一、预处理

　　

 预处理

 gcc -E Hello.c -o hello.i

 编译

 gcc -S hello.i -o hello.s

 汇编

 gcc -c hello.s -o hello.o

 链接

 gcc hello.o -o hello

Makefile

 #  "hello.c"

 #  "<built-in>"

 #  "<command-line>"

 #  "/usr/include/stdc-predef.h"

 #  "<command-line>"

 #  "hello.c"

 #  "/usr/include/stdio.h"

 #  "/usr/include/stdio.h"

 #  "/usr/include/features.h"

 #  "/usr/include/features.h"

 #  "/usr/include/i386-linux-gnu/sys/cdefs.h"

 #  "/usr/include/i386-linux-gnu/sys/cdefs.h"

 #  "/usr/include/i386-linux-gnu/bits/wordsize.h"

 #  "/usr/include/i386-linux-gnu/sys/cdefs.h"

 #  "/usr/include/features.h"

 #  "/usr/include/features.h"

 #  "/usr/include/i386-linux-gnu/gnu/stubs.h"   

 #  "/usr/include/i386-linux-gnu/gnu/stubs-32.h"

 #  "/usr/include/i386-linux-gnu/gnu/stubs.h"

 #  "/usr/include/features.h"

 #  "/usr/include/stdio.h"   

 #  "/usr/lib/gcc/i686-linux-gnu/4.8/include/stddef.h"

 #  "/usr/lib/gcc/i686-linux-gnu/4.8/include/stddef.h"

 typedef unsigned int size_t;

 #  "/usr/include/stdio.h"   

 #  "/usr/include/i386-linux-gnu/bits/types.h"

 #  "/usr/include/i386-linux-gnu/bits/types.h"

 #  "/usr/include/i386-linux-gnu/bits/wordsize.h"

 #  "/usr/include/i386-linux-gnu/bits/types.h"   

 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;

 __extension__ typedef signed long long int __int64_t;

 __extension__ typedef unsigned long long int __uint64_t;

 __extension__ typedef long long int __quad_t;

 __extension__ typedef unsigned long long int __u_quad_t;

 #  "/usr/include/i386-linux-gnu/bits/types.h"

 #  "/usr/include/i386-linux-gnu/bits/typesizes.h"

 #  "/usr/include/i386-linux-gnu/bits/types.h"   

 __extension__ typedef __u_quad_t __dev_t;

 __extension__ typedef unsigned int __uid_t;

 __extension__ typedef unsigned int __gid_t;

 __extension__ typedef unsigned long int __ino_t;

 __extension__ typedef __u_quad_t __ino64_t;

 __extension__ typedef unsigned int __mode_t;

 __extension__ typedef unsigned int __nlink_t;

 __extension__ typedef long int __off_t;

 __extension__ typedef __quad_t __off64_t;

 __extension__ typedef int __pid_t;

 __extension__ typedef struct { int __val[]; } __fsid_t;

 __extension__ typedef long int __clock_t;

 __extension__ typedef unsigned long int __rlim_t;

 __extension__ typedef __u_quad_t __rlim64_t;

 __extension__ typedef unsigned int __id_t;

 __extension__ typedef long int __time_t;

 __extension__ typedef unsigned int __useconds_t;

 __extension__ typedef long int __suseconds_t;

 __extension__ typedef int __daddr_t;

 __extension__ typedef int __key_t;

 __extension__ typedef int __clockid_t;

 __extension__ typedef void * __timer_t;

 __extension__ typedef long int __blksize_t;

 __extension__ typedef long int __blkcnt_t;

 __extension__ typedef __quad_t __blkcnt64_t;

 __extension__ typedef unsigned long int __fsblkcnt_t;

 __extension__ typedef __u_quad_t __fsblkcnt64_t;

 __extension__ typedef unsigned long int __fsfilcnt_t;

 __extension__ typedef __u_quad_t __fsfilcnt64_t;

 __extension__ typedef int __fsword_t;

 __extension__ typedef int __ssize_t;

 __extension__ typedef long int __syscall_slong_t;

 __extension__ typedef unsigned long int __syscall_ulong_t;

 typedef __off64_t __loff_t;

 typedef __quad_t *__qaddr_t;

 typedef char *__caddr_t;

 __extension__ typedef int __intptr_t;

 __extension__ typedef unsigned int __socklen_t;

 #  "/usr/include/stdio.h"

 #  "/usr/include/stdio.h"

 struct _IO_FILE;

 typedef struct _IO_FILE FILE;

 #  "/usr/include/stdio.h"

 typedef struct _IO_FILE __FILE;

