c++ 

<cmath>

     double abs (      double x );
      float abs (       float x );
long double abs ( long double x );

c

 int abs(int j);
 long int labs(long int j);
 long long int llabs(long long int j);

c 中要用 fabs
 double fabs(double x);
 float fabsf(float x);
 long double fabsl(long double x);