amba start.S 分析 .weak作用

我們用nm看動態庫時，會發現有些符號類型是"V"，手冊裡解釋如下：

"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.
說的是動態庫中的weak symbol，預設會被normal symbol替代，如果沒有定義，則該symbol的值為0。
很抽象，是不是，我一直想找一個簡單的例子。
             
最近看過一篇文章：
http://www.cs.virginia.edu/~wh5a/blog/2006/07/20/the-weak-attribute-of-gcc/


終於對所謂的weak symbol有了一點瞭解。
http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Function-Attributes.html


講了__attribute__的文法。 【weak.c】
extern void foo() __attribute__((weak));int main() {
  if (foo) foo();
}
程式居然能夠編譯通過，甚至成功執行！讓我們來看看是為什嗎？
首先聲明了一個符號foo()，屬性為weak，但並不定義它，這樣，連結器會將此未定義的weak symbol賦值為0，也就是說foo()並沒有真正被調用，試試看，去掉if條件，肯定core dump！ 【strong.c】
extern void foo() ;int main() {
  if (foo) foo();
}
這個是一般程式，編譯過不了：
strong.c: undefined reference to `foo'再添上一個定義檔案：【foo.c】
void foo() {
  printf("in foo./n");
}
OK！ 用nm檢查一下：
linux:~/test/weak # nm weak.o
         w foo
00000000 T main
linux:~/test/weak # nm foo.o
00000000 T foo
         U printf
連結時，前面那個weak symbol會被後面這個代替，如果沒有連結foo.o,也沒問題，對應符號為0。 這就是weak symbol的意義。-------------------------------------------------------------------------------------------------------------------------------------------

`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.