 #  "/usr/include/stdio.h"

 #  "/usr/include/libio.h"

 #  "/usr/include/libio.h"

 #  "/usr/include/_G_config.h"

 #  "/usr/include/_G_config.h"

 #  "/usr/lib/gcc/i686-linux-gnu/4.8/include/stddef.h"

 #  "/usr/include/_G_config.h"   

 #  "/usr/include/wchar.h"

 #  "/usr/include/wchar.h"

 typedef struct

 {

   int __count;

   union

   {

     unsigned int __wch;

     char __wchb[];

   } __value;

 } __mbstate_t;

 #  "/usr/include/_G_config.h"

 typedef struct

 {

   __off_t __pos;

   __mbstate_t __state;

 } _G_fpos_t;

 typedef struct

 {

   __off64_t __pos;

   __mbstate_t __state;

 } _G_fpos64_t;

 #  "/usr/include/libio.h"

 #  "/usr/include/libio.h"

 #  "/usr/lib/gcc/i686-linux-gnu/4.8/include/stdarg.h"

 #  "/usr/lib/gcc/i686-linux-gnu/4.8/include/stdarg.h"

 typedef __builtin_va_list __gnuc_va_list;

 #  "/usr/include/libio.h"

 #  "/usr/include/libio.h"

 struct _IO_jump_t; struct _IO_FILE;

 #  "/usr/include/libio.h"

 typedef void _IO_lock_t;

 struct _IO_marker {

   struct _IO_marker *_next;

   struct _IO_FILE *_sbuf;

   int _pos;

 #  "/usr/include/libio.h"

 };

 enum __codecvt_result

 {

   __codecvt_ok,

   __codecvt_partial,

   __codecvt_error,

   __codecvt_noconv

 };

 #  "/usr/include/libio.h"

 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[];

   _IO_lock_t *_lock;

 #  "/usr/include/libio.h"

   __off64_t _offset;

 #  "/usr/include/libio.h"

   void *__pad1;

   void *__pad2;

   void *__pad3;

   void *__pad4;

   size_t __pad5;

   int _mode;

   char _unused2[ * sizeof (int) -  * 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_;

 #  "/usr/include/libio.h"

 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);

 #  "/usr/include/libio.h"

 extern int __underflow (_IO_FILE *);

 extern int __uflow (_IO_FILE *);

 extern int __overflow (_IO_FILE *, int);

 #  "/usr/include/libio.h"

 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__));

 #  "/usr/include/libio.h"

 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__));

 #  "/usr/include/stdio.h"   

 typedef __gnuc_va_list va_list;

 #  "/usr/include/stdio.h"

 typedef __off_t off_t;

 #  "/usr/include/stdio.h"

 typedef __ssize_t ssize_t;

 typedef _G_fpos_t fpos_t;

 #  "/usr/include/stdio.h"

 #  "/usr/include/i386-linux-gnu/bits/stdio_lim.h"

 #  "/usr/include/stdio.h"   

 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) ;

 #  "/usr/include/stdio.h"

 extern char *tmpnam (char *__s) __attribute__ ((__nothrow__ , __leaf__)) ;

 extern char *tmpnam_r (char *__s) __attribute__ ((__nothrow__ , __leaf__)) ;

 #  "/usr/include/stdio.h"

 extern char *tempnam (const char *__dir, const char *__pfx)

      __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__malloc__)) ;

 extern int fclose (FILE *__stream);

 extern int fflush (FILE *__stream);

 #  "/usr/include/stdio.h"

 extern int fflush_unlocked (FILE *__stream);

 #  "/usr/include/stdio.h"  

 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) ;

 #  "/usr/include/stdio.h"  

 #  "/usr/include/stdio.h"

 extern FILE *fdopen (int __fd, const char *__modes) __attribute__ ((__nothrow__ , __leaf__)) ;

 #  "/usr/include/stdio.h"

 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__, , )));

 extern int vsnprintf (char *__restrict __s, size_t __maxlen,

         const char *__restrict __format, __gnuc_va_list __arg)

      __attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, , )));

 #  "/usr/include/stdio.h"

 extern int vdprintf (int __fd, const char *__restrict __fmt,

        __gnuc_va_list __arg)

      __attribute__ ((__format__ (__printf__, , )));

 extern int dprintf (int __fd, const char *__restrict __fmt, ...)

