让我们采用简单的“hello world！“C程序名为main.c

#include <stdio.h>
int main() {
   printf("Hello, World!");
   return 0;
}

预处理器

• 此阶段的输入文件是*.c文件。
• 它执行以下任务：
• 删除评论：它删除了所有的评论。
• 线路拼接（\）：反斜杠\用于行拼接，允许您将长代码行拆分为多行以提高可读性，并连接成一行。
• 头文件包含：例如，如果指令#include <stdio.h>在程序中可用，则预处理器解释该指令并将该指令替换为/usr/include/stdio.h文件的内容。
• • 宏 * 扩展：宏是一个预处理器指令，允许您为代码序列定义符号名称或标识符。它们用于创建可以轻松重用的代码段，并引入简单的文本替换。在C中，宏最常见的用法是通过#define指令，它定义了一个宏。它们可以是常量宏，例如#define BUFFER_SIZE 1024，也可以是类似函数的宏，例如#define MAX(a, b) ((a) > (b) ? (a) : (b))。
• 解析 * 条件编译指令 *：使用特殊的预处理指令，您可以根据各种条件包含或排除程序的某些部分。例如，#ifdef DEBUG printf("Debugging is enabled.\n"); #endif。其他条件编译是#ifndef，#if，#elif，#else。预处理器评估这些条件，并确定是否应将所包含的代码包含在预处理的输出文件中。
• • 线路控制 *：C预处理器通知C编译器源代码中每个标记的来源位置。C语言中的标记可以定义为C编程语言中对编译器有意义的最小单个元素。它是C程序的基本组成部分。它们可以是关键字（double，if，while，return，.）;标识符（变量和函数名）; Constants（const int c_var = 20;）;字符串.特殊符号（[]，()，{}，,，#，.）运算符（一元、二元和三元运算符）;
• 简而言之，预处理器扩展代码。
• 输出文件为*.i或预处理文件。这个预处理的文件就是所谓的translation unit（或者更随便地说是编译单元），它是C或C++编译器的最终输入，编译器将从中生成目标文件。我们的静态函数只在这个文件中可见。

gcc -E main.c -o main.i

• 选项-E在预处理阶段后停止;不要正确运行编译器。输出是以预处理的源代码的形式，它被发送到标准输出（或发送到带有-o选项的文件）。

也可以直接使用C（C预处理器）获取预处理后的文件：

cpp main.c -o main2.i

请注意，这两个文件完全相同

❯ cmp main.i main2.i && echo "Files are equal" || echo "Files are not equal"
  Files are equal

输出是

# 0 "main.c"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "main.c"
# 1 "/usr/include/stdio.h" 1 3 4
# 27 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/libc-header-start.h" 1 3 4
# 33 "/usr/include/x86_64-linux-gnu/bits/libc-header-start.h" 3 4
# 1 "/usr/include/features.h" 1 3 4
# 392 "/usr/include/features.h" 3 4
# 1 "/usr/include/features-time64.h" 1 3 4
# 20 "/usr/include/features-time64.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4
# 21 "/usr/include/features-time64.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 1 3 4
# 19 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4
# 20 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 2 3 4
# 22 "/usr/include/features-time64.h" 2 3 4
# 393 "/usr/include/features.h" 2 3 4
# 486 "/usr/include/features.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 1 3 4
# 559 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4
# 560 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/long-double.h" 1 3 4
# 561 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 2 3 4
# 487 "/usr/include/features.h" 2 3 4
# 510 "/usr/include/features.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/gnu/stubs.h" 1 3 4
# 10 "/usr/include/x86_64-linux-gnu/gnu/stubs.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/gnu/stubs-64.h" 1 3 4
# 11 "/usr/include/x86_64-linux-gnu/gnu/stubs.h" 2 3 4
# 511 "/usr/include/features.h" 2 3 4
# 34 "/usr/include/x86_64-linux-gnu/bits/libc-header-start.h" 2 3 4
# 28 "/usr/include/stdio.h" 2 3 4



# 1 "/usr/lib/gcc/x86_64-linux-gnu/11/include/stddef.h" 1 3 4
# 209 "/usr/lib/gcc/x86_64-linux-gnu/11/include/stddef.h" 3 4

# 209 "/usr/lib/gcc/x86_64-linux-gnu/11/include/stddef.h" 3 4
typedef long unsigned int size_t;
# 34 "/usr/include/stdio.h" 2 3 4

