Is it OK to cast an _Atomic Type * to void * and back?

Question

Some libraries require a callback function takes an void *(not _Atomic void *). Is it OK to cast an _Atomic Type * to void * and cast back to use it? Like the following code:

#include <stdio.h>
int Call(int(*f)(void*), void *arg){
  return f(arg);
}
int Use(void *arg){
  return *(_Atomic int *)arg;
}
_Atomic int g_x;
int main(int argc, const char **argv){
  g_x = argc;
  int y = Call(Use, (void*)&g_x);
  printf("%d\n",y);
}

Answer 1

C 2018 6.3.2.3 1 says:

A pointer to void may be converted to or from a pointer to any object type. A pointer to any object type may be converted to a pointer to void and back again; the result shall compare equal to the original pointer.

So there is no issue in merely converting the pointer to void * and back; the behavior is defined by the C standard. This specification of the conversions does not restrict them regarding atomic types in any way.

The properties regarding atomic types apply to accessing atomic objects. (In particular, C 2018 6.7.2.4 4 says “The properties associated with atomic types are meaningful only for expressions that are lvalues…”) As long as the void * is not used to access the object through some non-atomic type, it has no consequence on the use of the object.