.so File — Shared Object Library

SO (Shared Object) files are the Linux/Unix equivalent of Windows DLLs — dynamic libraries containing compiled code that can be loaded and linked by multiple programs at runtime. Shared objects use the ELF (Executable and Linkable Format) binary format and are fundamental to how Linux systems work. The C standard library (libc.so), OpenSSL (libssl.so), and virtually every system library exist as shared objects. SO files follow a versioning convention (libfoo.so.1.2.3) with symlinks (libfoo.so → libfoo.so.1 → libfoo.so.1.2.3) for ABI compatibility management. The dynamic linker (ld-linux.so) resolves and loads shared objects when programs start, searching paths defined by LD_LIBRARY_PATH, /etc/ld.so.conf, and RPATH/RUNPATH embedded in executables. SO files export symbols through ELF symbol tables and support lazy binding through the PLT (Procedure Linkage Table) and GOT (Global Offset Table). Shared objects enable code reuse, reduce memory usage through shared memory mappings, and allow library updates without recompiling dependent applications. They are used in plugin architectures, language bindings, and as the compiled output of C, C++, Rust, Go, and other compiled languages on Linux.