// output control points to a text file - if you want to see the 
// computations
void printPointArray (GLfloatPointArray P)
{
  outFile << "Size: " << P.num << endl << "Points:" << endl;
  for (int j = 0; j < P.num; j++)
    outFile << "(" << P.pt[j].x << "," << P.pt[j].y << ")" << endl;
}

const int MAXCONTPTS = 100;
int c[MAXCONTPTS];   // the binomial coefficients
//helper routines - compute the coefficient
void ComputeCoeff (int n)
{ int j,k;
  for (k=0;k<=n;k++)
  { //compute n! / (k!*(n-k)!)
    c[k] = 1;
    for (j = n;j>=k+1;j--)
      c[k] *=j;
    for (j = n-k;j>=2;j--)
      c[k] /= j;
  }
}
// compute the blending value
float BlendingValue (int n, int k, float t)
{ int j;
  float bv;
  // compute  c[k]*t^k * (1-t)^(n-k)
  bv = c[k];
  for (j=1; j<=k;j++)
    bv *= t;
  for (j = 1;j<=n-k;j++)
    bv *= (1-t);
  return bv;
}

// compute one point on the Bezier curve - fixed value of t
void ComputePoint (float t, int n, GLfloatPoint & p, 
                   GLfloatPointArray ctrlPts)
{ int k;
  float b;
  p.x = 0.0;
  p.y = 0.0;
  for (k = 0; k<=n;k++)
  {  b = BlendingValue (n,k,t);
     p.x += ctrlPts.pt[k].x*b;
     p.y  += ctrlPts.pt[k].y*b;
  }
}
// compute the array of Bezier points - drawing done separately
void Bezier ( GLfloatPointArray controlPts, int numInter, 
              GLfloatPointArray & curve)
{ // there are numContPts+1 control points and numInter t values to evaluate the curve
  int k;
  float t;
  ComputeCoeff (controlPts.num-1);
  curve.num = numInter+1;
  for (k=0; k<=numInter; k++)
  { t = (float) k / (float) numInter;
    ComputePoint (t, controlPts.num-1,curve.pt[k],controlPts);
  }
}

//************************ myDisplay 
void myDisplay(void)
{  
   int numbCurves;
   GLfloatPointArray ControlPts,BezCurve;
   colorType C = {0.0f,1.0f,0.0f};

   glClear(GL_COLOR_BUFFER_BIT);     // clear the screen 

   inFile >> numbCurves;
   for (int i = 0; i < numbCurves; i++)
   { 
     // read control points and draw them in red big points
     readControlPoints (ControlPts);
     glPointSize (4.0);
     for (int j = 0; j < ControlPts.num; j++)
       drawDot (ControlPts.pt[j].x,ControlPts.pt[j].y,1,0,0);
     glPointSize (1.0);
     // Compute the Bezier curve points and draw
     Bezier (ControlPts,50,BezCurve);
       // draw the Bezier curve
     drawFloatPolyLine (BezCurve,C);
     glFlush ();
   }
}

//**************************** main
void main(int argc, char** argv)
{
    glutInit(&argc, argv);          // initialize the toolkit
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); // set display mode
    glutInitWindowSize(640,480);     // set window size
    glutInitWindowPosition(100, 150); // set window position on screen
    glutCreateWindow("Bezier Curve drawing"); // open the screen window
    glutDisplayFunc(myDisplay);     // register redraw function
    myInit();                   
    glutMainLoop();              // go into a perpetual loop
}

Answer 1

I’m surprised the error message didn’t actually give you the name of the unresolved external.

However, there’s a good chance you didn’t link with the GLUT libraries. It’s not enough to just include the header files, all they do is inform the compiler what capabilities are provided by the library. They do not automatically figure out which file contains the code for that library.

From what I’ve seen, GLUT comes with a lib and a dll file. You will probably have to link with the library and ensure the DLL is accessible on the path somewhere.

---

I see from your update that the linker is actually telling you the unresolved function WinMain.

That means you’ve created a non-console mode application, I believe. Instead of main, your entry point in your code will have to be WinMain:

int WINAPI WinMain (
    HINSTANCE hInstance,
    HINSTANCE hPredInstance,
    LPSTR lpCmdLine,
    int nShowCmd)
{
    // Weave your magic here.
} 

Or, you could re-create your project as a console mode application so that main is the correct entry point.