File extension

.dylib

Mach-O dynamic library

.dylib files are dynamic libraries for macOS and other Apple platforms, stored in the Mach-O format and loaded by the dynamic linker (dyld).

Known meanings

Formats and reference material for .dylib

Mach-O dynamic library

Apple dynamic shared library in Mach-O format, loaded by dyld

  • binaries
  • Apple Inc.
Open link

Details

Format notes

DYLIB (Dynamic Library)

A .dylib is a dynamically linked library on macOS, iOS, and related Apple platforms. It plays the same role as .so on Linux and .dll on Windows: executable code that several programs can share and that is bound to them when they are launched or by explicit loading (dlopen). Related Apple containers include frameworks (.framework bundles that contain a dylib together with headers and resources) and plug-in bundles (.bundle).

Structure

A dylib is a Mach-O file whose file type is MH_DYLIB. A Mach-O file starts with a magic number: 0xFEEDFACF for 64-bit and 0xFEEDFACE for 32-bit images (stored in the byte order of the file). A header is followed by load commands describing segments, symbol tables, and dependencies. A dylib carries an “install name” recorded in the file, which is the path that programs linking against it will use to find it, often written with @rpath, @executable_path, or @loader_path prefixes. Universal (“fat”) files wrap several architecture slices, for instance Intel and Apple silicon, in one file with a different header magic.

Tooling

Apple’s otool and install_name_tool, as well as lipo for universal files, are used to inspect and modify dylibs. Recent macOS versions keep many system libraries in a shared cache instead of individual files.

Preservation And Security Notes

Binaries are tied to an architecture and operating system version. On current macOS, code signing and notarization apply, and modifying a signed dylib invalidates its signature. Treat unknown dylibs as executable code.

Further Reading

  • Overview of Dynamic Libraries (Apple): https://developer.apple.com/library/archive/documentation/DeveloperTools/Conceptual/DynamicLibraries/100-Articles/OverviewOfDynamicLibraries.html
  • Mach-O loader header (Apple open source): https://github.com/apple-oss-distributions/xnu/blob/main/EXTERNAL_HEADERS/mach-o/loader.h