.so vs. .dylib in macOS: What are the Key Differences and When Should I Use Each?

Clarifying the Differences between .so and .dylib File Extensions

Overview:

In macOS, dynamic libraries can be packaged either as .so shared objects or .dylib dynamic loadable modules. Understanding the differences between these two formats is crucial for effective library management.

Conceptual Differences:

• Object Types: .dylib files are Mach-O shared libraries (MH_DYLIB), while .so files are loadable modules (MH_BUNDLE) that typically serve as plug-ins or extend applications.
• Linking: .dylib files can be linked statically using regular linking flags, while .so bundles cannot be linked in this manner.

Choosing between .so and .dylib:

• Use .dylib: For shared libraries that are not intended for dynamic loading, such as system libraries or code shared between multiple applications.
• Use .so: For dynamically loaded plug-ins or application extensions that need to access the application's API.

Compilation:

• .dylib Shared Libraries: Use the -dynamiclib flag with the compiler.
• .so Loadable Bundles: Use the -bundle flag with the compiler.

Support and History:

• Dynamic Loading: .dylib was initially incompatible with dynamic loading, but support was added in macOS 10.4. Both file formats now support dlopen, dlclose, and other DL functionality.
• Legacy: Historically, the distinctions between .so and .dylib were more significant. .so bundles were used exclusively for dynamic loading, while .dylib shared libraries could only be linked statically. These limitations have been lifted over time, making the choice between file formats more nuanced.

Additional Information:

• In macOS, "bundle" can also refer to directories with a specific structure containing executable code and resources. These directories should not be confused with Mach-O bundles used for libraries and plug-ins.

References:

• Fink Porting Guide
• ld(1) and dlopen(3)
• Dynamic Library Programming Topics
• Mach-O Programming Topics

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