09.C语言：预处理（宏定义）、字节序、地址对齐

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

# 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/i386-linux-gnu/sys/cdefs.h" 1 3 4
# 385 "/usr/include/i386-linux-gnu/sys/cdefs.h" 3 4
# 1 "/usr/include/i386-linux-gnu/bits/wordsize.h" 1 3 4
# 386 "/usr/include/i386-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/i386-linux-gnu/gnu/stubs.h" 1 3 4






# 1 "/usr/include/i386-linux-gnu/gnu/stubs-32.h" 1 3 4
# 8 "/usr/include/i386-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/i686-linux-gnu/4.8/include/stddef.h" 1 3 4
# 212 "/usr/lib/gcc/i686-linux-gnu/4.8/include/stddef.h" 3 4
typedef unsigned int size_t;
# 34 "/usr/include/stdio.h" 2 3 4

# 1 "/usr/include/i386-linux-gnu/bits/types.h" 1 3 4
# 27 "/usr/include/i386-linux-gnu/bits/types.h" 3 4
# 1 "/usr/include/i386-linux-gnu/bits/wordsize.h" 1 3 4
# 28 "/usr/include/i386-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;




__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;
# 121 "/usr/include/i386-linux-gnu/bits/types.h" 3 4
# 1 "/usr/include/i386-linux-gnu/bits/typesizes.h" 1 3 4
# 122 "/usr/include/i386-linux-gnu/bits/types.h" 2 3 4


__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[2]; } __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;
# 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/i686-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/i686-linux-gnu/4.8/include/stdarg.h" 1 3 4
# 40 "/usr/lib/gcc/i686-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/i386-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/i386-linux-gnu/bits/sys_errlist.h" 1 3 4
# 26 "/usr/include/i386-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

# 9 "hello.c" 2

int main(int argc, char* argv[])
{
    printf("Hello , world !
");

    return 0;
}

    .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 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    andl    $-16, %esp
    subl    $16, %esp
    movl    $.LC0, (%esp)
    call    puts
    movl    $0, %eax
    leave
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    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

 　　　　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）

　　　　　　

　　　　　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
", PI);
    printf("%d
", M);
    printf("%s
", S);

    return 0;
}

　　 　　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 = 5, b =3;
    int c;

    c = fun(a, b);  //函数调用
    printf("c = %d
", 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
", f);
#else
    f = FUN(3.1, 4.5);  //Data type is correct
    printf("f = %f
", f);
#endif

    return 0;
}

　　　　　　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
");
}

void fun2(void){
    printf("This is a good idea !
");
}

//下面为测试代码
#ifdef  DEBUG
int main(int argc, char* argv[])
{
    fun1();
    fun2();

    return 0;
}
#endif

　　　　2》第二种方式：

　　　　

#ifdef __HEAD_H_
#define __HEAD_H_

#include <stdio.h>

int x = 100;

#endif

//该条件编译的作用是，防止头文件被多次（重复）包含。

　　　　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

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 + 0);
    unsigned char byte1 = *((unsigned char *)&word + 1);
    unsigned char byte2 = *((unsigned char *)&word + 2);
    unsigned char byte3 = *((unsigned char *)&word + 3);

    printf("%p---->0x%hhx
", &byte0, byte0);
    printf("%p---->0x%hhx
", &byte1, byte1);
    printf("%p---->0x%hhx
", &byte2, byte2);
    printf("%p---->0x%hhx
", &byte3, byte3);

    if(byte0 == 0x78){
        printf("the machine is little endian !
");
        ret = TRUE;
    }else if(byte0 == 0x12){
        printf("the machine is big endian !
");
        ret = FALSE;
    }

    return ret;
}

int main(int argc, char* argv[])
{
    if(is_little_endian() == TRUE){
        printf("Little endian is true !
");
    }

    return 0;
}

　　　　　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 !
");
    }else if(un.byte == 0x12){
        printf("the machine is big endian !
");
    }

    return 0;
}

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	sizeof p1 = %d
", sizeof(struct param), sizeof p1);

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

    return 0;
}

　　　执行结果：