4.2 Differences Between Unix and Windows
Unix and Windows use completely different
paradigms for run-time loading of code. Before you try to build a module that can be
dynamically loaded, be aware of how your system works.
In Unix, a shared object (.so)
file contains code to be used by the program, and also the names of functions and data that it
expects to find in the program. When the file is joined to the program, all references to
those functions and data in the file's code are changed to point to the actual locations in
the program where the functions and data are placed in memory. This is basically a link
operation.
In Windows, a dynamic-link library (.dll) file has no dangling
references. Instead, an access to functions or data goes through a lookup table. So the DLL
code does not have to be fixed up at runtime to refer to the program's memory; instead, the
code already uses the DLL's lookup table, and the lookup table is modified at runtime to point
to the functions and data.
In Unix, there is only one type of library
file (.a) which contains code from several object files (.o). During the link step to create a shared object file (.so), the linker may find that it doesn't know where an identifier is
defined. The linker will look for it in the object files in the libraries; if it finds it, it
will include all the code from that object file.
In Windows, there are two types of library, a static library and an import library (both
called .lib). A static library is like a Unix .a file; it contains
code to be included as necessary. An import library is basically used only to reassure the
linker that a certain identifier is legal, and will be present in the program when the DLL is
loaded. So the linker uses the information from the import library to build the lookup table
for using identifiers that are not included in the DLL. When an application or a DLL is
linked, an import library may be generated, which will need to be used for all future DLLs
that depend on the symbols in the application or DLL.
Suppose you are building two dynamic-load modules, B and C, which should share another
block of code A. On Unix, you would not
pass A.a to the linker for B.so and C.so; that would cause it to be included twice, so that B and C would each
have their own copy. In Windows, building A.dll will also build A.lib. You do pass A.lib to the linker
for B and C. A.lib does not contain code; it just contains
information which will be used at runtime to access A's code.
In Windows, using an import library is sort of like using "import
spam"; it gives you access to spam's names, but does not create a separate copy. On Unix, linking with a library is more like "from spam import *"; it does create a separate copy.
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