PointOnLinePosition
From Solid Graphics Wiki

The PointOnLinePosition is function inverse to the LinePositionToPoint function. It translates point's coordinates into point's position on a line value.
float PointOnLinePosition ( const float P0[ 3 ], const float P1[ 3 ], const float Q[ 3 ] );
double PointOnLinePosition ( const double P0[ 3 ], const double P1[ 3 ], const double Q[ 3 ] );
Parameters
P0  x,y,z coordinates of first point defining the line 
P1  x,y,z coordinates of second point defining the line 
Q  the x,y,z coordinates of a point lying on line defined by points P0 and P1 which coordinates are being translated by the the PointOnLinePosition function to the point's position on line value. 
See Also