论证在C头文件中定义的变量的奥秘

function.h

#ifndef _FUNCTION_H_
#define _FUNCTION_H_


void test_extern1_print();
void test_extern2_print();

#endif

header.h

#ifndef _HEADDER_H_
#define _HEADDER_H_

extern int test_extern1_val;
extern int test_extern2_val;
static int tmp_vale = 3;

#endif

 

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

o_file := test_extern1.o test_extern2.o

all: main
main : main.o $(o_file)
    $(CC) main.o $(o_file) -o main

clean:
    rm -rvf ./*.o main

 

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