CODEWITHSUNDEEP
ccompilation
Ken
Ken

Reputation: 2355

What is the deal with position-independent code (PIC)?

Could somebody explain why I should be interested in compiling position-independent code, and also why should I avoid it?

Upvotes: 8

Views: 1942

Answers (3)

dmckee --- ex-moderator kitten
dmckee --- ex-moderator kitten

Reputation: 101251

The gory details depend on how your loader works on on whether you are building a executable or a library, but there is a sense in which this is all a problem for the build system and the compiler, not for you.

If you really want to understand you need to consider where the code gets put in the address space before execution starts and what set of branching instructions your chip provides. Are branches relative or absolute? Is access to the data segment relative or absolute?

  • If branches are absolute, then the code must be loaded to a reliable address or it won't work. That's position dependent code. Many simple (or older) operating systems accommodate this by always loading a program to the same place.

  • Relative branches mean that the can be placed at any location in memory. That is position independent code.

Again, all you need to know is the recipe for invoking your compiler and linker on your platform.

Upvotes: 4

TJD
TJD

Reputation: 11896

PIC code usually has to be slightly larger because the compiler can't use instructions that encode relative address offsets. Without PIC, many addresses can be encoded with 16 or 8 bits relative to current PC. Sometimes in embedded systems, PIC is useful. For example if you want to have patch code that can run at various physical addresses.

Upvotes: 1

Kerrek SB
Kerrek SB

Reputation: 477388

Making code position-independent adds a layer of abstraction, which requires an additional lookup step at runtime for certain operations (usually pertaining to accessing variables with static storage).

So if you don't need it, don't use it!

There are specific situations where you must produce PIC (namely when creating run-time loadable code, such as a plug-in module or library), but the added flexibility comes at a price.

Upvotes: 5

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