[LLVMbugs] [Bug 23567] New: atomic_load() requiring a pointer to non-const is not helpful.

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23567

            Bug ID: 23567
           Summary: atomic_load() requiring a pointer to non-const is not
                    helpful.
           Product: clang
           Version: 3.6
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: toojays at toojays.net
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

jscott at citra:/tmp$ cat atomic.c 
/* The following lines work around Clang bug 22740, where Clang tries to use
 * GCC's stdatomic.h, and fails.
 */
#include <stddef.h>
#include <stdint.h>
#undef __STDC_HOSTED__
#define __STDC_HOSTED__ 0
#define __CLANG_STDINT_H
#include <stdatomic.h>
#undef __CLANG_STDINT_H
#undef __STDC_HOSTED__
#define __STDC_HOSTED__ 1

struct s
{
  atomic_int i;
};

int f (const struct s *p_s)
{
  return atomic_load(&p_s->i);
}
jscott at citra:/tmp$ clang -std=c11 -c atomic.c 
atomic.c:21:10: error: address argument to atomic operation must be a pointer
to non-const _Atomic type ('const atomic_int *' (aka 'const _Atomic(int) *')
invalid)
  return atomic_load(&p_s->i);
         ^           ~~~~~~~
/usr/lib/llvm-3.6/bin/../lib/clang/3.6.1/include/stdatomic.h:134:29: note:
expanded from macro 'atomic_load'
#define atomic_load(object) __c11_atomic_load(object, __ATOMIC_SEQ_CST)
                            ^
1 error generated.


My reading suggests that although this is a strictly correct interpretation of
the C11 standard, it isn't useful. There's no practical reason why
atomic_load() shouldn't accept a pointer to non-const.

Martin Sebor has submitted C defect report 459 about this.
<http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_459.htm>


jscott at citra:/tmp$ clang --version
Ubuntu clang version 3.6.1-svn236856-1~exp1 (branches/release_36) (based on
LLVM 3.6.1)
Target: x86_64-pc-linux-gnu
Thread model: posix

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