在学校学C语言的时候，很多人都不是很注重编译过程链，但是其实编译过程是项目过程中很重要的一部分，有时候有些语法诸如static、volatile等关键词不理解时大多数都是对整个C语言编译链没有进行过详细的了解的，今天就将整个编译过程链讲解下，首先需要了解的是编译是个逐步过程，每一步的输出都是作为下一步的输入。编译输出为某个平台上的可执行文件。这些可执行文件有不同格式被操作系统识别。linux识别ELF（可执行和链接格式）；windows识别PE/COFF（可移植可执行/通用对象文件格式）。这些格式都有特定文件头格式和关联偏移量，需要按指定规则去读取和理解文件头和相应部分。
编译过程链如下：
源码——————>预处理——————>编译——————>目标文件——————>链接——————>可执行文件
对于linux下编程，编译过程是通过GCC编译链来完成的，GCC通过后缀来识别输入文件的类型，下面为约定的文件后缀名的规则：
.c为后缀的文件，C语言源代码文件；
.a为后缀的文件，是由目标文件构成的档案库文件；
.C，.cc或.cxx 为后缀的文件，是C++源代码文件；
.h为后缀的文件，是程序所包含的头文件；
.i 为后缀的文件，是已经预处理过的C源代码文件；
.ii为后缀的文件，是已经预处理过的C++源代码文件；
.m为后缀的文件，是Objective-C源代码文件；
.o为后缀的文件，是编译后的目标文件；
.s为后缀的文件，是汇编语言源代码文件；
.S为后缀的文件，是经过预编译的汇编语言源代码文件。
下面以一个实际的例子进行理解
当然，这里还可以使用-save-temps标志将过程文件直接存储在当前目录中：
$ gcc -Wall -save-temps hello.c -o hello
1.预处理
这是源代码通过的第一个阶段。在这个阶段，完成以下任务：
- 宏观替代
- 评论被剥离
- 包含文件的扩展
我们可以打开hello.i文件进行查看
# 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 # 374 "/usr/include/features.h" 3 4 # 1 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 1 3 4 # 385 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 3 4 # 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4 # 386 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 2 3 4 # 375 "/usr/include/features.h" 2 3 4 # 398 "/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 # 399 "/usr/include/features.h" 2 3 4 # 28 "/usr/include/stdio.h" 2 3 4 # 1 "/usr/lib/gcc/x86_64-linux-gnu/4.8/include/stddef.h" 1 3 4 # 212 "/usr/lib/gcc/x86_64-linux-gnu/4.8/include/stddef.h" 3 4 typedef long unsigned int size_t; # 34 "/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 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 long int __quad_t; typedef unsigned long int __u_quad_t; # 121 "/usr/include/x86_64-linux-gnu/bits/types.h" 3 4 # 1 "/usr/include/x86_64-linux-gnu/bits/typesizes.h" 1 3 4 # 122 "/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 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 __quad_t *__qaddr_t; typedef char *__caddr_t; typedef long int __intptr_t; typedef unsigned int __socklen_t; # 36 "/usr/include/stdio.h" 2 3 4 # 44 "/usr/include/stdio.h" 3 4 struct _IO_FILE; typedef struct _IO_FILE FILE; # 64 "/usr/include/stdio.h" 3 4 typedef struct _IO_FILE __FILE; # 74 "/usr/include/stdio.h" 3 4 # 1 "/usr/include/libio.h" 1 3 4 # 31 "/usr/include/libio.h" 3 4 # 1 "/usr/include/_G_config.h" 1 3 4 # 15 "/usr/include/_G_config.h" 3 4 # 1 "/usr/lib/gcc/x86_64-linux-gnu/4.8/include/stddef.h" 1 3 4 # 16 "/usr/include/_G_config.h" 2 3 4 # 1 "/usr/include/wchar.h" 1 3 4 # 82 "/usr/include/wchar.h" 3 4 typedef struct { int __count; union { unsigned int __wch; char __wchb[4]; } __value; } __mbstate_t; # 21 "/usr/include/_G_config.h" 2 3 4 typedef struct { __off_t __pos; __mbstate_t __state; } _G_fpos_t; typedef struct { __off64_t __pos; __mbstate_t __state; } _G_fpos64_t; # 32 "/usr/include/libio.h" 2 3 4 # 49 "/usr/include/libio.h" 3 4 # 1 "/usr/lib/gcc/x86_64-linux-gnu/4.8/include/stdarg.h" 1 3 4 # 40 "/usr/lib/gcc/x86_64-linux-gnu/4.8/include/stdarg.h" 3 4 typedef __builtin_va_list __gnuc_va_list; # 50 "/usr/include/libio.h" 2 3 4 # 144 "/usr/include/libio.h" 3 4 struct _IO_jump_t; struct _IO_FILE; # 154 "/usr/include/libio.h" 3 4 typedef void _IO_lock_t; struct _IO_marker { struct _IO_marker *_next; struct _IO_FILE *_sbuf; int _pos; # 177 "/usr/include/libio.h" 3 4 }; enum __codecvt_result { __codecvt_ok, __codecvt_partial, __codecvt_error, __codecvt_noconv }; # 245 "/usr/include/libio.h" 3 4 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; # 293 "/usr/include/libio.h" 3 4 __off64_t _offset; # 302 "/usr/include/libio.h" 3 4 void *__pad1; void *__pad2; void *__pad3; void *__pad4; size_t __pad5; int _mode; char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)]; }; typedef struct _IO_FILE _IO_FILE; struct _IO_FILE_plus; extern struct _IO_FILE_plus _IO_2_1_stdin_; extern struct _IO_FILE_plus _IO_2_1_stdout_; extern struct _IO_FILE_plus _IO_2_1_stderr_; # 338 "/usr/include/libio.h" 3 4 typedef __ssize_t __io_read_fn (void *__cookie, char *__buf, size_t __nbytes); typedef __ssize_t __io_write_fn (void *__cookie, const char *__buf, size_t __n); typedef int __io_seek_fn (void *__cookie, __off64_t *__pos, int __w); typedef int __io_close_fn (void *__cookie); # 390 "/usr/include/libio.h" 3 4 extern int __underflow (_IO_FILE *); extern int __uflow (_IO_FILE *); extern int __overflow (_IO_FILE *, int); # 434 "/usr/include/libio.h" 3 4 extern int _IO_getc (_IO_FILE *__fp); extern int _IO_putc (int __c, _IO_FILE *__fp); extern int _IO_feof (_IO_FILE *__fp) __attribute__ ((__nothrow__ , __leaf__)); extern int _IO_ferror (_IO_FILE *__fp) __attribute__ ((__nothrow__ , __leaf__)); extern int _IO_peekc_locked (_IO_FILE *__fp); extern void _IO_flockfile (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__)); extern void _IO_funlockfile (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__)); extern int _IO_ftrylockfile (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__)); # 464 "/usr/include/libio.h" 3 4 extern int _IO_vfscanf (_IO_FILE * __restrict, const char * __restrict, __gnuc_va_list, int *__restrict); extern int _IO_vfprintf (_IO_FILE *__restrict, const char *__restrict, __gnuc_va_list); extern __ssize_t _IO_padn (_IO_FILE *, int, __ssize_t); extern size_t _IO_sgetn (_IO_FILE *, void *, size_t); extern __off64_t _IO_seekoff (_IO_FILE *, __off64_t, int, int); extern __off64_t _IO_seekpos (_IO_FILE *, __off64_t, int); extern void _IO_free_backup_area (_IO_FILE *) __attribute__ ((__nothrow__ , __leaf__)); # 75 "/usr/include/stdio.h" 2 3 4 typedef __gnuc_va_list va_list; # 90 "/usr/include/stdio.h" 3 4 typedef __off_t off_t; # 102 "/usr/include/stdio.h" 3 4 typedef __ssize_t ssize_t; typedef _G_fpos_t fpos_t; # 164 "/usr/include/stdio.h" 3 4 # 1 "/usr/include/x86_64-linux-gnu/bits/stdio_lim.h" 1 3 4 # 165 "/usr/include/stdio.h" 2 3 4 extern struct _IO_FILE *stdin; extern struct _IO_FILE *stdout; extern struct _IO_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__)); extern FILE *tmpfile (void) ; # 209 "/usr/include/stdio.h" 3 4 extern char *tmpnam (char *__s) __attribute__ ((__nothrow__ , __leaf__)) ; extern char *tmpnam_r (char *__s) __attribute__ ((__nothrow__ , __leaf__)) ; # 227 "/usr/include/stdio.h" 3 4 extern char *tempnam (const char *__dir, const char *__pfx) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__malloc__)) ; extern int fclose (FILE *__stream); extern int fflush (FILE *__stream); # 252 "/usr/include/stdio.h" 3 4 extern int fflush_unlocked (FILE *__stream); # 266 "/usr/include/stdio.h" 3 4 extern FILE *fopen (const char *__restrict __filename, const char *__restrict __modes) ; extern FILE *freopen (const char *__restrict __filename, const char *__restrict __modes, FILE *__restrict __stream) ; # 295 "/usr/include/stdio.h" 3 4 # 306 "/usr/include/stdio.h" 3 4 extern FILE *fdopen (int __fd, const char *__modes) __attribute__ ((__nothrow__ , __leaf__)) ; # 319 "/usr/include/stdio.h" 3 4 extern FILE *fmemopen (void *__s, size_t __len, const char *__modes) __attribute__ ((__nothrow__ , __leaf__)) ; extern FILE *open_memstream (char **__bufloc, size_t *__sizeloc) __attribute__ ((__nothrow__ , __leaf__)) ; 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))); # 412 "/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__)); # 443 "/usr/include/stdio.h" 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__)) ; # 463 "/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))); # 494 "/usr/include/stdio.h" 3 4 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))); # 522 "/usr/include/stdio.h" 3 4 extern int fgetc (FILE *__stream); extern int getc (FILE *__stream); extern int getchar (void); # 550 "/usr/include/stdio.h" 3 4 extern int getc_unlocked (FILE *__stream); extern int getchar_unlocked (void); # 561 "/usr/include/stdio.h" 3 4 extern int fgetc_unlocked (FILE *__stream); extern int fputc (int __c, FILE *__stream); extern int putc (int __c, FILE *__stream); extern int putchar (int __c); # 594 "/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) ; # 638 "/usr/include/stdio.h" 3 4 extern char *gets (char *__s) __attribute__ ((__deprecated__)); # 665 "/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); # 737 "/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); # 773 "/usr/include/stdio.h" 3 4 extern int fseeko (FILE *__stream, __off_t __off, int __whence); extern __off_t ftello (FILE *__stream) ; # 792 "/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); # 815 "/usr/include/stdio.h" 3 4 # 824 "/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); # 1 "/usr/include/x86_64-linux-gnu/bits/sys_errlist.h" 1 3 4 # 26 "/usr/include/x86_64-linux-gnu/bits/sys_errlist.h" 3 4 extern int sys_nerr; extern const char *const sys_errlist[]; # 854 "/usr/include/stdio.h" 2 3 4 extern int fileno (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ; extern int fileno_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) ; # 873 "/usr/include/stdio.h" 3 4 extern FILE *popen (const char *__command, const char *__modes) ; extern int pclose (FILE *__stream); extern char *ctermid (char *__s) __attribute__ ((__nothrow__ , __leaf__)); # 913 "/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__)); # 943 "/usr/include/stdio.h" 3 4 # 2 "hello.c" 2 int main(int argc,char**argv) { printf("hello,world! "); return 0; }
在上面的输出中，您可以看到源文件中充满了大量的信息，但在最后，我们可以看到我们编写的代码行。让我们先分析一下这些代码行。
- 第一个观察是printf（）的参数现在直接包含字符串“Hello World”而不是宏。事实上宏的定义和用法已经完全消失了。这证明了所有宏在预处理阶段被扩展的第一个任务。
- 第二个观察是，我们在原始代码中写的评论不存在。这证明所有的评论都被剥夺了。
- 第三个观察结果是，在#include这一行旁边，我们看到了大量的代码。所以它可以安全地得出结论，stdio.h已经被扩展并且被直接包含在我们的源文件中。因此，我们理解编译器如何能够看到printf（）函数的声明。
当我们搜索print.i文件时，我发现，函数printf被声明为：
extern int printf（__const char * __ restrict __format，...）;
关键字'extern'告诉函数printf（）在这里没有定义。这是该文件的外部。我们稍后将看到gcc如何得到printf（）的定义。
现在我们对预处理阶段发生的事情有了很好的理解。让我们进入下一阶段。
2.编译
编译器完成预处理器阶段之后。下一步是将print.i作为输入，编译并产生一个中间编译输出。这个阶段的输出文件是'print.s'。print.s中的输出是汇编级指令。
在编辑器中打开print.s文件并查看内容。
.file "hello.c" .section .rodata .LC0: .string "hello,world!" .text .globl main .type main, @function main: .LFB0: .cfi_startproc pushq %rbp .cfi_def_cfa_offset 16 .cfi_offset 6, -16 movq %rsp, %rbp .cfi_def_cfa_register 6 subq $16, %rsp movl %edi, -4(%rbp) movq %rsi, -16(%rbp) movl $.LC0, %edi call puts movl $0, %eax leave .cfi_def_cfa 7, 8 ret .cfi_endproc .LFE0: .size main, .-main .ident "GCC: (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4" .section .note.GNU-stack,"",@progbits
我们可以快速的看到，这个汇编语言输出是汇编程序能够理解并转换成机器语言的某种形式的指令。
3.组装
在这个阶段，将print.s文件作为输入并产生一个中间文件print.o。这个文件也被称为对象文件。
这个文件是由汇编程序生成的，它可以理解并将汇编指令中的“.s”文件转换为包含机器级指令的“.o”目标文件。在这个阶段只有现有的代码被转换成机器语言，像printf（）函数调用没有解决。
由于这个阶段的输出是一个机器级文件（print.o）。所以我们不能查看它的内容。如果您仍然尝试打开print.o并查看它，则会看到一些完全不可读的内容。
通过查看print.o文件我们唯一可以解释的是关于字符串ELF。
ELF代表可执行和可链接的格式。
这是由gcc生成的机器级对象文件和可执行文件的一种相对较新的格式。在此之前，使用了一种称为a.out的格式。据说ELF是比a.out更复杂的格式（我们可能会在以后的某篇文章中深入研究ELF格式）。
注意：如果编译代码时未指定输出文件的名称，则生成的输出文件具有名称“a.out”，但现在格式已更改为ELF。只是默认的可执行文件名保持不变。
4.连接
这是函数调用与其定义的所有链接完成的最后阶段。正如前面所讨论的，直到现在，gcc并不知道printf（）这样的函数的定义。在编译器确切地知道所有这些函数的实现之前，它只是使用占位符来进行函数调用。在这个阶段，printf（）的定义被解析，并且printf（）函数的实际地址被插入。
链接器在此阶段开始执行此任务。
链接器也做了一些额外的工作; 它将一些额外的代码结合到程序启动时和程序结束时所需的程序中。例如，有一些代码是建立运行环境的标准，如传递命令行参数，将环境变量传递给每个程序。类似地，一些标准的代码是将程序的返回值返回给系统所必需的。
编译器的上述任务可以通过一个小实验来验证。由于现在我们已经知道链接器将.o文件（print.o）转换为可执行文件（print）。
所以如果我们比较print.o和print文件的文件大小，我们会看到不同之处。