      __attribute__ ((__format__ (__printf__, , )));

 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__));

 #  "/usr/include/stdio.h"

 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__))

                       ;

 #  "/usr/include/stdio.h"  

 extern int vfscanf (FILE *__restrict __s, const char *__restrict __format,

       __gnuc_va_list __arg)

      __attribute__ ((__format__ (__scanf__, , ))) ;

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

      __attribute__ ((__format__ (__scanf__, , ))) ;

 extern int vsscanf (const char *__restrict __s,

       const char *__restrict __format, __gnuc_va_list __arg)

      __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__format__ (__scanf__, , )));

 #  "/usr/include/stdio.h"

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

      __attribute__ ((__format__ (__scanf__, , ))) ;

 extern int vscanf (const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vscanf")

      __attribute__ ((__format__ (__scanf__, , ))) ;

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

      __attribute__ ((__format__ (__scanf__, , )));

 #  "/usr/include/stdio.h"  

 extern int fgetc (FILE *__stream);

 extern int getc (FILE *__stream);

 extern int getchar (void);

 #  "/usr/include/stdio.h"

 extern int getc_unlocked (FILE *__stream);

 extern int getchar_unlocked (void);

 #  "/usr/include/stdio.h"

 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);

 #  "/usr/include/stdio.h"

 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)

      ;

 #  "/usr/include/stdio.h"

 extern char *gets (char *__s) __attribute__ ((__deprecated__));

 #  "/usr/include/stdio.h"

 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);

 #  "/usr/include/stdio.h"

 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);

 #  "/usr/include/stdio.h"

 extern int fseeko (FILE *__stream, __off_t __off, int __whence);

 extern __off_t ftello (FILE *__stream) ;

 #  "/usr/include/stdio.h"  

 extern int fgetpos (FILE *__restrict __stream, fpos_t *__restrict __pos);

 extern int fsetpos (FILE *__stream, const fpos_t *__pos);

 #  "/usr/include/stdio.h"  

 #  "/usr/include/stdio.h"  

 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);

 #  "/usr/include/i386-linux-gnu/bits/sys_errlist.h"

 #  "/usr/include/i386-linux-gnu/bits/sys_errlist.h"

 extern int sys_nerr;

 extern const char *const sys_errlist[];

 #  "/usr/include/stdio.h"   

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

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

 #  "/usr/include/stdio.h"

 extern FILE *popen (const char *__command, const char *__modes) ;

 extern int pclose (FILE *__stream);

 extern char *ctermid (char *__s) __attribute__ ((__nothrow__ , __leaf__));

 #  "/usr/include/stdio.h"

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

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

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

 #  "/usr/include/stdio.h"  

 #  "hello.c" 

 int main(int argc, char* argv[])

 {

     printf("Hello , world !\n");

     return ;

 }

hello.i

     .file    "hello.c"

     .section    .rodata

 .LC0:

     .string    "Hello , world !"

     .text

     .globl    main

     .type    main, @function

 main:

 .LFB0:

     .cfi_startproc

     pushl    %ebp

     .cfi_def_cfa_offset

     .cfi_offset , -

     movl    %esp, %ebp

     .cfi_def_cfa_register

     andl    $-, %esp

     subl    $, %esp

     movl    $.LC0, (%esp)

     call    puts

     movl    $, %eax

     leave

     .cfi_restore

     .cfi_def_cfa ,

     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

hello.s

　　　　gcc编译过程：http://lib.csdn.net/article/c/66184

　　1.去掉程序中的注释；

　　2.文件包含：当对C源程序进行预处理时，文件包含指的是将.c文件包含的头文件内容复制到当前.c文件中。

　　　　1》、第一种：#include <stdio.h>　　直接去系统头文件目录下寻找所包含的头文件。

　　　　2》、第二种：#include "head.h"　　 先在当前目录下寻找所包含的头文件，如果找不到，则再去系统头文件目录下寻找所包含的头文件。

　　3.宏替换

　　　　当对C源程序进行预处理时，如果程序中有事先定义好的宏，则会将所有的宏后面的字符串替换，例如：

　　　　　

　　　　1》预定义宏：　　　

 __FILE__　　　　　　 //正在编译的文件名（字符串常量）　　

 __LINE__　　　　　　 //文件当前的行号（整型常量）

 __FUNCTION__　　　 //当前所在的函数名（字符串常量）

 __DATE__　　　　　　//预编译文件的日期（字符串常量）

 __TIME__　　　　　　//预编译文件的时间（字符串常量）

 __STDC__　　　　　　//判断编译器是否遵循ANSI C，是则为1（整型常量）

Predefine Macros

　　　　　　

