How do I produce a Negative Buffer of a polygon in GEOS? - buffer

Question

I am trying to produce a negative buffer (offset) of a polygon using GEOS (with the c api).
So far I have accomplished a positive offset (see image), however when I try to make the width '-'ve, it produces nothing. I suspect that 'offset' requires clipping somehow prior to retrieving the exterior ring.
Had a good look through documentation but can't solve it, any help would be greatly appreciated!!
My code is below if that helps at all:
#include <geos_c.h>
// Define coordinate sequence
int noPoints = 6 + 1; // +1 because it is a loop
GEOSCoordSequence* points = GEOSCoordSeq_create(noPoints, 2 /*# ordinates*/);
GEOSCoordSeq_setX(points, 0, 0);
GEOSCoordSeq_setY(points, 0, 0);
GEOSCoordSeq_setX(points, 1, 0);
GEOSCoordSeq_setY(points, 1, 50);
GEOSCoordSeq_setX(points, 2, 50);
GEOSCoordSeq_setY(points, 2, 50);
GEOSCoordSeq_setX(points, 3, 50);
GEOSCoordSeq_setY(points, 3, 0);
GEOSCoordSeq_setX(points, 4, 30);
GEOSCoordSeq_setY(points, 4, 15);
GEOSCoordSeq_setX(points, 5, 20);
GEOSCoordSeq_setY(points, 5, 15);
GEOSCoordSeq_setX(points, noPoints-1, 0);
GEOSCoordSeq_setY(points, noPoints-1, 0);
// Define linear ring
GEOSGeometry* ploop = GEOSGeom_createLinearRing(points);
// Define offset
double width = 15;
int quadsegs = 100;
int endCapStyle = 1;
int joinStyle = 1;
double mitreLimit = 1.0;
const GEOSGeometry* offset = GEOSBufferWithStyle(ploop, width, quadsegs, endCapStyle, joinStyle, mitreLimit);
// Get exterior ring
const GEOSGeometry* exteriorRing = GEOSGetExteriorRing(offset);
// Convert to coord sequence and draw points
const GEOSCoordSequence *coordSeq = GEOSGeom_getCoordSeq(exteriorRing);
uint numPoints = GEOSGeomGetNumPoints(exteriorRing);
double xCoord, yCoord;
for (uint p = 0; p < numPoints; p++) {
GEOSCoordSeq_getX(coordSeq, p, &xCoord);
GEOSCoordSeq_getY(coordSeq, p, &yCoord);
printf("x: %g\ty:%g\n", xCoord, yCoord);
drawPoint(xCoord, yCoord);
}
Thanks in advance!

Answer 1

Finally solved it! the geometry I was feeding in was a linear ring, rather than a polygon. For anyone with a similar issue in the future:
Replace:
GEOSGeometry* ploop = GEOSGeom_createLinearRing(points);
With:
GEOSGeometry* rloop = GEOSGeom_createLinearRing(points);
const GEOSGeometry* ploop = GEOSGeom_createPolygon(rloop, NULL, 0);