Question

I am reviewing some optimisation libraries and came across the function signature

double solvopt(unsigned short n,
           double x[], 
           double fun(), 
           void grad(),
           double options[],
           double func(),
           void gradc()
          )

note that fun() and gard() are passed as function. My question is if this is valid standard C grammar.

Thanks.

Answer 1

The use of double fun() rather than double (*fun)() is an archaic form, that was only valid in standard C and never in C++, and if I remember correctly, only when declaring a function argument. (much like ary[] which is legal for a function argument, but not for an uninitialized variable)

Since it isn’t possible (in C) to pass a function by value to another function, the compiler just took double fun() to mean a pointer to a function that returned a double.

So this is valid (but archaic. has fallen out of favor)