　　　　　2》不带参数的宏：#define 　　宏名　　字符串

 /*******************************************************************

  *   > File Name: noParamMacros.c

  *   > Author: fly

  *   > Mail: XXXXXXXX@icode.com

  *   > Create Time: Sun 17 Sep 2017 01:00:07 PM CST

  ******************************************************************/

 #include <stdio.h>

 #define PI  3.14*2

 #define M   60

 #define S   "HelloWorld"

 int main(int argc, char* argv[])

 {

     printf("%f\n", PI);

     printf("%d\n", M);

     printf("%s\n", S);

     return ;

 }

noParamMacros.c

　　 　　3》带参数的宏：可以像函数一样被调用。

 /*******************************************************************

  *   > File Name: paramMacros.c

  *   > Author: fly

  *   > Mail: XXXXXXXX@icode.com

  *   > Create Time: Sun 17 Sep 2017 01:40:15 PM CST

  ******************************************************************/

 #include <stdio.h>

 #define FUN(a, b)   a+b*a

 #define     FALSE   0

 #define     TRUE    1

 int fun(int a, int b){

     return a+b*a;

 }

 int main(int argc, char* argv[])

 {

     int a = , b =;

     int c;

     c = fun(a, b);  //函数调用

     printf("c = %d\n", c);

     c = FUN(a, b);  //宏调用

     float f;

 #if (DataType == FALSE)

     f = fun(3.1, 4.5);  //Data type is error

     printf("f = %f\n", f);

 #else

     f = FUN(3.1, 4.5);  //Data type is correct

     printf("f = %f\n", f);

 #endif

     return ;

 }

paramMacros.c

　　　　　　在上述例子中，由于传参时，可能会传一个表达式，这是调用带参宏就会导致结果不正确，所以一般在定义带参宏时，都会如下去定义：　　#define　　FUN(a, b)　　(a)+(b)*(a)

　　4.条件编译：在编译程序之前，按照某个条件来选择需要编译哪一段代码。

　　　　1》第一种方式：

　　　　

 /*******************************************************************

  *   > File Name: ifdef.c

  *   > Author: fly

  *   > Mail: XXXXXXXX@icode.com

  *   > Create Time: Sun 17 Sep 2017 02:51:43 PM CST

  ******************************************************************/

 #include <stdio.h>

 #define     DEBUG

 //有如下两个函数，如果要测试这两个函数是否正确，则必须写主函数测试

 void fun1(void){

     printf("hello world\n");

 }

 void fun2(void){

     printf("This is a good idea !\n");

 }

 //下面为测试代码

 #ifdef  DEBUG

 int main(int argc, char* argv[])

 {

     fun1();

     fun2();

     return ;

 }

 #endif

ifdef.c

　　　　2》第二种方式：

　　　　

 #ifdef __HEAD_H_

 #define __HEAD_H_

 #include <stdio.h>

 int x = ;

 #endif

 //该条件编译的作用是，防止头文件被多次（重复）包含。

ifndef

　　　　3》第三种方式：

　　　　

 //用于注释一段代码

 #if （0）

 #include "head.h"

 #include "head.h"

 #include "head.h"

 #include "head.h"

 #include "head.h"

 #include "head.h"

 #include "head.h"

 #include "head.h"

 #endif

if.c

二、字节序　

　　1.计算机在存储多字节数据时，数据内部各个字节的存储顺序称为字节序；

　　　　　　4字节：unsigned int word = 0x12345678；

　　　　　　　　

　　2.端序

　　　　1》小端序（little-endian）:最高有效位所在的字节放在最高字节位置，其他字节依次放在低字节位置，则该字节序称为高位优先。

　　　　2》大端序（big-endian）:最低有效位所在的字节放在最高字节位置，其他字节依次放在低字节位置，则该字节序称为低位优先。

 /*******************************************************************

  *   > File Name: endian.c

  *   > Author: fly

  *   > Mail: XXXXXXXX@icode.com

  *   > Create Time: Sun 17 Sep 2017 04:34:11 PM CST

  ******************************************************************/

 #include <stdio.h>

 #include <stdbool.h>

 #define FALSE   0

 #define TRUE    1

 bool is_little_endian(void){

     bool ret;

     unsigned int word = 0x12345678;

     unsigned char byte0 = *((unsigned char *)&word + );

     unsigned char byte1 = *((unsigned char *)&word + );

     unsigned char byte2 = *((unsigned char *)&word + );

     unsigned char byte3 = *((unsigned char *)&word + );

     printf("%p---->0x%hhx\n", &byte0, byte0);

     printf("%p---->0x%hhx\n", &byte1, byte1);

     printf("%p---->0x%hhx\n", &byte2, byte2);

     printf("%p---->0x%hhx\n", &byte3, byte3);

     if(byte0 == 0x78){

         printf("the machine is little endian !\n");

         ret = TRUE;

