In computing, position-independent code (PIC) or position-independent executable (PIE) is a body of machine code that, being placed somewhere in the primary memory, executes properly regardless of its absolute address. PIC is commonly used for shared libraries, so that the same library code can be loaded in a location in each program address space where it will not overlap any other uses of memory (for example, other shared libraries). PIC was also used on older computer systems lacking an MMU, so that the operating system could keep applications away from each other even within the single address space of an MMU-less system.
Position-independent code can be copied to any memory location and executed without modification. This differs from relocatable code, which requires special processing by a link editor or program loader to make it suitable for execution at a given location. Position independent code must adhere to a specific set of semantics in the source code and compiler support is required. Instructions that refer to specific memory addresses, such as absolute branches, must be replaced with equivalent program counter relative instructions. The extra indirection may cause PIC to be less efficient, although modern processors make the difference practically negligible.
Read more about Position-independent Code: History, Technical Details, Windows DLLs, Position-independent Executables
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