.so vs. .dylib on macOS: What\'s the Difference?

Understanding the Differences: .so vs. .dylib on macOS

Shared code and dynamically loadable modules are both fundamental components in software development. On macOS, these entities are represented by .so and .dylib files, respectively. Yet, the distinction between the two file types can be obscure.

Conceptual Differences

Mach-O (Mach Object) is the underlying file format for both .so and .dylib files. However, they belong to different Mach-O object file types. .dylib files are classified as shared libraries (MH_DYLIB), while .so files belong to the category of dynamically loaded modules (MH_BUNDLE).

When to Use Each Type

Shared Libraries (.dylib):

• Can be linked against directly via static linker flags (e.g., -lfoo)
• Typically used for libraries that are part of the system or commonly used by multiple applications

Dynamically Loaded Modules (.so):

• Cannot be linked against like shared libraries
• Commonly employed for plugins or extensions that dynamically extend the functionality of an existing application

Compilation Techniques

In macOS, the appropriate compiler flags for creating a shared library are:

-dynamiclib

To generate a loadable module, use these flags instead:

-bundle

Unlike in Linux, where shared code can be used for both libraries and dynamic loading, macOS strictly separates these functions.

Historical Evolution

Historically, the distinction between .so and .dylib files was more pronounced. In early versions of macOS, dylibs could only be loaded through specific APIs. Support for dlopen with dylibs was introduced gradually over several OS versions.

Additional Considerations

• On macOS, "bundle" also refers to a standardized directory structure for executable code and resources.
• Frameworks are another type of bundle used to package related code and resources in macOS.

References:

• [Fink Porting Guide](https://sourceware.org/fink/doc/porting/porting-guide.html)
• [man ld(1)](https://developer.apple.com/documentation/xcode/ld/1/reference/argument-list)
• [man dlopen(3)](https://developer.apple.com/documentation/xcode/core-foundation/1214327-dlopen)
• [Dynamic Library Programming Topics](https://developer.apple.com/documentation/macos/dynamic_library_programming_topics/managing_object_files_for_dynamic_libraries)
• [Mach-O Programming Topics](https://developer.apple.com/documentation/xcode/mach-o_programming_topics/

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