How do global variables get initialized by the elf loader

允我心安 提交于 2019-12-04 07:31:58

An int-sized space is made in a data section, with the value 5 encoded in it and a global non-function symbol named 'aglobal' is added to the symbol table pointing at it. References to aglobal are turned into relocations that are resolved at link-time to point to that data block, so in a fully-linked image instructions will load directly from that spot in memory that holds the 5 value

For example, the (x86) assembly might look something like:

.data
.globl aglobal
aglobal: .long 5

.text
main:
    mov eax, aglobal

In an object file, the mov instruction will turn into mov eax, 0 with a relocation R_386_32 aglobal+0, because the object file doesn't know for sure where the data section will be in memory.

In a fully-linked image, it might be something like:

mov eax, 0x804a010

Now the actual address of the 4 bytes in the data section is known, so it's specified directly

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