# 1 "/usr/lib/gcc/x86_64-linux-gnu/11/include/stdarg.h" 1 3 4
# 40 "/usr/lib/gcc/x86_64-linux-gnu/11/include/stdarg.h" 3 4
typedef __builtin_va_list __gnuc_va_list;
# 37 "/usr/include/stdio.h" 2 3 4

# 1 "/usr/include/x86_64-linux-gnu/bits/types.h" 1 3 4
# 27 "/usr/include/x86_64-linux-gnu/bits/types.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4
# 28 "/usr/include/x86_64-linux-gnu/bits/types.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 1 3 4
# 19 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4
# 20 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 2 3 4
# 29 "/usr/include/x86_64-linux-gnu/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;




typedef __int8_t __int_least8_t;
typedef __uint8_t __uint_least8_t;
typedef __int16_t __int_least16_t;
typedef __uint16_t __uint_least16_t;
typedef __int32_t __int_least32_t;
typedef __uint32_t __uint_least32_t;
typedef __int64_t __int_least64_t;
typedef __uint64_t __uint_least64_t;


typedef long int __quad_t;
typedef unsigned long int __u_quad_t;





typedef long int __intmax_t;
typedef unsigned long int __uintmax_t;
# 141 "/usr/include/x86_64-linux-gnu/bits/types.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/typesizes.h" 1 3 4
# 142 "/usr/include/x86_64-linux-gnu/bits/types.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/time64.h" 1 3 4
# 143 "/usr/include/x86_64-linux-gnu/bits/types.h" 2 3 4

typedef unsigned long int __dev_t;
typedef unsigned int __uid_t;
typedef unsigned int __gid_t;
typedef unsigned long int __ino_t;
typedef unsigned long int __ino64_t;
typedef unsigned int __mode_t;
typedef unsigned long int __nlink_t;
typedef long int __off_t;
typedef long int __off64_t;
typedef int __pid_t;
typedef struct { int __val[2]; } __fsid_t;
typedef long int __clock_t;
typedef unsigned long int __rlim_t;
typedef unsigned long int __rlim64_t;
typedef unsigned int __id_t;
typedef long int __time_t;
typedef unsigned int __useconds_t;
typedef long int __suseconds_t;
typedef long int __suseconds64_t;

typedef int __daddr_t;
typedef int __key_t;

typedef int __clockid_t;

typedef void * __timer_t;

typedef long int __blksize_t;



typedef long int __blkcnt_t;
typedef long int __blkcnt64_t;

typedef unsigned long int __fsblkcnt_t;
typedef unsigned long int __fsblkcnt64_t;

typedef unsigned long int __fsfilcnt_t;
typedef unsigned long int __fsfilcnt64_t;

typedef long int __fsword_t;

typedef long int __ssize_t;

typedef long int __syscall_slong_t;

typedef unsigned long int __syscall_ulong_t;


typedef __off64_t __loff_t;
typedef char *__caddr_t;

typedef long int __intptr_t;

typedef unsigned int __socklen_t;



typedef int __sig_atomic_t;
# 39 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/__fpos_t.h" 1 3 4



# 1 "/usr/include/x86_64-linux-gnu/bits/types/__mbstate_t.h" 1 3 4
# 13 "/usr/include/x86_64-linux-gnu/bits/types/__mbstate_t.h" 3 4
typedef struct
{
  int __count;
  union
  {
    unsigned int __wch;
    char __wchb[4];
  } __value;
} __mbstate_t;
# 6 "/usr/include/x86_64-linux-gnu/bits/types/__fpos_t.h" 2 3 4



typedef struct _G_fpos_t
{
  __off_t __pos;
  __mbstate_t __state;
} __fpos_t;
# 40 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/__fpos64_t.h" 1 3 4
# 10 "/usr/include/x86_64-linux-gnu/bits/types/__fpos64_t.h" 3 4
typedef struct _G_fpos64_t
{
  __off64_t __pos;
  __mbstate_t __state;
} __fpos64_t;
# 41 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/__FILE.h" 1 3 4


struct _IO_FILE;
typedef struct _IO_FILE __FILE;
# 42 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/FILE.h" 1 3 4


struct _IO_FILE;

typedef struct _IO_FILE FILE;
# 43 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/struct_FILE.h" 1 3 4
# 35 "/usr/include/x86_64-linux-gnu/bits/types/struct_FILE.h" 3 4
struct _IO_FILE;
struct _IO_marker;
struct _IO_codecvt;
struct _IO_wide_data;



