In the source code :

tol1=2.0*EPS*fabs(b)+0.5*tol;
xm=0.5*(c-b);
if(fabs(xm)<=tol1 || fb==0) return b;

I don't know what's the meaning of 2.0*EPS*fabs(b),and why fabs(xm)<=tol1,then return b as the root?