dylib

A dylib is a dynamic shared library format used on Apple platforms such as macOS and iOS. It contains code and data intended to be loaded into one or more processes at run time, enabling multiple programs to share common functionality without embedding it. Dylibs typically have the .dylib extension, though system libraries on macOS are Mach-O binaries that may be packaged differently.

On macOS, dylibs are Mach-O executables that are loaded by the dynamic linker, dyld. Each dylib carries an install_name that identifies its own location, and executables declare dependencies via Mach-O load commands. At load time, dyld resolves symbols, often lazily, and uses search paths such as @rpath, @loader_path, and @executable_path to locate libraries. Dylibs are built as position-independent code to allow flexible mapping in memory.

Versioning and compatibility are managed through version metadata, including a current_version and a compatibility_version, which communicate ABI stability. This enables applications to link against a range of dylib versions without recompilation, provided the ABI remains compatible. The install_name and path search settings influence how libraries are found and updated.

Usage and deployment: applications may link against dylibs at build time or load them at run time using APIs like dlopen and dlsym. Apple packaging such as frameworks bundles dylibs with resources. Modern macOS also imposes code signing and Library Validation requirements, which affect loading dylibs from non-default or untrusted locations.

Comparison: dylibs are dynamic libraries, distinct from static libraries that are embedded into executables at link time. Similar concepts exist on other platforms, where shared libraries use extensions such as .so (Linux) or .dll (Windows).