typedef void _IO_lock_t;



struct _IO_FILE
{
  int _flags;

  char *_IO_read_ptr;
  char *_IO_read_end;
  char *_IO_read_base;
  char *_IO_write_base;
  char *_IO_write_ptr;
  char *_IO_write_end;
  char *_IO_buf_base;
  char *_IO_buf_end;

  char *_IO_save_base;
  char *_IO_backup_base;
  char *_IO_save_end;

  struct _IO_marker *_markers;

  struct _IO_FILE *_chain;

  int _fileno;
  int _flags2;
  __off_t _old_offset;

  unsigned short _cur_column;
  signed char _vtable_offset;
  char _shortbuf[1];

  _IO_lock_t *_lock;





  __off64_t _offset;

  struct _IO_codecvt *_codecvt;
  struct _IO_wide_data *_wide_data;
  struct _IO_FILE *_freeres_list;
  void *_freeres_buf;
  size_t __pad5;
  int _mode;

  char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
};
# 44 "/usr/include/stdio.h" 2 3 4
# 52 "/usr/include/stdio.h" 3 4
typedef __gnuc_va_list va_list;
# 63 "/usr/include/stdio.h" 3 4
typedef __off_t off_t;
# 77 "/usr/include/stdio.h" 3 4
typedef __ssize_t ssize_t;




typedef __fpos_t fpos_t;
# 133 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/stdio_lim.h" 1 3 4
# 134 "/usr/include/stdio.h" 2 3 4
# 143 "/usr/include/stdio.h" 3 4
extern FILE *stdin;
extern FILE *stdout;
extern FILE *stderr;




extern int remove (const char *__filename) __attribute__ ((__nothrow__ , __leaf__));

extern int rename (const char *__old, const char *__new) __attribute__ ((__nothrow__ , __leaf__));


extern int renameat (int __oldfd, const char *__old, int __newfd,
       const char *__new) __attribute__ ((__nothrow__ , __leaf__));
# 178 "/usr/include/stdio.h" 3 4
extern int fclose (FILE *__stream);
# 188 "/usr/include/stdio.h" 3 4
extern FILE *tmpfile (void)
  __attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;
# 205 "/usr/include/stdio.h" 3 4
extern char *tmpnam (char[20]) __attribute__ ((__nothrow__ , __leaf__)) ;



extern char *tmpnam_r (char __s[20]) __attribute__ ((__nothrow__ , __leaf__)) ;
# 222 "/usr/include/stdio.h" 3 4
extern char *tempnam (const char *__dir, const char *__pfx)
   __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__malloc__)) __attribute__ ((__malloc__ (__builtin_free, 1)));




extern int fflush (FILE *__stream);
# 239 "/usr/include/stdio.h" 3 4
extern int fflush_unlocked (FILE *__stream);
# 258 "/usr/include/stdio.h" 3 4
extern FILE *fopen (const char *__restrict __filename,
      const char *__restrict __modes)
  __attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;



extern FILE *freopen (const char *__restrict __filename,
        const char *__restrict __modes,
        FILE *__restrict __stream) ;
# 293 "/usr/include/stdio.h" 3 4
extern FILE *fdopen (int __fd, const char *__modes) __attribute__ ((__nothrow__ , __leaf__))
  __attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;
# 308 "/usr/include/stdio.h" 3 4
extern FILE *fmemopen (void *__s, size_t __len, const char *__modes)
  __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;



extern FILE *open_memstream (char **__bufloc, size_t *__sizeloc) __attribute__ ((__nothrow__ , __leaf__))
  __attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;
# 328 "/usr/include/stdio.h" 3 4
extern void setbuf (FILE *__restrict __stream, char *__restrict __buf) __attribute__ ((__nothrow__ , __leaf__));


extern int setvbuf (FILE *__restrict __stream, char *__restrict __buf,
      int __modes, size_t __n) __attribute__ ((__nothrow__ , __leaf__));



extern void setbuffer (FILE *__restrict __stream, char *__restrict __buf,
         size_t __size) __attribute__ ((__nothrow__ , __leaf__));

extern void setlinebuf (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));





extern int fprintf (FILE *__restrict __stream,
      const char *__restrict __format, ...);



extern int printf (const char *__restrict __format, ...);

