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What’s a DSO?

by sonthonax 5y ago

What’s a DSO?

mjw1007 5y ago
Dynamic Shared Object (in this context, probably a .so file).
- sonthonax 5y ago
  Ah!
wahern 5y ago
Dynamic shared object, or what's usually understood generically (in non-language specific terms) as a [binary] module or in GUI apps a plugin--something loaded and linked dynamically by the program, not by the linker at compile time or at program startup. In the Unix/C ecosystem "DSO" is a common term to describe binary modules, without reference to specific binary formats like ELF or mach-O. Especially in the land of Unix where dynamic linking has become very sophisticated and automated, many of these terms are losing their distinctive meanings as the various ways to link and load converge on the same underlying mechanisms which you can mix-and-match independently. But normally the difference between a DSO and a shared library is that shared library symbols (functions, global variables) are by default visible globally (at least on ELF; mach-O story is more complex), making them implicitly visible to subsequently loaded code, whether a shared library or DSO. On the other hand, you usually want all DSO symbols to remain private to the DSO by default, requiring the application at runtime to explicitly request a pointer to specific symbols as different DSOs might have symbols of the same name (especially true for plugins), leaving the application logic to decide which to use and when. Also, especially pertinent to Lua, Perl, Python, Ruby, and similar languages, a DSO usually implicitly relies on the main program to have loaded and exported certain core APIs, rather than the DSO explicitly linking to a shared library to provide them. (This still often requires special compile-time flags when building the DSO, and possibly also special flags to the main binary.) That ensures the VM/interpreter/engine and all DSOs are always calling into the same core runtime implementation, and ensures the main binary controls which that is. DSOs do often dynamically link to other shared libraries, but because of default symbol visibility semantics this can cause problems, such as two DSOs or a DSO and the main program compiled against different releases of OpenSSL with incompatible ABIs. (If you can manage to load two different version of OpenSSL, you have the reverse issue of accidentally passing objects between the two implementations.) Mitigations and resolutions for that problem differ between ELF-based and mach-O-based systems. (AIX is an outlier, which like Windows uses a PE-derived binary format that is significantly different than ELF or mach-O. Many of the semantic distinctions and jargon that developed around ELF don't make much sense in the context of PE.)

What’s a DSO?

mjw1007 5y ago

Dynamic Shared Object (in this context, probably a .so file).

sonthonax 5y ago
Ah!

wahern 5y ago

Dynamic shared object, or what's usually understood generically (in non-language specific terms) as a [binary] module or in GUI apps a plugin--something loaded and linked dynamically by the program, not by the linker at compile time or at program startup. In the Unix/C ecosystem "DSO" is a common term to describe binary modules, without reference to specific binary formats like ELF or mach-O. Especially in the land of Unix where dynamic linking has become very sophisticated and automated, many of these terms are losing their distinctive meanings as the various ways to link and load converge on the same underlying mechanisms which you can mix-and-match independently. But normally the difference between a DSO and a shared library is that shared library symbols (functions, global variables) are by default visible globally (at least on ELF; mach-O story is more complex), making them implicitly visible to subsequently loaded code, whether a shared library or DSO. On the other hand, you usually want all DSO symbols to remain private to the DSO by default, requiring the application at runtime to explicitly request a pointer to specific symbols as different DSOs might have symbols of the same name (especially true for plugins), leaving the application logic to decide which to use and when. Also, especially pertinent to Lua, Perl, Python, Ruby, and similar languages, a DSO usually implicitly relies on the main program to have loaded and exported certain core APIs, rather than the DSO explicitly linking to a shared library to provide them. (This still often requires special compile-time flags when building the DSO, and possibly also special flags to the main binary.) That ensures the VM/interpreter/engine and all DSOs are always calling into the same core runtime implementation, and ensures the main binary controls which that is. DSOs do often dynamically link to other shared libraries, but because of default symbol visibility semantics this can cause problems, such as two DSOs or a DSO and the main program compiled against different releases of OpenSSL with incompatible ABIs. (If you can manage to load two different version of OpenSSL, you have the reverse issue of accidentally passing objects between the two implementations.) Mitigations and resolutions for that problem differ between ELF-based and mach-O-based systems. (AIX is an outlier, which like Windows uses a PE-derived binary format that is significantly different than ELF or mach-O. Many of the semantic distinctions and jargon that developed around ELF don't make much sense in the context of PE.)