cabs, cabsf,
  cabsl —
return a complex absolute value
#include <complex.h>
double
  
  cabs(double
    complex z);
float
  
  cabsf(float
    complex z);
long double
  
  cabsl(long
    double complex z);
#include
    <tgmath.h>
complex-floating
  
  cabs(complex-floating
    z);
These functions compute the complex absolute value (also called norm, modulus,
  or magnitude) of z.
These functions return the complex absolute value.
IEEE Std 1003.1-2001 (“POSIX.1”)
  ⟨complex.h⟩
Portions of this text are reprinted and reproduced in electronic form from IEEE
  Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
  Operating System Interface (POSIX), The Open Group Base Specifications Issue
  6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
  Engineers, Inc and The Open Group. In the event of any discrepancy between
  this version and the original IEEE and The Open Group Standard, the original
  IEEE and The Open Group Standard is the referee document. The original
  Standard can be obtained online at http://www.opengroup.org/unix/online.html .