I am new to shader programming. I am trying to draw a circle with glsl. My os is Ubuntu 12.04.I used a point with a Size and tried to filter out the points outside the radius.(Altering the alpha value).
The code is as follows:

Fragment Shader:
varying vec2 textureCoordinate;
const float circleBorderWidth = 0.08;//for anti aliasing
void main() {
float d = smoothstep(circleBorderWidth,0.1, 1.0-length(textureCoordinate));
gl_FragColor = vec4(0.0, 1.0, 0.0, d);
}

Vertex Shader:

attribute vec4 coord3d;
attribute vec2 varPos;
varying highp vec2 textureCoordinate;
void
main()
{
textureCoordinate = varPos;
gl_FrontColor = gl_Color;
gl_Position = vec4(coord3d.xyz,1.);
gl_PointSize = coord3d.w;
}

Data:

float pos[] = {
-1, -1,
-1, 1,
1, 1,
1, -1,
};

float vertices[]={0.0,0.0f,0.0f,100.0f};

My Draw Scene Method:

void drawScene() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
program->makeCurrent();
glEnable(GL_POINT_SMOOTH);
glEnable(GL_VERTEX_PROGRAM_POINT_SIZE);
glEnable(GL_BLEND);
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

if (varPosAttrib>=0) {
glVertexAttribPointer( varPosAttrib, 2, GL_FLOAT, GL_FALSE,
0, pos );-->varPos in Vertex SHader.
glEnableVertexAttribArray( varPosAttrib );
}

if (posAttrib>=0) {
glVertexAttribPointer(posAttrib, 4, GL_FLOAT, GL_FALSE, 0, vertices); ->coord3d in vertex shader
glEnableVertexAttribArray(posAttrib);
glDrawArrays(GL_POINTS, 0, 1);
}
glDisable(GL_POINT_SMOOTH);
glDisable(GL_VERTEX_PROGRAM_POINT_SIZE);
glDisable(GL_BLEND);
program->release();
glutSwapBuffers(); //Send the 3D scene to the screen
}

This results in drawing a square if I replace d with 1.0 in the following line(in fragment shader):
gl_FragColor = vec4(0.0, 1.0, 0.0, d); -> if d is replace by 1.0

I tried to replace the x and y values in gl_FragColor with textureCoordinate.x and textureCoordinate.y. Result was black(so I assume the values are 0.0). Thing which I dont understand is that if I take the length of textureCoordinate than it is always 1.0.(experimented by replacing the value in gl_fragcolor). I am unable to figure out as to what I am doing wrong here. I was expecting the textureCoordinate value to interpolate w.r.t the passed in data(varPos).
Thanks for any pointers.