New compressed LINKEDIT Segment Format

The traditional format of the LINKEDIT segment is modeled on the symbol table entries and relocations created by the assembler when making object files. This format is not ideal for the dynamic loader to process at runtime: It is large and has little locality. OS X v10.6 introduces a new format for the data the dynamic loader needs from the LINKEDIT segment. A process can contain Mach-O images in both the new format or traditional format. The dynamic loader allows intermixing of the formats, but uses the new format when available.

The new format is described by the LC_DYLD_INFO load command. It has five chunks: rebasing, binding, weak binding, lazy binding, and exported symbols. The format is described in <mach-o/loader.h>. The command-line tool dyldinfo(1) can be used to display that information.

When building a product targeted at OS X v10.6, the linker, ld, creates Macho-O images that contain only the new LINKEDIT format; these programs do not run on OS X v10.5 and earlier. When targeting OS X v10.5, the linker creates images that contain both the new and traditional LINKEDIT segment format; this practice allows programs to run on OS X v10.5 and run very efficiently on OS X v10.6. (When targeting OS X v10.4 or earlier, the linker produces only the traditional format.)

Faster Weak-symbol Coalescing

C++ uses weak definitions to solve a number of issues. But if weak symbols are not hidden, the dynamic loader needs to make them unique at runtime. The previous algorithm for doing that was expensive. For every weak symbol in each image, the dynamic loader would look up that symbol in every other image. In OS X v10.6 the algorithm is more efficient: It assumes that weak-symbol duplicates are rare. First, all symbols are bound by their normal two-level namespace binding. Then there is as a separate pass that takes an alphabetized list of weak symbols from each image and iterates through just those weak symbols looking for duplicates. The dynamic loader updates only the duplicates. The new LINKEDIT segment format is optimized for this algorithm.

New dlsym Handle

There is a new pseudo handle you can pass to dlsym called RTLD_MAIN_ONLY. It tells the dynamic loader to search for the symbol only in the main executable.

Change to update_dyld_shared_cache Command-line Tool

This tool creates the dynamic loader shared cache, which improves launch performance. In OS X v10.5, the dynamic loader would automatically start it if it appeared that the shared cache was outdated. In OS X v10.6, update_dyld_shared_cache(1) is not automatically run. The Installer runs it when you first install the operating system; Software Update runs it when it updates the operating system. If for some reason you swap out an system dynamic library, you should manually run update_dyld_shared_cache.

Restarting the computer in Safe Boot mode deletes the dynamic-loader shared cache.

If you suspect the shared cache is somehow corrupt, you can run:

sudo update_dyld_shared_cache -verify

which re-creates the cache and compares the result with your actual cache file.

The launch time of Cocoa programs has been improved by caching some Objective-C runtime fix ups in the shared cache. Thus the Objective-C runtime has less work to do during launch.