dyld

dyld is Apple's dynamic linker, the component responsible for loading and linking dynamic libraries for programs on Darwin-based systems such as macOS and iOS. It works with the Mach-O binary format to bring together the executable and its dependent libraries at runtime, preparing the process for execution.

During process startup, dyld locates the required dynamic libraries listed in the executable’s load commands, loads them into memory, and performs symbol resolution and relocation. It supports both eager binding, where symbols are resolved up front, and lazy binding, where resolution is deferred until a symbol is first referenced. This process enables applications to share code efficiently and to load libraries only as needed.

A key performance feature of dyld is the use of a shared cache of system libraries, which speeds startup by reducing the amount of work required to map and relocate common components. This dyld shared cache is designed to improve launch times and memory efficiency across many processes.

Security and system integrity considerations influence how dyld operates. macOS and iOS impose restrictions on dynamic library injection and the use of certain environment variables for loading libraries, especially under System Integrity Protection and code-signing requirements. Developers can influence library loading with environment variables such as path searches and run-time search paths, but these capabilities are tightly controlled in modern systems.

Over time, dyld has evolved through multiple generations (often referred to as dyld1, dyld2, and dyld3), incorporating changes to caching, binding strategies, and load commands to improve performance, security, and compatibility across Apple platforms.