C语言 链接期间对全局变量的未定义引用
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Undefined reference to global variable during linking
提问by freieschaf
I am trying to compile a program which is divided into 3 modules, corresponding to 3 source files: a.c, b.c, and z.c. z.ccontains the main()function, which calls functions in a.cand b.c. Furthermore, a function in a.ccalls a function in b.c, and viceversa. Finally, there is a global variable countwhich is used by the three modules and is defined in a separate header file, global.h.
我试图编译被划分成3个模块，对应于3个源文件的程序：a.c，b.c，和z.c。z.c包含main()调用a.c和中的函数的函数b.c。此外，函数 ina.c调用函数 in b.c，反之亦然。最后，有一个count由三个模块使用的全局变量，定义在单独的头文件global.h.
The code of the source files is the following:
源文件的代码如下：
a.c
a.c
#include "global.h"
#include "b.h"
#include "a.h"
int functAb() {
functB();
functA();
return 0;
}
int functA() {
count++;
printf("A:%d\n", count);
return 0;
}
b.c
b.c
#include "global.h"
#include "a.h"
#include "b.h"
int functBa() {
functA();
functB();
return 0;
}
int functB() {
count++;
printf("B:%d\n", count);
return 0;
}
z.c
z.c
#include "a.h"
#include "b.h"
#include "global.h"
int main() {
count = 0;
functAb();
functBa();
return 0;
}
The header files:
头文件：
a.h
a.h
#ifndef A_H
#define A_H
#include <stdio.h>
int functA();
int functAb();
#endif
b.h
b.h
#ifndef B_H
#define B_H
#include <stdio.h>
int functB();
int functBa();
#endif
global.h
global.h
#ifndef GLOBAL_H
#define GLOBAL_H
extern int count;
#endif
And, finally, the makefilethat reproduces my error:
最后，makefile重现我的错误：
CC = gcc
CFLAGS = -O3 -march=native -Wall -Wno-unused-result
z: a.o b.o z.o global.h
$(CC) -o z a.o b.o z.o $(CFLAGS)
a.o: a.c b.h global.h
$(CC) -c a.c $(CFLAGS)
b.o: b.c a.h global.h
$(CC) -c b.c $(CFLAGS)
z.o: z.c a.h global.h
$(CC) -c z.c $(CFLAGS)
With this, I can compile the objects a.o, b.o, and z.ofine, but, when linking with make z, I get undefined reference to 'count'in all of them:
有了这个，我可以编译对象a.o、b.o和z.o很好，但是，当与 链接时make z，我得到undefined reference to 'count'了所有对象：
z.o: In function `main':
z.c:(.text.startup+0x8): undefined reference to `count'
a.o: In function `functAb':
a.c:(.text+0xd): undefined reference to `count'
a.c:(.text+0x22): undefined reference to `count'
a.o: In function `functA':
a.c:(.text+0x46): undefined reference to `count'
a.c:(.text+0x5b): undefined reference to `count'
b.o:b.c:(.text+0xd): more undefined references to `count' follow
collect2: ld returned 1 exit status
I managed to reproduce the error in my actual code in this minimal example, so I guess there is a problem in the dependencies between modules, but I can't spot it. Can anyone point me in the right direction?
在这个最小的例子中，我设法在我的实际代码中重现了错误，所以我猜模块之间的依赖关系存在问题，但我无法发现它。任何人都可以指出我正确的方向吗？
回答by Mohit Jain
Change your z.cto
改变你z.c的
#include "a.h"
#include "b.h"
#include "global.h"
int count; /* Definition here */
int main() {
count = 0;
functAb();
functBa();
return 0;
}
From global.h, all your files inherit the declarationof variable countbut the definitionis missing from all files.
从global.h，您的所有文件都继承了变量的声明，count但所有文件中都缺少该定义。
You must add the definition to one of the file as int count = some_value;
您必须将定义添加到文件之一 int count = some_value;
回答by Sourav Ghosh
You have declaredcount, not definedit.
您已声明计数，而不是定义它。
externis a part of declaration, not a definition.
extern是声明的一部分，而不是定义。
To be explicit, externis a storage-class specifier and used at declaration.
明确地说，extern是一个存储类说明符，用于声明。
You need to defineint countsomewhere in your source files.
您需要在源文件中的某处定义int count。
回答by LPs
You have to add int count;to your z.c file.
This because of declaring a variable in header file as externtells to the compiler that the variable will be declared in another file, but the variable is not declared yet and will be resolved b linker.
您必须添加int count;到您的 zc 文件中。这是因为在头文件中声明了一个变量，因为它extern告诉编译器该变量将在另一个文件中声明，但该变量尚未声明并将在链接器中解析。
Then you need to declare the variable somewhere.
然后你需要在某处声明变量。