extern int sprintf (char *__restrict __s,
      const char *__restrict __format, ...) __attribute__ ((__nothrow__));



extern int vfprintf (FILE *__restrict __s, const char *__restrict __format,
       __gnuc_va_list __arg);



extern int vprintf (const char *__restrict __format, __gnuc_va_list __arg);

extern int vsprintf (char *__restrict __s, const char *__restrict __format,
       __gnuc_va_list __arg) __attribute__ ((__nothrow__));


extern int snprintf (char *__restrict __s, size_t __maxlen,
       const char *__restrict __format, ...)
     __attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, 3, 4)));

extern int vsnprintf (char *__restrict __s, size_t __maxlen,
        const char *__restrict __format, __gnuc_va_list __arg)
     __attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, 3, 0)));
# 403 "/usr/include/stdio.h" 3 4
extern int vdprintf (int __fd, const char *__restrict __fmt,
       __gnuc_va_list __arg)
     __attribute__ ((__format__ (__printf__, 2, 0)));
extern int dprintf (int __fd, const char *__restrict __fmt, ...)
     __attribute__ ((__format__ (__printf__, 2, 3)));





extern int fscanf (FILE *__restrict __stream,
     const char *__restrict __format, ...) ;



extern int scanf (const char *__restrict __format, ...) ;

extern int sscanf (const char *__restrict __s,
     const char *__restrict __format, ...) __attribute__ ((__nothrow__ , __leaf__));



# 1 "/usr/include/x86_64-linux-gnu/bits/floatn.h" 1 3 4
# 119 "/usr/include/x86_64-linux-gnu/bits/floatn.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/floatn-common.h" 1 3 4
# 24 "/usr/include/x86_64-linux-gnu/bits/floatn-common.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/long-double.h" 1 3 4
# 25 "/usr/include/x86_64-linux-gnu/bits/floatn-common.h" 2 3 4
# 120 "/usr/include/x86_64-linux-gnu/bits/floatn.h" 2 3 4
# 431 "/usr/include/stdio.h" 2 3 4


extern int fscanf (FILE *__restrict __stream, const char *__restrict __format, ...) __asm__ ("" "__isoc99_fscanf")

                               ;
extern int scanf (const char *__restrict __format, ...) __asm__ ("" "__isoc99_scanf")
                              ;
extern int sscanf (const char *__restrict __s, const char *__restrict __format, ...) __asm__ ("" "__isoc99_sscanf") __attribute__ ((__nothrow__ , __leaf__))

                      ;
# 459 "/usr/include/stdio.h" 3 4
extern int vfscanf (FILE *__restrict __s, const char *__restrict __format,
      __gnuc_va_list __arg)
     __attribute__ ((__format__ (__scanf__, 2, 0))) ;



extern int vscanf (const char *__restrict __format, __gnuc_va_list __arg)
     __attribute__ ((__format__ (__scanf__, 1, 0))) ;

extern int vsscanf (const char *__restrict __s,
      const char *__restrict __format, __gnuc_va_list __arg)
     __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__format__ (__scanf__, 2, 0)));



extern int vfscanf (FILE *__restrict __s, const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vfscanf")


     __attribute__ ((__format__ (__scanf__, 2, 0))) ;
extern int vscanf (const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vscanf")

     __attribute__ ((__format__ (__scanf__, 1, 0))) ;
extern int vsscanf (const char *__restrict __s, const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vsscanf") __attribute__ ((__nothrow__ , __leaf__))


     __attribute__ ((__format__ (__scanf__, 2, 0)));
# 513 "/usr/include/stdio.h" 3 4
extern int fgetc (FILE *__stream);
extern int getc (FILE *__stream);



extern int getchar (void);




extern int getc_unlocked (FILE *__stream);
extern int getchar_unlocked (void);
# 538 "/usr/include/stdio.h" 3 4
extern int fgetc_unlocked (FILE *__stream);
# 549 "/usr/include/stdio.h" 3 4
extern int fputc (int __c, FILE *__stream);
extern int putc (int __c, FILE *__stream);



extern int putchar (int __c);
# 565 "/usr/include/stdio.h" 3 4
extern int fputc_unlocked (int __c, FILE *__stream);





extern int putc_unlocked (int __c, FILE *__stream);
extern int putchar_unlocked (int __c);




extern int getw (FILE *__stream);

extern int putw (int __w, FILE *__stream);





extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream)
     __attribute__ ((__access__ (__write_only__, 1, 2)));
# 632 "/usr/include/stdio.h" 3 4
extern __ssize_t __getdelim (char **__restrict __lineptr,
                             size_t *__restrict __n, int __delimiter,
                             FILE *__restrict __stream) ;
extern __ssize_t getdelim (char **__restrict __lineptr,
                           size_t *__restrict __n, int __delimiter,
                           FILE *__restrict __stream) ;





extern __ssize_t getline (char **__restrict __lineptr,
                          size_t *__restrict __n,
                          FILE *__restrict __stream) ;





extern int fputs (const char *__restrict __s, FILE *__restrict __stream);



extern int puts (const char *__s);




extern int ungetc (int __c, FILE *__stream);




extern size_t fread (void *__restrict __ptr, size_t __size,
       size_t __n, FILE *__restrict __stream) ;



extern size_t fwrite (const void *__restrict __ptr, size_t __size,
        size_t __n, FILE *__restrict __s);
# 702 "/usr/include/stdio.h" 3 4
extern size_t fread_unlocked (void *__restrict __ptr, size_t __size,
         size_t __n, FILE *__restrict __stream) ;
extern size_t fwrite_unlocked (const void *__restrict __ptr, size_t __size,
          size_t __n, FILE *__restrict __stream);





extern int fseek (FILE *__stream, long int __off, int __whence);



extern long int ftell (FILE *__stream) ;



extern void rewind (FILE *__stream);
# 736 "/usr/include/stdio.h" 3 4
extern int fseeko (FILE *__stream, __off_t __off, int __whence);



extern __off_t ftello (FILE *__stream) ;
# 760 "/usr/include/stdio.h" 3 4
extern int fgetpos (FILE *__restrict __stream, fpos_t *__restrict __pos);



extern int fsetpos (FILE *__stream, const fpos_t *__pos);
# 786 "/usr/include/stdio.h" 3 4
extern void clearerr (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));

extern int feof (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;

extern int ferror (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;


extern void clearerr_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));
extern int feof_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
extern int ferror_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;





extern void perror (const char *__s);



extern int fileno (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;



extern int fileno_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;
# 823 "/usr/include/stdio.h" 3 4
extern int pclose (FILE *__stream);



extern FILE *popen (const char *__command, const char *__modes)
  __attribute__ ((__malloc__)) __attribute__ ((__malloc__ (pclose, 1))) ;




extern char *ctermid (char *__s) __attribute__ ((__nothrow__ , __leaf__))
  __attribute__ ((__access__ (__write_only__, 1)));
# 867 "/usr/include/stdio.h" 3 4
extern void flockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));


extern int ftrylockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ;

extern void funlockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__));
# 885 "/usr/include/stdio.h" 3 4
extern int __uflow (FILE *);
extern int __overflow (FILE *, int);
# 902 "/usr/include/stdio.h" 3 4

# 2 "main.c" 2

# 2 "main.c"
int main() {
   printf("Hello, World!");
   return 0;
}

编译

• 此阶段的输入文件是*.i文件。
• 一个名为 compiler 的工具获取预处理器的输出并将其转换为汇编语言。
• 汇编代码，通常被称为汇编语言或只是汇编，是一种低级编程语言，它使用助记符（例如，MOV表示“移动”，ADD表示“添加”）来表示CPU操作（所谓的操作码），从而使代码可读。
• 这与您直接使用汇编语言对微控制器（例如8051微控制器）进行编程时使用的代码完全相同。
• 输出文件为*.s或*.asm文件。

gcc -S main.i -o main.s

• -S：编译完成后停止;

严格地说，这是编译过程中最关键的部分，除了gcc编译器之外，没有人可以完成。当我们使用gcc直接生成可执行文件时（不分解所有步骤），幕后发生的事情是：gcc将所有其他编译步骤（预处理、汇编和链接）分别委托给cpp、as和ld，只执行编译步骤。通过这样做，我们C开发人员可以不必担心调用这些工具，因为gcc已经完成了。因此，我们可以使用一个独特的命令接口（gcc）来执行所有的编译过程。
输出是

