Question

test.c

int main () {
   void a;
   return 0;
}

I use gcc to compile but it gives me an error:

error: variable or field ‘a’ declared void

From what I read here,
I thought I can declare void variable without problem.

Answer 1

As your link states:

A variable that is itself declared void (such as my_variable above) is useless; it cannot be assigned a value, cannot be cast to another type, in fact, cannot be used in any way.

This means that although syntactically correct, a void declaration is not useful to anything, this is why GCC could consider it an error. Even if it would compile, you won’t be able to do anything with the variable, so I guess your question is just related to testing this behavior.

In fact void is useful just when we’re talking about pointers (void*) since it allows you to declare a generic pointer without specifying the type.