.so vs. .dylib on macOS: When to Use Which Shared Library Type?

Differences Between .so and .dylib on macOS

In the realm of shared libraries and dynamic loading on macOS, the use of .so and .dylib file extensions can be confusing. This article aims to clarify the distinctions between these two file formats and guide their appropriate use.

Conceptual Differences

macOS employs two types of files for shared code: .dylib (Mach-O shared libraries) and .so (bundles). Shared libraries are linked statically using flags like -lfoo for libfoo.dylib. On the other hand, bundles, also known as loadable modules, have the file type MH_BUNDLE and extension .bundle (or .so for compatibility). They are typically used for plug-ins that extend applications.

When to Use One Over the Other

In general, use .dylib when linking shared code that will be dynamically loaded into a program (e.g., libraries). For plug-ins that extend an application, .so (bundles) is the preferred choice.

Compilation Tips

To create a .dylib shared library, use the -dynamiclib flag to the compiler. For .so bundles, use the -bundle flag. It's important to note that pre-compiled .so files from other platforms may not work on macOS, as bundles require a specific bundle structure.

Historical Context

In early versions of macOS, bundles were introduced before dylibs for dynamic loading. Later, dlopen support was added to dylibs, providing equivalent functionality.

Contrast with ELF Systems

On Linux-based systems using ELF (Executable and Linkable Format), libraries and dynamically loaded code share the same file format. However, on macOS, the use of separate file formats for these purposes creates a distinction between shared libraries (.dylib) and bundles (.so).

Conclusion

Understanding the differences between .so and .dylib helps navigate the complexities of managing shared code on macOS. By following the guidelines provided, developers can leverage these file formats effectively to create and use dynamic libraries and plug-ins on the platform.

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