Re: Switch to std::atomic?

[John W. Eaton]

On 9/26/19 9:53 PM, Rik wrote:

> Maybe my misinterpretation, but I thought to include the C++ header file
> <atomic>, rather than the C header file <stdatomic.h>.  I was looking at
> this reference: http://www.cplusplus.com/reference/atomic/ which seems to
> show that atomic_fetch_add exists under C++, but that the prototype we need
> to use may require casting to volatile.
>
> However, I couldn't get it to compile after making those changes, so it is
> harder than I think.

For

#include <atomic>

int
main (void)
{
  int x = 1;
  volatile int *xp = &x;
  std::atomic_fetch_add (xp, 1);
  return 0;
}

G++ issues the following error:

atom.cc: In function ‘int main()’:
atom.cc:8:31: error: no matching function for call to 
‘atomic_fetch_add(volatile int*&, int)’
    8 |   std::atomic_fetch_add (xp, 1);
      |                               ^
In file included from atom.cc:1:
/usr/include/c++/9/atomic:1417:5: note: candidate: ‘template<class _ITp> 
_ITp std::atomic_fetch_add(std::atomic<_ITp>*, std::__atomic_diff_t<_ITp>)’
 1417 |     atomic_fetch_add(atomic<_ITp>* __a,
      |     ^~~~~~~~~~~~~~~~
/usr/include/c++/9/atomic:1417:5: note:   template argument 
deduction/substitution failed:
atom.cc:8:31: note:   mismatched types ‘std::atomic<_ITp>’ and ‘volatile 
int’
    8 |   std::atomic_fetch_add (xp, 1);
      |                               ^
In file included from atom.cc:1:
/usr/include/c++/9/atomic:1423:5: note: candidate: ‘template<class _ITp> 
_ITp std::atomic_fetch_add(volatile std::atomic<_ITp>*, 
std::__atomic_diff_t<_ITp>)’
 1423 |     atomic_fetch_add(volatile atomic<_ITp>* __a,
      |     ^~~~~~~~~~~~~~~~
/usr/include/c++/9/atomic:1423:5: note:   template argument 
deduction/substitution failed:
atom.cc:8:31: note:   mismatched types ‘volatile std::atomic<_ITp>’ and 
‘volatile int’
    8 |   std::atomic_fetch_add (xp, 1);
      |
                  ^

The candidate functions both accept std::atomic objects, not bare 
pointers.  That's consistent with the prototypes shown here:

  https://en.cppreference.com/w/cpp/atomic/atomic_fetch_add

It looks like the C interface is not available in C++.  Is the info 
available at cplusplus.com out of date?

jwe