.bss

其实就是**Uninitialized data segment** 。

wikipedia .bss

In computer programming, the block starting symbol (abbreviated to .bss or bss) is the portion of an object file, executable, or assembly language code that contains statically-allocated variables that are declared but have not been assigned a value yet. It is often referred to as the "bss section" or "bss segment".

NOTE:

static、uninitialized

Typically only the length of the bss section, but no data, is stored in the object file. The program loader allocates memory for the bss section when it loads the program. By placing variables with no value in the .bss section, instead of the .data or .rodata section which require initial value data, the size of the object file is reduced.

On some platforms, some or all of the bss section is initialized to zeroes. Unix-like systems and Windows initialize the bss section to zero, allowing C and C++ statically allocated variables initialized to values represented with all bits zero to be put in the bss segment. Operating systems may use a technique called zero-fill-on-demand to efficiently implement the bss segment.[1] In embedded software, the bss segment is mapped into memory that is initialized to zero by the C run-time system before main() is entered. Some C run-time systems may allow part of the bss segment not to be initialized; C variables must explicitly be placed into that portion of the bss segment.[2]

BSS in C

In C, statically allocated objects without an explicit initializer are initialized to zero (for arithmetic types) or a null pointer (for pointer types). Implementations of C typically represent zero values and null pointer values using a bit pattern consisting solely of zero-valued bits (though this is not required by the C standard). Hence, the BSS segment typically includes all uninitialized objects (both variables and constants) declared at file scope (i.e., outside any function) as well as uninitialized static local variables (local variables declared with the static keyword); static local constants must be initialized at declaration, however, as they do not have a separate declaration, and thus are typically not in the BSS section, though they may be implicitly or explicitly initialized to zero. An implementation may also assign statically-allocated variables and constants initialized with a value consisting solely of zero-valued bits to the BSS section.