This question confused me a lot. As far as I know, .bss section is for saving data that initialized but not used yet. But I don't understand what 'content' here mean and why there is no content here?
Thanks for any helps!
There's no content in the BSS (Block started By Symbol) section because it would be wasted storage. The contents of the BSS is all zeros and it is cleared by the startup code before main is called. Think of the BSS as a run-length compressed block of bytes. All you need to know to uncompress that block is the value (0) and the length, which is stored in the ELF entry for the BSS.
Your notion of "data that [is] initialized but not used yet" is a bit off. Consider that all sections in an ELF file are somehow "not used yet". The text segment may or may not become used (it may contain dead/unreachable code). The data segment may or may not be used at all (you can define objects never used by code).