cabs

double cabs(double complex z);

cabs() compute the complex absolute value … also called modules, norm, or magnitude of z.

#include <stdio.h> /* including standard library */
#include <complex.h>
 
void main(void)
{
   double complex z=4.2 + I*2.34;
   double result;
 
   result = cabs(z);
   printf("cabs(%f + I*%f) = %f\n",creal(z), cimag(z), result);
   return 0;
}

  cabs(4.200000 + I*2.340000) = 4.807869