"invalid use of void expression"

Question

I need to write a macro that operates on bitarray looking like this:

   array[0] = number of bits in bitarray (integer) 
   array[1..n] = bits

Macro must look like:

GetBit(pointer, index)
Macro *must* return 0,1 or call function similar to exit().

This is my (working) inline-function version of macro that I'm supposed to write:

static inline unsigned long GetBit(BitArray_t array, unsigned long index)
{
        if ((index) >= array[0])
                        exit(EXIT_FAILURE);
        else
                return (GetBit_wo_boundary_checks(array,index));
}

This is what I've got:

#define GetBit(array,index)\
                (((index) < array[0] || exit(EXIT_FAILURE)) ?\
                        GetBit_wo_boundary_checks(array,index) : 0)

My problem is that this has to do index boundary check (i < p[0]) and exit before it tries to access undefined memory with GetBit_wo_boundary_checks(p,i).

I thought that I could work around this by putting exit into short-circuit evaluated condition, but I get: "invalid use of void expression".

Is there any way to make this expression macro transparently exit() when index is higher than maximum defined in array[0]?

Answer 1

One option would be to use the comma operator with your exit() call, like:

(exit(EXIT_FAILURE), 0)

It's a little hack-ish, but this will make the expression return the value 0 and should satisfy the compiler.

Answer 2

You could use the , operator to do this:

#define GetBit(array,index) \
            (((index) >= array[0]) \
             ? exit(EXIT_FAILURE), -1 \
             : GetBit_wo_boundary_checks(array,(index)))

Note that index is evaluated twice, which is not safe.