Without the star in front of the parenthesis. I think tribe did that. That's what I get for not previewing.

This compiles fine, at least on its own (notice that I got rid of the semicolon at the end of the for also, this was an empty for followed by one single segmentLength execution). If there is any other problem it is in the caller of polygonPerimiter or within segmentLength. Try to follow all the other changes and figure out why I did them. Each one is important.

float segmentLength(float, float, float, float);
float polygonPerimeter (float *x, float *y, int pnts)
{
float d=0.0;
int i;
for (i=0; i<pnts; i++)
d += segmentLength (x[i], x[i+1], y[i], y[i+1]);

return d;
}