how to represent complex number in C?

How about

typedef struct Complex { double r; double i;} Complex;

And define all the needed function, e.g.

Complex add(Complex x, Complex y) { Complex z; z.r = x.r+y.r; z.i=x.i+y.i; return z; }

Etc.
Usage:

Complex a...; Complex b...; Complex c = add(a, b);

Cheers
Andi

PS: or you use the predefined <complex.h> implementation of the latest C standard as suggested in solution #2.