C Macro with varargs - c

Question

I am trying to write a macro which returns the smallest value of several integers. When I compile the following code, it throws an error "expected expression". I don't know what's wrong there. Could anyone point out the issues with this code?
#define SMALLEST (nums, (ret_val), ...) \
do { \
int i, val; \
va_list vl; \
va_start(vl,nums); \
(*ret_val) = va_arg(vl, int); \
for (i = 1; i < nums; i++) \
{ \
val=va_arg(vl, int); \
if ((*ret_val) > val) \
(*ret_val) = val; \
} \
va_end(vl); \
} while(0)
int main ()
{
int nums = 3;
int ret_val = 0;
SMALLEST(nums, &ret_val, 1, 2, 3);
return 0;
}
I am just curious about how to do it with Macro.

Answer 1

I am just curious about how to do it with Macro.
You can't. va_list is a way for a variadic function to access its arguments. What you have written is a variadic macro. They are not the same (in particular the variadic macro is still only a syntactic convenience that does not let you process individual arguments). The only way to do what you want is to call a variadic function of your own design inside the variadic macro (and then you might as well eliminate the macro).
However, if you really insist on using a variadic macro, it turns out that you are lucky that the same separator , is used in macro arguments and in array initializers, so you can try something like:
#define F(X, ...) \
do { \
int t[] = { __VA_ARGS__ }; \
for (int i = 0; i < sizeof t / sizeof t[0]; i++) \
… \
} while (0)

Answer 2

I don't think you can. From the gcc manual (https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html) the best you can do in a standard way is write __VA_ARGS__, which will expand the arguments in place (for example to pass to a function).
It then goes on to define other non-standard extensions, which you might be able to use, but wouldn't be standard.
Why not do it with a function?

Answer 3

The way you deal with argument lists in variadic macros is not the same as the way you deal with them in variadic functions. Instead of using va_list and its related macros, you use __VA_ARGS__.
That is pretty much the extent of it: you cannot write a macro that processes variadic list from the beginning to the end; you are limited to passing the arguments through to a variadic function, which performs the actual processing.
Note: Your implementation is incorrect, too: you should be using va_start(vl,ret_val) instead of va_start(vl,nums), because you are supposed to pass the last argument before ... to va_start.
If I were to rewrite this as a function, though, I would drop the ret_val pointer, and make a function that returns a value the regular way.

Answer 4

It makes no sense to do that with macro. That's what functions are for.
You get an error because the SMALLEST symbol in your main function gets replaced by the whole body of the function you've defined. AFAIK you can't define a function inside another function in C.

Answer 5

Is there any particular reason why you want to use a macro here? You seem to be confusing macro syntax and standard syntax (the reason for your error).
You should use a function to achieve this - this is what a function is for. The following code should get you what you want:
int Smallest( int iNumberOfIntegers, ... )
{
va_list args = NULL;
int i = 0;
int iSmallestValue = 0;
int iCurrentValue = 0;
va_start( args, iNumberOfIntegers );
iSmallestValue = va_arg( args, int );
for(i = 0; i < iNumberOfIntegers - 1; i++)
{
iCurrentValue = va_arg( args, int );
if(iSmallestValue > iCurrentValue)
{
iSmallestValue = iCurrentValue;
}
}
return iSmallestValue;
}
Of note, you you need to pass the size of the variadic argument if you are going to loop over it in this manner. This is not necessary in format strings because the compiler can infer the number from the format string specifiers.
We subtract 1 from the loop to account for a 0 offset.
Edit: And, as others have said, you can't use a variadic macro in the way you were trying.