.file   "main.c"
    .text
    .section    .rodata
.LC0:
    .string "Hello, World!"
    .text
    .globl  main
    .type   main, @function
main:
.LFB0:
    .cfi_startproc
    endbr64
    pushq   %rbp
    .cfi_def_cfa_offset 16
    .cfi_offset 6, -16
    movq    %rsp, %rbp
    .cfi_def_cfa_register 6
    leaq    .LC0(%rip), %rax
    movq    %rax, %rdi
    movl    $0, %eax
    call    printf@PLT
    movl    $0, %eax
    popq    %rbp
    .cfi_def_cfa 7, 8
    ret
    .cfi_endproc
.LFE0:
    .size   main, .-main
    .ident  "GCC: (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0"
    .section    .note.GNU-stack,"",@progbits
    .section    .note.gnu.property,"a"
    .align 8
    .long   1f - 0f
    .long   4f - 1f
    .long   5
0:
    .string "GNU"
1:
    .align 8
    .long   0xc0000002
    .long   3f - 2f
2:
    .long   0x3
3:
    .align 8
4:

组装

• 此阶段的输入文件是*.asm或*.s文件。

• 如果您使用gcc组装文件，则必须将扩展名命名为.s。否则，gcc的-c选项将尝试链接文件，而不是组装文件。

• 每个汇编指令通常代表一条机器码指令，它是一组0和1。这个二进制序列的一部分是操作码，而其余部分通常是变量或数值的内存地址。虽然汇编语言是一种独立于CPU架构的编程语言，但它所代表的机器码是特定于目标CPU的。换句话说，机器代码是高度平台特定的，这意味着它是针对特定的计算机体系结构和操作系统定制的。一种类型的CPU的机器指令可能不会在不修改的情况下在不同的CPU架构上运行。

• 将汇编代码转换为机器代码的工具称为 * 汇编程序 *，其任务称为 * 汇编 *。

• 在低级微控制器编程中，汇编程序将汇编代码转换为二进制序列（通常以十六进制表示）并将其嵌入微控制器。微处理器现在可以运行了。

• 另一方面，在C/C++中，汇编程序将汇编代码转换为.o中间文件，称为目标文件。此中间表示包含机器级代码，即表示代码中定义的函数和变量的非人类可读指令。不过，.o文件并不是最终的可执行代码。也就是说，它包含机器可读的指令，但不能直接由操作系统执行。这是因为缺少了操作系统可执行文件所需的一些信息，例如函数和变量的内存地址，这些信息在链接阶段解决。

• 输出文件为*.o或*.obj文件。

gcc -c main.s -o main.o

或者，您可以运行

as main.s -o main2.o

其中as是GNU汇编程序。同样，您可以检查这两种方法是否产生相同的输出：

❯ cmp main.o main2.o && echo "Files are equal" || echo "Files are not equal"
  Files are equal

链接

• 此阶段的输入文件是*.o文件。
• 链接器将来自多个模块的所有目标代码合并到一个模块中。如果我们正在使用库中的函数，链接器将把我们的代码与该库函数代码链接起来。
• C标准库通常与gcc编译器捆绑在一起。当您安装gcc或任何其他C编译器时，它包含编译和链接C程序所需的必要标准库。这些库是编译器发行版的组成部分。因此，它们并不作为一个单独的共享库文件存在于您的系统中，您不会通过查找.a（静态库）文件来找到它。共享库（.so文件）在编译时不链接，而仅在运行时链接。
• 对于C标准库之外的库，必须手动链接。
• 输出不可读。

gcc main.o -o myprogram

其中myprogram是可执行文件。或者，您可以直接使用ld命令，但猜猜看会发生什么？虽然这不是最关键的部分，手动将目标文件链接到可执行文件是复杂得要命。在这一点上，您实际上是在玩弄比特，以完美地匹配您的机器的硬件和操作系统规范。你必须了解的东西，如计算机组织和体系结构，endianness，文件类型，动态链接器/加载器路径，BuildID，操作系统兼容性，幻数，剥离和非剥离可执行文件，等等。幸运的是，gcc already knows all these stuffs and wonderfully does this hard work for us通过内部调用ld并设置所有必要的选项。我想ld选项超出了这个问题的范围。同样重要的是，在实践中，强烈建议不要直接使用ld。您应该使用gcc（或其他编译器）。
最后

❯ ./myprogram
Hello, World!

在实际编译过程中传递的信息要多得多，因为它比仅仅预处理编译和链接更复杂。编译是一个多阶段的过程，Godbolt给了你一个简单的机会来看看那里发生了什么（也可以看到中间结果）：
https://godbolt.org/z/b6vdzb8d9