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(python2.1-ext.info)Differences Between UNIX and Windows

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Differences Between UNIX and Windows
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This manual section was written by Chris Phoenix <cphoenix@best.com>.
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.