Load-time relocation of shared libraries (2011)

Summary

This article's aim is to explain how a modern operating system makes it possible to use shared libraries with load-time relocation. It focuses on the Linux OS running on 32-bit x86, but the general principles apply to other OSes and CPUs as well. Note that shared libraries have many names - shared libraries, shared objects, dynamic shared objects (DSOs), dynamically linked libraries (DLLs - if you're coming from a Windows background). For the sake of consistency, I will try to just use the name "shared library" throughout this article. Loading executables Linux, similarly to other OSes with virtual memory support, loads executables to a fixed memory address. If we examine the ELF header of some random executable, we'll see an Entry point address: $ readelf -h /usr/bin/uptime ELF Header: Magic: 7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 Class: ELF32 [...] some header fields Entry point address: 0x8048470 [...] some header fields This is placed by the linker to tell the OS where to start executing the executable's code . And indeed if we then load the executable with GDB and examine the address 0x8048470, we'll see the first instructions of the executable's .text segment there. What this means is that the linker, when linking the executable, can fully resolve all internal symbol references (to functions and data) to fixed and final locations. The linker does some relocations of its own , but eventually the output it produces contains no additional relocations. Or does it? Note that I emphasized the word internal in the previous paragraph. As long as the executable needs no shared libraries , it needs no relocations. But if it does use shared libraries (as do the vast majority of Linux applications), symbols taken from these shared libraries need to be relocated, because of how shared libraries are loaded. Loading shared libraries Unlike executables, when shared libraries are being built, the linker can't assume a known load address for their code. The reason for ...