CODEWITHSUNDEEP
shared-librariesshared-memorydynamic-linkingvirtual-memorydynamic-loading
shao
shao

Reputation: 25

How can process share shared library with load-time relocation?

When shared library was not complied as PIC, it still can be linked with the executable thorugh load-time relocation.

If I understand correctly, dynamic loader would look for entries listed in relocation table, and modifies them according to the memory mapping. That is, the code of shared library was adapted for the current process during load time.

My question is, how can another process uses the same shared library at the same time, dose the loader guarantee the memory mapping of the two processes are consistent? Or the library cannot be shared, and the OS would just load another copy of shared library into the memory?

Upvotes: 1

Views: 372

Answers (1)

Employed Russian
Employed Russian

Reputation: 213626

When shared library was not complied as PIC, it still can be linked with the executable thorugh load-time relocation.

This statement needs qualifiers: this is only possible on some platforms (e.g. ELF32 ix86), but not others (e.g. ELF64 x86_64 with default medium memory model).

dynamic loader would look for entries listed in relocation table, and modifies them according to the memory mapping. That is, the code of shared library was adapted for the current process during load time.

Correct. Note that any memory pages that the loader had to update become unshared.

how can another process uses the same shared library at the same time

The other process will have its own copies of the loader-updated pages, but will share any unmodified pages with the first process.

dose the loader guarantee the memory mapping of the two processes are consistent?

No.

Or the library cannot be shared, and the OS would just load another copy of shared library into the memory?

Not quite: depending on how many pages need to be modified by the loader, some sharing can still happen.

P.S. When you build the shared library with -fPIC, the number of pages that need updating by the loader is minimized (all the places to be updated are grouped together in the .got section, instead of having these places spread throughout the .text section).

Upvotes: 2

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