在 C 标识符中使用下划线的规则是什么？
What are the rules about using an underscore in a C identifier?
在 C（和其他语言）中为变量和函数的名称使用前缀和后缀是很常见的。特别是，偶尔会看到在“适当的”标识符之前或之后使用下划线，例如_x 和 _y 变量，或 _print 等。但是，还有避免名称以下划线开头的常识，以免与 C 标准库实现冲突。
那么，在什么地方、什么地方可以使用下划线呢？
足够好的经验法则
Don't start your identifier with an underscore.
就是这样。您可能仍然与某些特定于文件的定义有冲突（见下文），但这些只会给您一条错误消息，您可以自行处理。
安全，略有限制，经验法则
Don't start your identifier with:
- An underscore.
- Any 1-3 letter prefix, followed by an underscore, which isn't a proper word (e.g.
a_, st_)
memory_ or atomic_.
and don't end your identifier with either _MIN or _MAX.
此规则涵盖的内容比实际保留的要多，但更容易记住。
更详细的规则
这是基于C2x standard draft (and thus covers previous standards' reservations) and the glibc documentation。
不要使用：
- 前缀
__（两个下划线）。
- 一个下划线后跟一个大写字母的前缀（例如
_D）。
- 对于在文件范围内可见的标识符 - 前缀
_.
- 以下带下划线的前缀，当后跟小写字母时：
atomic_、memory_、memory_order_、cnd_、mtx_、thrd_, tss_
- 以下带下划线的前缀，后跟大写字符：
LC_、SIG_、ATOMIC、TIME_
- 后缀
_t（这是一个 POSIX 限制；对于 C 本身，您可以使用此后缀，除非您的标识符以 int 或 uint 开头）
附加限制是每个库头文件而不是通用的（其中一些是 POSIX 限制）：
If you use header file...
You can't use identifiers with ...
dirent.hPrefix d_
fcntl.hPrefixes l_, F_, O_, and S_
grp.hPrefix gr_
limits.hSuffix _MAX (also probably _MIN)
pwd.hPrefix pw_
signal.hPrefixes sa_ and SA_
sys/stat.hPrefixes st_ and S_
sys/times.hPrefix tms_
termios.hPrefix c_
当然还有不涉及下划线的额外限制。
C 标准，库章节，保留某些标识符（强调我的）：
C17 7.1.3 保留标识符
— All identifiers that begin with an underscore and either an uppercase letter or another
underscore are always reserved for any use.
— All identifiers that begin with an underscore are always reserved for use as identifiers
with file scope in both the ordinary and tag name spaces.
— Each macro name in any of the following subclauses (including the future library
directions) is reserved for use as specified if any of its associated headers is included;
unless explicitly stated otherwise (see 7.1.4).
— All identifiers with external linkage in any of the following subclauses (including the
future library directions) and errno are always reserved for use as identifiers with
external linkage.184)
— Each identifier with file scope listed in any of the following subclauses (including the
future library directions) is reserved for use as a macro name and as an identifier with
file scope in the same name space if any of its associated headers is included.
其中“为任何用途保留”是指为 compiler/standard 库保留，请参阅 “为实施保留”也意味着为 compiler/standard 库保留。
此外，未来的库方向C17.31保留了很多标识符-这是一个很大的章节，我只引用最值得注意的部分：
7.31.10 Integer types <stdint.h>
Typedef names beginning with int or uint and ending with _t may be added to the
types defined in the <stdint.h> header. Macro names beginning with INT or UINT
and ending with _MAX, _MIN, or _C may be added to the macros defined in the
<stdint.h> header.
7.31.12 General utilities <stdlib.h>
Function names that begin with str and a lowercase letter may be added to the
declarations in the <stdlib.h> header.
7.31.13 String handling <string.h>
Function names that begin with str, mem, or wcs and a lowercase letter may be added to the declarations in the <string.h> header.
直接回答你的问题：
So, where and where is it ok to use underscores?
