What is BSS segment in C?

Answer 1

The BSS segment is where all uninitialised static variables are allocated at runtime. BSS is an acronym for Block Started by Symbol, however it can also be referred to as the BES segment (Block Ended by Symbol) depending on how it is physically implemented. Regardless of how it is implemented, we can simply refer to it as the uninitialised data segment.

Unlike the data segment which is initialised directly from a bitmap image stored within the object file itself, the uninitialised data segment is dynamically instantiated at load time as a contiguous block of zero-initialised memory. Only the length of the block need be stored in the object file.

Uninitialised static variables are simply those that have not been explicitly initialised. Static variables may be declared globally (at file scope, outside of any function) or locally (inside a function):

int x; // uninitialised global variable

void foo () {

static int y; // uninitialised local variable

}

Both x and y will be allocated in the uninitialised data segment.

Although all constant variables must be initialised at the point of declaration in C, a constant variable that is initialised with the value zero may be allocated in the uninitialised data segment rather than in the initialised data segment where all other initialised static variables and constant variables are allocated. Similarly with static variables initialised to zero. This helps reduce the size of the object file and thus improves load time.

In memory, the text segment (.text) contains the executable code and is loaded first, followed by the data segment (.data) and finally the BSS segment (.bss). The object file header allows the program loader to allocate memory to all three segments as a single contiguous allocation. The .text and .data segments are initialised from bitmap images stored within the object file itself while the .bss segment is typically zero-filled-on-demand. However, in embedded systems software, the C runtime must initialise the BSS segment prior to entering main().