Regarding the bss segment and data segment in Unix

Question

I read in my unix text book that bss is used to store the uninitialized variables(global declarations). This would mean that the static variables are stored separately and not in the bss. The only other place is the data segment.

Here is my question: Is bss segment a part of the data segment or are they two entirely different set of memory segments? Also, why keep the global variables separately from the static variables?

Answer 1

The original idea is still there, although there are layers of obfuscation on top of it for local symbols, small references, shared libraries, and such.

But everything is still based on the original simple model and so it is worth knowing. The idea is that there are three segments.

• .text
  This has program code.
• .data
  This has initialized data.
• .bss
  This has uninitialized data.

Bss is special: .bss objects don't take any space in the object file, and by grouping all the symbols that are not specifically intialized together, they can be easily zeroed out at once.

Static has a couple of meanings. In one sense it is just a symbol attribute that tells the linker whether it is local to an object module or can be globally linked with other symbols of the same name. It is also used to specify storage duration, to distinguish objects within a function from automatic (stack allocated) objects.

So a static variable can be in either data or bss depending on whether it is initialized or not, as can a global.