这是weakref的正确用法吗？

Question

我想允许在头文件中定义的.c文件中重新定义一个函数。根据GCC手册的weakref属性：

The effect is equivalent to moving all references to the alias to a separate translation unit, renaming the alias to the aliased symbol, declaring it as weak, compiling the two separate translation units and performing a reloadable link on them.

这听起来正是我想要做的。 然而，下面的例子不会错误编译：

tpp.c:18:13: error: redefinition of ‘foo’ tpp.c:6:13: note: previous definition of ‘foo’ was here

#include <sys/types.h> 
#include <stdio.h> 

/* this will be in a header file */ 
static void foo(void) __attribute__ ((weakref ("_foo"))); 

static void _foo(void) 
{ 
    printf("default foo\n"); 
} 

/* in a .c file #including the header mentioned above */ 
#define CUSTOM_FOO 

#ifdef CUSTOM_FOO 
static void foo(void) 
{ 
    printf("user defined foo.\n"); 
} 
#endif 

int main(int argc, char **argv) 
{ 
    printf("calling foo.\n"); 
    foo(); 
} 

我使用这是否正确？我错过了什么？

gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5)

Answer 1

就我所知，您需要将该函数定义为extern。 然后，它为我工作如下：

user@horst:$ cat weakref.c 

#include <sys/types.h> 
#include <stdio.h> 

/* this will be in a header file */ 
extern void foo(void) __attribute__ ((weak, alias ("_foo"))); 

void _foo(void) 
{ 
    printf("default foo\n"); 
} 

int main(int argc, char **argv) 
{ 
    printf("calling foo.\n"); 
    foo(); 
} 

user@horst:$ gcc weakref.c 
user@horst:$ ./a.out 
calling foo. 
default foo 
user@horst:$ cat weakrefUser.c 
#include <stdio.h> 
/* in a .c file #including the header mentioned above */ 
#define CUSTOM_FOO 

#ifdef CUSTOM_FOO 
void foo(void) 
{ 
    printf("user defined foo.\n"); 
} 
#endif 
user@horst:$ gcc -c weakrefUser.c 
user@horst:$ gcc -c weakref.c 
user@horst:$ gcc weakref.o weakrefUser.o 
user@horst:$ ./a.out 
calling foo. 
user defined foo. 

注1：它不与静态函数工作，对弱者的属性，它必须是全球性的。注意2：ELF目标仅支持弱符号。