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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 937
CHAP. 11
The implementation of DLLs is simple in concept.
Instead of the compiler
emitting code that calls directly to subroutines in the same executable image, a
level of indirection is introduced: the
Import Address Table
). When an ex-
ecutable is loaded it is searched for the list of DLLs that must also be loaded (this
will be a graph in general, as the listed DLLs will themselves will generally list
other DLLs needed in order to run).
The required DLLs are loaded and the IAT is
filled in for them all.
The reality is more complicated.
Another problem is that the graphs that
represent the relationships between DLLs can contain cycles, or have nondetermin-
istic behaviors, so computing the list of DLLs to load can result in a sequence that
does not work. Also, in Windows the DLL libraries are given a chance to run code
whenever they are loaded into a process, or when a new thread is created.
ally, this is so they can perform initialization, or allocate per-thread storage, but
many DLLs perform a lot of computation in these
routines. If any of the
functions called in an
routine needs to examine the list of loaded DLLs, a
deadlock can occur, hanging the process.
DLLs are used for more than just sharing common code.
They enable a
model for extending applications.
Internet Explorer can download and link to
DLLs called
ActiveX controls
At the other end of the Internet, Web servers also
load dynamic code to produce a better Web experience for the pages they display.
Applications like Microsoft
link and run DLLs to allow
to be used as
a platform for building other applications.
The COM (component object model)
style of programming allows programs to dynamically find and load code written
to provide a particular published interface, which leads to in-process hosting of
DLLs by almost all the applications that use COM.
All this dynamic loading of code has resulted in even greater complexity for
the operating system, as library version management is not just a matter of match-
ing executables to the right versions of the DLLs, but sometimes loading multiple
versions of the same DLL into a process—which Microsoft calls
single program can host two different dynamic code libraries, each of which may
want to load the same Windows library—yet have different version requirements
for that library.
A better solution would be hosting code in separate processes.
But out-of--
process hosting of code results has lower performance, and makes for a more com-
plicated programming model in many cases. Microsoft has yet to develop a good
solution for all of this complexity in user mode.
It makes one yearn for the relative
simplicity of kernel mode.
One of the reasons that kernel mode has less complexity than user mode is that
it supports relatively few extensibility opportunities outside of the device-driver
model. In Windows, system functionality is extended by writing user-mode ser-
vices. This worked well enough for subsystems, and works even better when only
a few new services are being provided rather than a complete operating system per-
There are few functional differences between services implemented in the

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