论证在C头文件中定义的变量的奥秘-阿里云开发者社区

论证在C头文件中定义的变量的奥秘

2011-12-02 820

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简介： function.h #ifndef _FUNCTION_H_ #define _FUNCTION_H_ void test_extern1_print(); void test_extern2_print(); #endif header.

function.h

header.h

test_extern1.c

#include stdio.h>
#include "header.h"
int test_extern1_val = 1;
void test_extern1_print()
{
printf("In test_extern1.c:\n");
printf("______________________________________________________________________________________________\n");
printf("| test_extern1_val | address | test_extern2_val | address | tmp_vale | address |\n");
printf("______________________________________________________________________________________________\n");
printf("| %d | %p | %d | %p | %d | %p |\n",\
test_extern1_val,&test_extern1_val,test_extern2_val,&test_extern2_val,tmp_vale,&tmp_vale);
printf("______________________________________________________________________________________________\n");
}

test_extern2.c

#include stdio.h>
#include "header.h"
int test_extern2_val = 2;
void test_extern2_print()
{
printf("In test_extern2.c:\n");
printf("______________________________________________________________________________________________\n");
printf("| test_extern1_val | address | test_extern2_val | address | tmp_vale | address |\n");
printf("______________________________________________________________________________________________\n");
printf("| %d | %p | %d | %p | %d | %p |\n",\
test_extern1_val,&test_extern1_val,test_extern2_val,&test_extern2_val,tmp_vale,&tmp_vale);
printf("______________________________________________________________________________________________\n");
}

main.c

#include stdio.h>
#include stdlib.h>
#include "founctions.h"
#include "header.h"
int main()
{
system("clear");
printf("\n\n****************************************** Head *****************************************\n\n");
test_extern1_print();
printf("\n\n");
test_extern2_print();
printf("\n\n");
printf("In main.c:\n");
printf("______________________________________________________________________________________________\n");
printf("| test_extern1_val | address | test_extern2_val | address | tmp_vale | address |\n");
printf("______________________________________________________________________________________________\n");
printf("| %d | %p | %d | %p | %d | %p |\n",\
test_extern1_val,&test_extern1_val,test_extern2_val,&test_extern2_val,tmp_vale,&tmp_vale);
printf("______________________________________________________________________________________________\n");
printf("\n\n");
printf("\n\n****************************************** Tail *****************************************\n\n");
return 0;
}

Makefile

编译效果：

****************************************** Head *****************************************
In test_extern1.c:
______________________________________________________________________________________
| test_extern1_val | address | test_extern2_val | address | tmp_vale | address |
______________________________________________________________________________________
| 1 | 0x8049ca0 | 2 | 0x8049ca8 | 3 | 0x8049c9c |
______________________________________________________________________________________
In test_extern2.c:
______________________________________________________________________________________
| test_extern1_val | address | test_extern2_val | address | tmp_vale | address |
______________________________________________________________________________________
| 1 | 0x8049ca0 | 2 | 0x8049ca8 | 3 | 0x8049ca4 |
______________________________________________________________________________________
In main.c:
______________________________________________________________________________________
| test_extern1_val | address | test_extern2_val | address | tmp_vale | address |
______________________________________________________________________________________
| 1 |0x8049ca0 | 2 | 0x8049ca8 | 3 | 0x8049c98 |
______________________________________________________________________________________
****************************************** Tail *****************************************
[root@localhost test_extern]#

上面注意到，tmp_vale值虽然一样，但是地址是不一样的。

所以，在头文件中定义static变量，代码中好像使用了相同的变量，但是实际上使用的是不同的变量，在每个源文件中都有单独的变量。会造成变量多次定义，造成内存空间的浪费，而且也不是真正的全局变量。应该避免使用这种定义方式

论证在C头文件中定义的变量的奥秘