SOME HIDE DETAILES OF MACRO IN C

macro看似只进行简单的文本替换，在预处理阶段它会根据你所定义的macro生成一个hash，稍后再处理的时候，根据遇到的macro名查找hash，得到宏的内容，使用hash存储速度非常快。

下面我们进行一个简单的实验，猜猜程序的运行结果？
macro.c

#include <assert.h>
#define WIDTH  40
#define HEIGHT  80
#define AREA    2*(WIDTH* HEIGHT)
#define MAX(a,b)          ( ((a) > (b))?(a): (b))
int main(int argc,char ** argv){
    int i=1;
    int j=2;
    int c;
    c = MAX(i  ,j  );
    assert(j == 4);
    assert( AREA == 6400);
}

after post preprocessor:

我们观察一下程序预处理后的信息
gcc -E macro.c > macro.i

# 1 "macro.c"
# 1 ""
# 1 ""
# 1 "macro.c"
# 1 "/usr/include/assert.h" 1 3 4
# 37 "/usr/include/assert.h" 3 4
# 1 "/usr/include/features.h" 1 3 4
# 323 "/usr/include/features.h" 3 4
# 1 "/usr/include/i386-linux-gnu/bits/predefs.h" 1 3 4
# 324 "/usr/include/features.h" 2 3 4
# 356 "/usr/include/features.h" 3 4
# 1 "/usr/include/i386-linux-gnu/sys/cdefs.h" 1 3 4
# 353 "/usr/include/i386-linux-gnu/sys/cdefs.h" 3 4
# 1 "/usr/include/i386-linux-gnu/bits/wordsize.h" 1 3 4
# 354 "/usr/include/i386-linux-gnu/sys/cdefs.h" 2 3 4
# 357 "/usr/include/features.h" 2 3 4
# 388 "/usr/include/features.h" 3 4
# 1 "/usr/include/i386-linux-gnu/gnu/stubs.h" 1 3 4



# 1 "/usr/include/i386-linux-gnu/bits/wordsize.h" 1 3 4
# 5 "/usr/include/i386-linux-gnu/gnu/stubs.h" 2 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
# 389 "/usr/include/features.h" 2 3 4
# 38 "/usr/include/assert.h" 2 3 4
# 68 "/usr/include/assert.h" 3 4



extern void __assert_fail (__const char *__assertion, __const char *__file,
      unsigned int __line, __const char *__function)
     __attribute__ ((__nothrow__)) __attribute__ ((__noreturn__));


extern void __assert_perror_fail (int __errnum, __const char *__file,
      unsigned int __line,
      __const char *__function)
     __attribute__ ((__nothrow__)) __attribute__ ((__noreturn__));




extern void __assert (const char *__assertion, const char *__file, int __line)
     __attribute__ ((__nothrow__)) __attribute__ ((__noreturn__));



# 2 "macro.c" 2




int main(int argc,char ** argv){
 int i=1;
 int j=2;
 int c;
 c = ( ((i  ) > (j  ))?(i  ): (j  ));
 ((j == 4) ? (void) (0) : __assert_fail ("j !== 4", "macro.c", 11, __PRETTY_FUNCTION__));
 ((2*(40 *80) == 6400) ? (void) (0) : __assert_fail ("2*(40 *80) == 6400", "macro.c", 12, __PRETTY_FUNCTION__));
}

需要注意的是在宏扩展以后，asser宏已经被替换成了真正的条件表达式，不过该条件表达是比较奇怪，

assert( j == 4)
expand
((j == 4) ? (void) (0) : __assert_fail ("j == 4", "macro.c", 11, __PRETTY_FUNCTION__));

?表达式的条件部分很好立即，

条件结果为真是，执行的空操作 (void) (0) 这里看起来比较奇怪，其实也就是一个空操作

条件结果为假时   执行如下操作：

__assert_fail ("j == 4", "macro.c", 11, __PRETTY_FUNCTION__)

该函数在宏扩展以后，我们已经看到了该函数的prototype，

compile and link  and then run
对该程序进行编译，运行发现程序运行正常。

那C的结果为什么是4而不是3呢？这正是macro的诡异之处，

通过观察macro展开的代码不难发现

c = ( ((i  ) > (j  ))?(i  ): (j  ));

不论那一个条件为真，较大的值都执行了两次自加操作，因此J的结果为4而不是3，

这一点要求我们在对两个函数的返回结果进行比较时要注意，

两个函数比较大小，函数总共执行3次，函数的大小对你程序的性能影响还是比较大的。

note

下面附上ASSERT.H的实现，感兴趣的筒子可以看一下。
something about assert.h

/* Copyright (C) 1991,1992,1994-2001,2003,2004,2007
   Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, write to the Free
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
   02111-1307 USA.  */

