/* csos.c - circular buffer implementation of a single SOS */void wrap();double csos(a, b, w, p, x)                      /* \(a,b,w\) are 3-dimensional */double *a, *b, *w, **p, x;                      /* \(p\) is circular pointer to \(w\) */{       double y, s0;       *(*p) = x;                               /* read input sample \(x\) */       s0  = *(*p)++;           wrap(2, w, p);       s0 -= a[1] * (*(*p)++);  wrap(2, w, p);       s0 -= a[2] * (*(*p)++);  wrap(2, w, p);       *(*p) = s0;                              /* \(p\) has wrapped around once */       y  = b[0] * (*(*p)++);  wrap(2, w, p);       y += b[1] * (*(*p)++);  wrap(2, w, p);       y += b[2] * (*(*p));                     /* \(p\) now points to \(s\sb{2}\) */       return y;}