What does KEEP mean in a linker script?

Question

The LD manual does not explain what the KEEP command does. Below is a snippet from a third-party linker script that features KEEP. What does the KEEP command do in ld?

SECTIONS
{  
    .text :
    {
        . = ALIGN(4);
        _text = .;
        PROVIDE(stext = .);
        KEEP(*(.isr_vector))
        KEEP(*(.init))
        *(.text .text.*)        
        *(.rodata .rodata.*)        
        *(.gnu.linkonce.t.*)
        *(.glue_7)
        *(.glue_7t)
        *(.gcc_except_table)
        *(.gnu.linkonce.r.*)
        . = ALIGN(4);
        _etext = .;
        _sidata = _etext;
        PROVIDE(etext = .);   
            _fini = . ;
                *(.fini)

    } >flash

Answer 1

Afaik LD keeps the symbols in the section even if symbols are not referenced. (--gc-sections).

Usually used for sections that have some special meaning in the binary startup process, more or less to mark the roots of the dependency tree.

---

(For Sabuncu below)

Dependency tree:

If you eliminate unused code, you analyze the code and mark all reachable sections (code+global variables + constants).

So you pick a section, mark it as "used" and see what other section it references, then you mark those section as "used", and check what they reference etc.

The section that are not marked "used" are then redundant, and can be eliminated.

Since a section can reference multiple other sections (e.g. one procedure calling three different other ones), if you would draw the result you get a tree.

Roots:

The above principle however leaves us with a problem: what is the "first" section that is always used? The first node (root) of the tree so to speak? This is what "keep()" does, it tells the linker which sections (if available) are the first ones to look at. As a consequence these are always linked in.

Typically these are sections that are called from the program loader to perform tasks related to dynamic linking (can be optional, and OS/fileformat dependent), and the entry point of the program.