如何解释 C 预处理器输出中的#前缀行？

Question

下面是hello.c的代码：

#include <stdio.h>

int
main (void)
{
  printf ("Hello, world!\n");
  return 0;
}

我使用命令 gcc -E hello.c 对其进行预处理，得到以下输出：

# 1 "hello.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "hello.c"
# 1 "/usr/include/stdio.h" 1 3 4
# 27 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/features.h" 1 3 4
# 375 "/usr/include/features.h" 3 4
# 1 "/usr/include/sys/cdefs.h" 1 3 4
# 392 "/usr/include/sys/cdefs.h" 3 4
# 1 "/usr/include/bits/wordsize.h" 1 3 4
# 393 "/usr/include/sys/cdefs.h" 2 3 4
# 376 "/usr/include/features.h" 2 3 4
# 399 "/usr/include/features.h" 3 4
# 1 "/usr/include/gnu/stubs.h" 1 3 4
# 10 "/usr/include/gnu/stubs.h" 3 4
# 1 "/usr/include/gnu/stubs-64.h" 1 3 4
# 11 "/usr/include/gnu/stubs.h" 2 3 4
# 400 "/usr/include/features.h" 2 3 4
# 28 "/usr/include/stdio.h" 2 3 4





# 1 "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include/stddef.h" 1 3 4
# 212 "/usr/lib/gcc/x86_64-redhat-linux/4.8.5/include/stddef.h" 3 4
typedef long unsigned int size_t;
# 34 "/usr/include/stdio.h" 2 3 4

# 1 "/usr/include/bits/types.h" 1 3 4
# 27 "/usr/include/bits/types.h" 3 4
# 1 "/usr/include/bits/wordsize.h" 1 3 4
# 28 "/usr/include/bits/types.h" 2 3 4


typedef unsigned char __u_char;
typedef unsigned short int __u_short;
typedef unsigned int __u_int;
typedef unsigned long int __u_long;


typedef signed char __int8_t;
typedef unsigned char __uint8_t;
typedef signed short int __int16_t;
typedef unsigned short int __uint16_t;
typedef signed int __int32_t;
typedef unsigned int __uint32_t;

typedef signed long int __int64_t;
typedef unsigned long int __uint64_t;
////I omitted a lot in the following.

我知道这些信息可用于调试符号。但是我想知道每个FIELD的含义。但是，如果您能够回答以下问题，您的回答也将被接受。

# 1 "<built-in>" and # 1 "<command-line>"是什么意思？为什么它给了我完全不相关的内置和命令行消息？
#1 "/usr/include/stdc-predef.h" 1 3 4是什么意思？ 1 3 4 是什么意思？

谢谢。请注意，我知道预处理器将包含头文件。我想知道预处理输出的每个FIELD的含义。

Answer 1

预处理器输出的字段记录在 9 Preprocessor Output GNU cpp 手册。

字段是：

# linenum filename [flags]
...

这样的一行意味着后面的内容（...）起源于行文件 filename 中的编号 linenum。可选的 flags 是：-

‘1’

This indicates the start of a new file.

‘2’

This indicates returning to a file (after having included another file).

‘3’

This indicates that the following text comes from a system header file, so certain warnings should be suppressed.

‘4’

This indicates that the following text should be treated as being wrapped in an implicit extern "C" block.

您看到嵌套来自 #include 指令，例如

# 1 "hello.c"
# 1 "/usr/include/stdio.h" 1 3 4
# 27 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/features.h" 1 3 4
# 375 "/usr/include/features.h" 3 4
# 1 "/usr/include/sys/cdefs.h" 1 3 4
...

告诉我们：

在 "hello.c" 的第 1 行开始一个新文件 "/usr/include/stdio.h",
在第 27 行开始另一个新文件 "/usr/include/features.h"
在第 375 行开始另一个新文件 "/usr/include/sys/cdefs.h"
...

读起来不容易

特殊的“文件名”<built-in> 和 <command-line> 总是会出现在类似于以下的上下文中：-

# 1 "hello.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "hello.c"

在任何文件的预处理输出的顶部。详细信息可能因 GCC 版本而异。

如您所料，<built-in> 和 <command-line> 不是真实文件。他们是 non-file 需要以某种方式表示的预处理器标记的来源在输出格式中，因此它们被视为 就好像 它们是文件一样。叫他们 pseudo-files.

<built-in> 是包含编译器的 built-in 宏的 pseudo-file 定义。要查看此 pseudo-file 的内容，请浏览至 GCC dump preprocessor defines.

<command-line> 当然是预处理器（通常是 GCC）命令行，被视为宏定义的来源（可能 un-definitions），例如

gcc ... -DFOO=1 -UBAR ...

所以你的例子：

# 1 "hello.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "hello.c"
# 1 "/usr/include/stdio.h" 1 3 4
...

代表这个过程：

开始阅读hello.c，然后
立即读取built-in宏pseudo-file，然后
立即阅读命令行pseudo-file，然后
立即读取pre-defined宏文件/usr/include/stdc-predef.h 好像它是-include /usr/include/stdc-predef.h在命令行上pre-included，然后
继续阅读命令行pseudo-file，然后
继续阅读hello.c，然后
开始阅读/usr/include/stdio.h
...

如何解释 C 预处理器输出中的#前缀行？

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