什么是weak symbol?

weak symbol

 

我们用nm看动态库时，会发现有些符号类型是"V"，手册里解释如下：
"V" The symbol is a weak object.  When a weak defined symbol is linked with a normal  defined  symbol, the normal defined symbol is used with no error. When a weak undefined symbol is linked and the symbol is not defined, the value of the weak symbol becomes zero with no error.
说的是动态库中的weak symbol，缺省会被normal symbol替代，如果没有定义，则该symbol的值为0。
很抽象，是不是，我一直想找一个简单的例子。
             
最近看过一篇文章：
http://www.cs.virginia.edu/~wh5a/blog/2006/07/20/the-weak-attribute-of-gcc/
终于对所谓的weak symbol有了一点了解。
http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Function-Attributes.html
讲了__attribute__的语法。

 

【weak.c】
extern void foo() __attribute__((weak));

int main() {
  if (foo) foo();
}
程序居然能够编译通过，甚至成功执行！让我们来看看是为什么？
首先声明了一个符号foo()，属性为weak，但并不定义它，这样，链接器会将此未定义的weak symbol赋值为0，也就是说foo()并没有真正被调用，试试看，去掉if条件，肯定core dump！

 

【strong.c】
extern void foo() ;

int main() {
  if (foo) foo();
}
这个是一般程序，编译过不了：
strong.c: undefined reference to `foo'

再添上一个定义文件：

【foo.c】
void foo() {
  printf("in foo./n");
}
OK！

 

用nm检查一下：
linux:~/test/weak # nm weak.o
         w foo
00000000 T main
linux:~/test/weak # nm foo.o
00000000 T foo
         U printf
链接时，前面那个weak symbol会被后面这个代替，如果没有链接foo.o,也没问题，对应符号为0。

 

这就是weak symbol的意义。

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`V' The symbol is a weak object.  When a weak defined symbol is linked with a normal defined  symbol, the  normal defined symbol is used with no error.  When a weak undefined symbol is linked and the symbol is not defined, the value of the weak symbol becomes zero with no error.
`W' The symbol is a weak symbol that has not been specifically tagged as a weak object symbol.   When a  weak  defined symbol is linked with a normal defined symbol, the normal defined symbol is used with no error.  When a weak undefined symbol is linked and the symbol is not defined,  the  value of the weak symbol becomes zero with no error.