clanguage-lawyeratomicincomplete-typequalifiers

Is applying _Atomic type qualifier to an incomplete type valid?

Is applying _Atomic type qualifier to an incomplete type valid?

Example:

_Atomic void* p;
_Atomic struct S* p1;
_Atomic struct S x;
struct S { int i; };

Invocations:

$ gcc t0.c -std=c11 -pedantic -Wall -Wextra -c
<nothing>

$ clang t0.c -std=c11 -pedantic -Wall -Wextra -c
<source>:1:1: error: _Atomic cannot be applied to incomplete type 'void' 
<source>:2:1: error: _Atomic cannot be applied to incomplete type 'struct S' 
<source>:3:1: error: _Atomic cannot be applied to incomplete type 'struct S'

In the C11 standard (as well as in the C2x draft n2596.pdf) I couldn't find any constraints / semantics, which forbid / restrict applying _Atomic to an incomplete type.

Solution

  • From https://github.com/llvm/llvm-project/issues/36585:

    it does appear to be valid