carg, cargf,
  cargl —
complex argument functions
#include <complex.h>
double
  
  carg(double
    complex z);
float
  
  cargf(float
    complex z);
long double
  
  cargl(long
    double complex z);
These functions compute the argument (also called phase angle) of
  z, with a branch cut along the negative real axis.
These functions return the value of the argument in the interval
  [-pi, +pi].
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 .