#include <complex.h>
long double complex
foo(long double complex z)
{
   return (ctanl(z));
}

cc -c foo.c
foo.c:7:12: warning: implicitly declaring library function 'ctanl' with type
      '_Complex long double (_Complex long double)'
      [-Wimplicit-function-declaration]
   return (ctanl(z));
           ^
foo.c:7:12: note: include the header <complex.h> or explicitly provide a
      declaration for 'ctanl'
1 warning generated.