Editing Position-independent code (section)

== SunOS 4.x and ELF ==
Procedure calls inside a shared library are typically made through small procedure linkage table (PLT) [[method stub|stub]]s, which then call the definitive function. This notably allows a shared library to inherit certain function calls from previously loaded libraries rather than using its own versions.<ref name="sunos 4.x">{{cite conference |url=https://www.cs.cornell.edu/courses/cs414/2001FA/sharedlib.pdf |title=Shared Libraries in SunOS |first1=Robert A. |last1=Gingell |first2=Meng |last2=Lee |first3=Xuong T. |last3=Dang |first4=Mary S. |last4=Weeks |conference=1987 Summer USENIX Technical Conference and Exhibition |pages=131–146}}</ref>

Data references from position-independent code are usually made indirectly, through [[Global Offset Table]]s (GOTs), which store the addresses of all accessed [[global variable]]s. There is one GOT per compilation unit or object module, and it is located at a fixed offset from the code (although this offset is not known until the library is [[Library linking|linked]]). When a [[Linker (computing)|linker]] links modules to create a shared library, it merges the GOTs and sets the final offsets in code. It is not necessary to adjust the offsets when loading the shared library later.<ref name="sunos 4.x" />

Position-independent code that accesses global data does so by fetching the address for the global variable from its entry in the GOT. As the GOT is at a fixed offset from the code, the offset between the address of a given instruction in the code and the address of a GOT entry for a given global variable is also fixed, so that the offset does not need to be changed depending on the address at which the position-independent code is loaded. An instruction that fetches the GOT entry for a global variable would use an [[addressing mode]] that contains an offset relative to some instruction in the code; this might be a [[addressing mode#PC-relative 2|PC-relative]] addressing mode if the [[instruction set architecture]] supports it, or a [[addressing mode#Base plus offset, and variations|register-relative]] addressing mode, with functions loading that register with the address of an instruction in the [[function prologue]].<ref name="sunos 4.x" /><ref>{{cite book |title=System V Application Binary Interface Motorola 68000 Processor Family Supplement |date=1990 |url=https://people.debian.org/~glaubitz/m68k-sysv-abi.pdf |pages=3{{hyp}}32-3{{hyp}}35 |publisher=Prentice-Hall |isbn=0-13-877663-6}}</ref><ref>{{cite book |title=System V Application Binary Interface i386 Architecture Processor Supplement |edition=Fourth |url=https://www.sco.com/developers/devspecs/abi386-4.pdf |pages=3{{hyp}}35-3{{hyp}}39}}</ref><ref>{{cite book |title=System V Application Binary Interface AMD64 Architecture Processor Supplement (With LP64 and ILP32 Programming Models) Version 1.0 |url=https://cs61.seas.harvard.edu/site/2022/pdf/x86-64-abi-20210928.pdf |date=September 28, 2021}}</ref>