What is the .bss section and why does it exist?

.bss holds uninitialized (zero) globals and statics; it takes no space in the file — just a recorded size — and the loader zero-fills it in memory.

* .data carries real bytes on disk; .bss carries only a size — the loader materialises its zeros in memory at startup. *

// Goes in .data
int initialized = 42;
// Goes in .bss
int uninitialized;
// Goes in .bss
static int also_bss;

Why have a separate section?

The .bss section occupies no space in the object file - only the section header records how much memory is needed.

Space savings example:

char buffer[1000000];  // 1MB uninitialized
// In .bss: adds ~0 bytes to file
// In memory: 1MB of zeros allocated at load time

The loader:

1. Reads .bss size from section header
2. Allocates that much memory
3. Fills it with zeros

Tip: This is why uninitialized globals are guaranteed to be zero in C - the OS zeros the .bss pages.

Go deeper:

• .bss (Wikipedia) — the section that stores a size, not the zeros themselves.