Is this valid standard c?

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)

Answer 2

func() and gradc() are in the proper form for functions with unknown parameters. I'm pretty sure this was an acceptable syntax even for Unix 6 circa 1975.

The [] parameters are the same as *, now and in the past. In fact, I remember a 1980s dispute over which was more proper:

int main (int argc, char **argv, char **envp)

or

int main (int argc, char *argv[], char *envp[])

There is no different in effect. The dispute is which is more correct according to semantics.

Answer 3

Your array parameters (x and options) should probably be pointers.

Referring to a function with parenthesis, like "func()" or "gradc()" calls the function. The name of the function alone is a code pointer that can be dereferenced to call the function in question.

When in doubt, try compiling this with an ANSI C compiler - a lot of compilers provide an ANSI compatibility switch to enforce standards compliance. Also, the K&R book is your friend.

Is this a homework question?