Using C11 `stdnoreturn.h` causes compiler warnings in headers

[Jonathon Anderson]

The public headers introduce a compiler warning related to the noreturn attribute if stdnoreturn.h is included before they are. This prevents -Werror builds from passing for XED users using C11 features:

# gcc --version
gcc (Ubuntu 13.2.0-23ubuntu4) 13.2.0
Copyright (C) 2023 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

# cat example.c
#include <stdnoreturn.h>
#include <xed/xed-interface.h>
# gcc example.c -I kits/xed-*/include -Werror
In file included from kits/xed-install-base-2024-09-05-lin-x86-64/include/xed/xed-decoded-inst.h:26,
                 from kits/xed-install-base-2024-09-05-lin-x86-64/include/xed/xed-decode.h:24,
                 from kits/xed-install-base-2024-09-05-lin-x86-64/include/xed/xed-interface.h:40,
                 from example.c:2:
kits/xed-install-base-2024-09-05-lin-x86-64/include/xed/xed-util.h:120:1: error: '_Noreturn' attribute directive ignored [-Werror=attributes]
  120 | XED_NORETURN XED_NOINLINE XED_DLL_EXPORT void xed_internal_assert(const char* s, const char* file, int line);
      | ^~~~~~~~~~~~
cc1: all warnings being treated as errors

[Jonathon Anderson]

Unfortunately I can't help but be highly suspicious of a hidden bit.ly link posted not even a second after posting my issue. 🫤

@z3t444 If this isn't a bot spam or hacked account, could you post the fix as a PR as per the contribution guidelines instead of through these unconventional means?

[Maor Arjevani]

Thank you for reporting the issue.

The XED library uses the __attribute__((noreturn)) directive to mark functions that do not return, instead of the _Noreturn keyword. This decision was made to maintain backward compatibility with C99 standards (before C11) where _Noreturn is not supported.

When you include <stdnoreturn.h> (a C11 header) before the XED library, GCC encounters a potential conflict between the _Noreturn keyword (introduced in C11) and the __attribute__((noreturn)) attribute used in XED. Both serve the same purpose, but GCC may issue warnings when it detects both attributes in the same code, sometimes ignoring one.

To resolve this issue, I recommend swapping the order in which you include headers. By including the XED library headers before including <stdnoreturn.h>, you ensure that GCC correctly handles the __attribute__((noreturn)) directive used by the XED library, while still allowing you to use the _Noreturn keyword in your own code if needed.

You can adjust it to:

#include <xed/xed-interface.h>
#include <stdnoreturn.h>

Please let us know if this solution works for you.

[Jonathon Anderson]

Thank you for the comprehensive response. Swapping the order of #includes does work in this simple example, however in our full codebase we include external C11 headers that do or may in the future include stdnoreturn.h themselves. Combined with #include sorting it is generally difficult for us to ensure Xed headers are always #included before stdnoreturn.h.

Please fix the headers to work in a C11 codebase. For C99 compatibility you can use the __attribute__((__noreturn__)) alias, but I recommend using the standardized noreturn or [[noreturn]] attributes when compiling in C11 or C23 mode respectively.

[Maor Arjevani]

A fix will be available with the upcoming external release.