     }else if(byte0 == 0x12){

         printf("the machine is big endian !\n");

         ret = FALSE;

     }

     return ret;

 }

 int main(int argc, char* argv[])

 {

     if(is_little_endian() == TRUE){

         printf("Little endian is true !\n");

     }

     return ;

 }

　　　　　执行结果：　

 /*******************************************************************

  *   > File Name: endian1.c

  *   > Author: fly

  *   > Mail: XXXXXXXX@icode.com

  *   > Create Time: Sun 17 Sep 2017 05:02:58 PM CST

  ******************************************************************/

 #include <stdio.h>

 union A{

     unsigned int word;

     unsigned char byte;

 };

 int main(int argc, char* argv[])

 {

     union A un;

     un.word = 0x12345678;

     if(un.byte == 0x78){

         printf("the machine is little endian !\n");

     }else if(un.byte == 0x12){

         printf("the machine is big endian !\n");

     }

     return ;

 }

使用结构体判断大小端序

三、地址对齐

　　为了提高CPU从内存中存取数据的效率，在给数据分配内存空间时，会有意的将数据放在某些地址位置，这种分配空间的方式称为地址对齐。　　

　　1》自然对齐：在给数据分配空间是时，如果数据的起始地址能够被数据的长度整除，则该分配空间的方式为自然对齐。

　　2》数据的M值：

　　　　对于每一个数据都有M值，M值如下：

　　　　　　1）对于基本数据类型：

　　　　　　　　如果该数据的长度小于机器字字长，则它的M值为自身的长度；

　　　　　　　　如果该数据的长度大于或者等于机器字长，则它的M值为机器字长；

　　　　　　2）对于数组：

　　　　　　　　M值为元素的M值；

　　　　　　3）对于结构体，共用体：

　　　　　　　　M值为成员中最大的M值；

　　3》适当对齐：在给数据分配空间时，如果数据的起始地址能够被数据的M值整除，则该分配空间的方式为适当对齐。

　　4》结构体的存储：

 /*******************************************************************

  *   > File Name: align.c

  *   > Author: fly

  *   > Mail: XXXXXXXX@icode.com

  *   > Create Time: Sun 17 Sep 2017 05:15:23 PM CST

  ******************************************************************/

 #include <stdio.h>

 struct param{

     char a;

     short b;

     int c;

 };

 struct param1{

     char a;

     int b;

     short c;

 };

 int main(int argc, char* argv[])

 {

     struct param p1;

     printf("sizeof(struct param) = %d\tsizeof p1 = %d\n", sizeof(struct param), sizeof p1);

     struct param1 p2;

     printf("sizeof(struct param1) = %d\tsizeof p2 = %d\n", sizeof(struct param1), sizeof p2);

     return ;

 }

structAlign.c

　　　执行结果：

最新文章

  1. call,apply,bind的用法
  2. js框架设计1.1命名空间笔记
  3. Spark入门实战系列--8.Spark MLlib（上）--机器学习及SparkMLlib简介
  4. gdb调试汇编堆栈分析
  5. Linux阵列 RAID详解
  6. 禁止 IOS 系统 数字 变超链 （自动识别为电话号码）
  7. struts2文件下载 出现Can not find a java.io.InputStream with the name的错误
  8. WCF与WebService之间的异同
  9. c语言_帮助别人
  10. iOS相机权限、相册权限、定位权限判断
  11. 关于SQL语言的优化(Oracle)
  12. Qt编写文件一键命名软件
  13. vueThink权限配置
  14. ITU-T Technical Paper： QoS 的参数（非常的全，共计88个）
  15. CSS网页中导入特殊字体@font-face属性详解
  16. Liferay7 BPM门户开发之3: Activiti开发环境搭建
  17. python之OrderedDict类
  18. Boostrap本地导入js文件
  19. List 的一个有用的高效的操作 removeAll
  20. Handlebars.js 预编译(转)

热门文章

  1. [IOI2005]Riv 河流
  2. nginx开发（一） 源码－编译
  3. MySQL 备份和恢复数据
  4. bzoj 1800: [Ahoi2009]fly 飞行棋【枚举】
  5. P3469 [POI2008]BLO-Blockade(Tarjan 割点)
  6. [C++ STL] list使用详解
  7. hdu61272017杭电多校第七场1008Hard challenge
  8. Window对象与DOM
  9. 经典矩阵dp寻找递增最大长度
  10. C#学习-EF在三层中使用