为什么 Windows 上的 Clang 运行 不符合 C（__STDC__ 未定义为 1）？

Question

示例代码（t127.c）：

#include <stdio.h>

int main(void)
{
    int ret;
#if __STDC__ == 1
    printf("Has C conformance to version ");
#if __STDC_VERSION__
    printf("%ld", __STDC_VERSION__);
#else
    printf("1989");
#endif
    printf(" OR has no C conformance but __STDC__ is defined to 1\n");
    ret = 0;
#else
    printf("Has no C conformance\n");
    ret = 1;
#endif
    return ret;
}

调用：

$ clang t127.c -std=c11 -Wall -Wextra -pedantic && ./a.exe
Has no C conformance

# comparison with gcc
$ gcc t127.c -std=c11 -Wall -Wextra -pedantic && ./a.exe
Has C conformance to version 201112 OR has no C conformance but __STDC__ is defined to 1

$ clang --version
clang version 11.0.1
Target: x86_64-pc-windows-msvc
Thread model: posix

$ gcc --version
gcc (GCC) 10.2.0
Copyright (C) 2020 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.

$ systeminfo
OS Name:                   Microsoft Windows 10 Pro
OS Version:                10.0.19041 N/A Build 19041

问题：

1. 为什么 Windows 上的 Clang 运行 不符合 C（__STDC__ 未定义为 1）？ IE。什么（技术）障碍阻止“Windows 上的 Clang 运行”将 __STDC__ 定义为 1？
2. 需要指定哪些额外选项才能使 Clang C 符合 Windows？

Answer 1

原来__STDC__的定义依赖于clang的分布

@M.M

works fine for me, target x86_64-w64-windows-gnu . Sounds like you are using a non-conforming build, perhaps you could file a report with wherever you got this build from

感谢您的确认和想法。

在上面的测试中使用了clang version 11.0.1。它是从 LLVM-11.0.1-win64.exe 获得的，发货时间为 https://github.com/llvm/llvm-project/releases/tag/llvmorg-11.0.1。

比较通过 Cygwin 安装程序安装的 clang (setup-x86_64.exe) 显示 C 一致性：

$ clang t127.c -std=c11 -Wall -Wextra -pedantic && ./a.exe
Has C conformance to version 201112 OR has no C conformance but __STDC__ is defined to 1

$ clang --version
clang version 8.0.1 (tags/RELEASE_801/final)
Target: x86_64-unknown-windows-cygnus
Thread model: posix

@克利福德

What happens if you change the test to #if STDC == 1 || defined STDC_VERSION?

#if __STDC__ == 1 || defined __STDC_VERSION__

调用：

$ /cygdrive/d/LLVM/11.0.1/bin/clang t127.c -std=gnu11 -Wall -Wextra -pedantic && ./a.exe
Has C conformance to version 201112 OR has no C conformance but __STDC__ is defined to 1

所以，出于某些（什么？）原因，LLVM-11.0.1-win64.exe 中的 clang version 11.0.1 没有将 __STDC__ 定义为 1.