LCLOADDYLIB

LC_LOAD_DYLIB is a Mach-O load command used by macOS and iOS binaries to declare a dependency on a dynamic library (dylib). It is part of the dynamic linking mechanism in the Mach-O format and is read by the dynamic linker, dyld, at launch time to locate and load required shared libraries.

Structure and location: An LC_LOAD_DYLIB entry comprises a load command header (cmd and cmdsize) followed by a dylib structure. The dylib record contains a name offset, version numbers, and a library path. The actual pathname is a C string located after the header, and its position is determined by the name offset. The library path can be an absolute path or use install-name conventions such as @rpath, @loader_path, or @executable_path. The dylib’s own install name is used by other binaries that link to it.

Purpose and behavior: During program startup, dyld parses all LC_LOAD_DYLIB commands to build the dependency graph for the executable. It searches for each listed library, loads it, and resolves symbols before the main image runs. The current_version and compatibility_version fields indicate version constraints that affect whether the library is considered compatible. If a required library cannot be found or is incompatible, the executable may fail to launch. There is a related mechanism, LC_LOAD_WEAK_DYLIB, which allows weak linking and may permit continued execution with null pointers if a library is unavailable.

Tools and manipulation: The command’s effect can be inspected with tools such as otool -L, which lists the dynamic libraries a binary uses. The install_name_tool utility can modify LC_LOAD_DYLIB entries to relocate or rename dependencies. Runtime search paths can be influenced by environment variables like DYLD_LIBRARY_PATH and related DYLD_PATHs.

See also: dyld, LC_LOAD_WEAK_DYLIB, install_name_tool, @rpath, @loader_path.