File extension

.so

Shared object (ELF dynamic library)

.so files are shared objects: ELF dynamic libraries used on Linux and most Unix-like systems, loaded by the dynamic linker at program start or on demand.

Known meanings

Formats and reference material for .so

Shared object (ELF dynamic library)

ELF shared library loaded by the dynamic linker on Linux, BSD, and other Unix-like systems

  • binaries
  • Unix System Laboratories / Tool Interface Standards Committee
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Details

Format notes

SO (Shared Object)

A .so file is a shared library for Unix-like systems. Programs that depend on it reference it by name and the dynamic linker (ld.so on Linux) maps it into memory when the program starts, or later through dlopen. Many processes can share the same read-only code pages, and libraries can be updated independently of the programs that use them. The equivalent formats are .dll on Windows and .dylib on macOS.

Structure

A shared object is an ELF file, beginning with the magic bytes 0x7F 'E' 'L' 'F', with file type ET_DYN. The dynamic section lists needed libraries, the library’s own name (SONAME), and the symbol tables used for linking. Library files are usually versioned through the file name, for example libfoo.so.1.2.3, with symbolic links such as libfoo.so.1 (the SONAME) and libfoo.so (used when linking at build time). Android native libraries are also .so files.

Tooling

Common tools for inspecting shared objects are readelf, objdump, nm, and ldd. Search paths are controlled by system configuration and variables such as LD_LIBRARY_PATH.

Preservation And Security Notes

A shared object is built for a specific processor architecture, operating system ABI, and set of dependent libraries; record these when archiving. Environment variables such as LD_PRELOAD and LD_LIBRARY_PATH can cause unintended libraries to be loaded, so they should be handled carefully in privileged programs. Treat unknown .so files as executable code.

Further Reading

  • ELF specification (Tool Interface Standards): https://refspecs.linuxfoundation.org/elf/elf.pdf
  • ld.so manual page: https://man7.org/linux/man-pages/man8/ld.so.8.html