C/C++标准有没有规定未初始化的全局变量,装载时自动设为0?

donet8

如题。我知道全局变量有初始值的放在数据段，没有初始值的放在bss段。

但是C/C++标准有没有规定，装载bss大小的数据段的时候，全局变量所用的那些空间应该被默认初始化为全0?
谢谢。

幻の上帝

ISO C/C++没有.bss的概念。
C的文件作用域对象，C++的全局（命名空间作用域）变量具有静态存储期：
ISO C11(N1570)
6.2.4
3 An object whose identifier is declared without the storage-class specifier _Thread_local, and either with external or internal linkage or with the storage-class specifier static, has static storage duration. Its lifetime is the entire execution of the program and its stored value is initialized only once, prior to program startup.
ISO C++11
3.7.1 Static storage duration [basic.stc.static]
1 All variables which do not have dynamic storage duration, do not have thread storage duration, and are not local have static storage duration. The storage for these entities shall last for the duration of the program (3.6.2, 3.6.3).
如果没有显式初始化，则默认行为可以总结为填0（但关于实际的存储值则有些差别，例如null pointer不必然保证是0）：
ISO C11(N1570) 6.7.9
10 If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate. If an object that has static or thread storage duration is not initialized explicitly, then:
— if it has pointer type, it is initialized to a null pointer;
— if it has arithmetic type, it is initialized to (positive or unsigned) zero;
— if it is an aggregate, every member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;
— if it is a union, the first named member is initialized (recursively) according to these rules, and any padding is initialized to zero bits;
关于ISO C++11见http://bbs.chinaunix.net/thread-3710317-4-1.html的40L。

bruceteen

1。C/C++ 估计没有所谓 bbs 等概念
2。全局变量默认初始化的，因为没法保证赋值在存取之前完成

folklore

no, os do it....
the bss section store the variants that its initialize value is not 0.