/*
 *    ISO C99 Standard: 7.2 Diagnostics    <assert.h>
 */

#ifdef    _ASSERT_H

# undef    _ASSERT_H
# undef    assert
# undef __ASSERT_VOID_CAST

# ifdef    __USE_GNU
#  undef assert_perror
# endif

#endif /* assert.h    */

#define    _ASSERT_H    1
#include

#if defined __cplusplus && __GNUC_PREREQ (2,95)
# define __ASSERT_VOID_CAST static_cast
#else
# define __ASSERT_VOID_CAST (void)
#endif

/* void assert (int expression);

   If NDEBUG is defined, do nothing.
   If not, and EXPRESSION is zero, print an error message and abort.  */

#ifdef    NDEBUG

# define assert(expr)        (__ASSERT_VOID_CAST (0))

/* void assert_perror (int errnum);

   If NDEBUG is defined, do nothing.  If not, and ERRNUM is not zero, print an
   error message with the error text for ERRNUM and abort.
   (This is a GNU extension.) */

# ifdef    __USE_GNU
#  define assert_perror(errnum)    (__ASSERT_VOID_CAST (0))
# endif

#else /* Not NDEBUG.  */

#ifndef _ASSERT_H_DECLS
#define _ASSERT_H_DECLS
__BEGIN_DECLS

/* This prints an "Assertion failed" message and aborts.  */
extern void __assert_fail (__const char *__assertion, __const char *__file,
               unsigned int __line, __const char *__function)
     __THROW __attribute__ ((__noreturn__));

/* Likewise, but prints the error text for ERRNUM.  */
extern void __assert_perror_fail (int __errnum, __const char *__file,
                  unsigned int __line,
                  __const char *__function)
     __THROW __attribute__ ((__noreturn__));


/* The following is not at all used here but needed for standard
   compliance.  */
extern void __assert (const char *__assertion, const char *__file, int __line)
     __THROW __attribute__ ((__noreturn__));


__END_DECLS
#endif /* Not _ASSERT_H_DECLS */

# define assert(expr)                            \
  ((expr)                                \
   ? __ASSERT_VOID_CAST (0)                        \
   : __assert_fail (__STRING(expr), __FILE__, __LINE__, __ASSERT_FUNCTION))

# ifdef    __USE_GNU
#  define assert_perror(errnum)                        \
  (!(errnum)                                \
   ? __ASSERT_VOID_CAST (0)                        \
   : __assert_perror_fail ((errnum), __FILE__, __LINE__, __ASSERT_FUNCTION))
# endif

/* Version 2.4 and later of GCC define a magical variable `__PRETTY_FUNCTION__'
   which contains the name of the function currently being defined.
   This is broken in G   before version 2.6.
   C9x has a similar variable called __func__, but prefer the GCC one since
   it demangles C   function names.  */
# if defined __cplusplus ? __GNUC_PREREQ (2, 6) : __GNUC_PREREQ (2, 4)
#   define __ASSERT_FUNCTION    __PRETTY_FUNCTION__
# else
#  if defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L
#   define __ASSERT_FUNCTION    __func__
#  else
#   define __ASSERT_FUNCTION    ((__const char *) 0)
#  endif
# endif

#endif /* NDEBUG.  */

通过下面这个实验你可能还会进步一的了解macro的本质，

ubuntu@ubuntu-virtual-machine:~/Desktop/detail$ cat a.h
int a(int * a,int * b);
ubuntu@ubuntu-virtual-machine:~/Desktop/detail$ cat b.h
int b(int * a,int b) ;
ubuntu@ubuntu-virtual-machine:~/Desktop/detail$ cat c.h
int c(int a,int * b);
ubuntu@ubuntu-virtual-machine:~/Desktop/detail$ cat ma.c
#include "a.h"
#include  "b.h"
#include "c.h"
int main(int argc,char * argv]){
    return 0;
}

ubuntu@ubuntu-virtual-machine:~/Desktop/detail$ gcc -E ma.c
# 1 "ma.c"
# 1 ""
# 1 ""
# 1 "ma.c"
# 1 "a.h" 1
int a(int * a,int * b);
# 2 "ma.c" 2
# 1 "b.h" 1
int b(int * a,int b) ;
# 3 "ma.c" 2
# 1 "c.h" 1
int c(int a,int * b);
# 4 "ma.c" 2
int main(int argc,char * argv]){
 return 0;
}