严格来说：无处可去。你永远不应该声明以下划线开头的标识符，因为它们可能与标准库或语言关键字等冲突。尽管如上面粗体文本所暗示的那样，你可以在本地命名空间中使用一个下划线后跟一个小写字母。
在 C（和其他语言）中为变量和函数的名称使用前缀和后缀是很常见的。特别是，偶尔会看到在“适当的”标识符之前或之后使用下划线，例如_x 和 _y 变量，或 _print 等。但是，还有避免名称以下划线开头的常识，以免与 C 标准库实现冲突。
那么，在什么地方、什么地方可以使用下划线呢？
足够好的经验法则
Don't start your identifier with an underscore.
就是这样。您可能仍然与某些特定于文件的定义有冲突（见下文），但这些只会给您一条错误消息，您可以自行处理。
安全，略有限制，经验法则
Don't start your identifier with:
- An underscore.
- Any 1-3 letter prefix, followed by an underscore, which isn't a proper word (e.g.
a_,st_)memory_oratomic_.and don't end your identifier with either
_MINor_MAX.
此规则涵盖的内容比实际保留的要多，但更容易记住。
更详细的规则
这是基于C2x standard draft (and thus covers previous standards' reservations) and the glibc documentation。
不要使用：
- 前缀
__（两个下划线）。 - 一个下划线后跟一个大写字母的前缀（例如
_D）。 - 对于在文件范围内可见的标识符 - 前缀
_. - 以下带下划线的前缀，当后跟小写字母时：
atomic_、memory_、memory_order_、cnd_、mtx_、thrd_,tss_ - 以下带下划线的前缀，后跟大写字符：
LC_、SIG_、ATOMIC、TIME_ - 后缀
_t（这是一个 POSIX 限制；对于 C 本身，您可以使用此后缀，除非您的标识符以int或uint开头）
附加限制是每个库头文件而不是通用的（其中一些是 POSIX 限制）：
| If you use header file... | You can't use identifiers with ... |
|---|---|
dirent.h |
Prefix d_ |
fcntl.h |
Prefixes l_, F_, O_, and S_ |
grp.h |
Prefix gr_ |
limits.h |
Suffix _MAX (also probably _MIN) |
pwd.h |
Prefix pw_ |
signal.h |
Prefixes sa_ and SA_ |
sys/stat.h |
Prefixes st_ and S_ |
sys/times.h |
Prefix tms_ |
termios.h |
Prefix c_ |
当然还有不涉及下划线的额外限制。
C 标准，库章节，保留某些标识符（强调我的）：
C17 7.1.3 保留标识符
— All identifiers that begin with an underscore and either an uppercase letter or another underscore are always reserved for any use.
— All identifiers that begin with an underscore are always reserved for use as identifiers with file scope in both the ordinary and tag name spaces.
— Each macro name in any of the following subclauses (including the future library directions) is reserved for use as specified if any of its associated headers is included; unless explicitly stated otherwise (see 7.1.4).
— All identifiers with external linkage in any of the following subclauses (including the future library directions) and errno are always reserved for use as identifiers with external linkage.184)
— Each identifier with file scope listed in any of the following subclauses (including the future library directions) is reserved for use as a macro name and as an identifier with file scope in the same name space if any of its associated headers is included.
其中“为任何用途保留”是指为 compiler/standard 库保留，请参阅
此外，未来的库方向C17.31保留了很多标识符-这是一个很大的章节，我只引用最值得注意的部分：
7.31.10 Integer types <stdint.h> Typedef names beginning with int or uint and ending with _t may be added to the types defined in the <stdint.h> header. Macro names beginning with INT or UINT and ending with _MAX, _MIN, or _C may be added to the macros defined in the <stdint.h> header.
7.31.12 General utilities <stdlib.h>
Function names that begin with str and a lowercase letter may be added to the declarations in the <stdlib.h> header.7.31.13 String handling <string.h>
Function names that begin with str, mem, or wcs and a lowercase letter may be added to the declarations in the <string.h> header.
直接回答你的问题：
So, where and where is it ok to use underscores?
严格来说：无处可去。你永远不应该声明以下划线开头的标识符，因为它们可能与标准库或语言关键字等冲突。尽管如上面粗体文本所暗示的那样，你可以在本地命名空间中使用一个下划线后跟一个小写字母。