What is the basic difference between unix and windows
operating systems?
Answer Posted / muthu
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.
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