src.cpp: In function 'int main()':
src.cpp:7:62: error: 'sqrt' was not declared in this scope
#define area(s, a, b, c) sqrt(s * (s - a) * (s - b) * (s - c))
^
src.cpp:11:18: note: in expansion of macro 'area'
printf("%.3lf", area(S(a, b, c), a, b, c));
^