DLL and LIB files - what and why

Question

I know very little about DLL s and LIB s other than that they contain vital code required for a program to run properly - libraries  But why do compilers generate them at all  Wouldn t it be easier to just include all the code in a single executable  And what s the difference between DLL s and LIB s

User · Answer

A DLL is a library of functions that are shared among other executable programs. Just look in your windows/system32 directory and you will find dozens of them. When your program creates a DLL it also normally creates a lib file so that the application *.exe program can resolve symbols that are declared in the DLL.

A .lib is a library of functions that are statically linked to a program -- they are NOT shared by other programs. Each program that links with a *.lib file has all the code in that file. If you have two programs A.exe and B.exe that link with C.lib then each A and B will both contain the code in C.lib.

How you create DLLs and libs depend on the compiler you use. Each compiler does it differently.

User · Answer

One important reason for creating a DLL LIB rather than just compiling the code into an executable is reuse and relocation   The average Java or  NET application  for example  will most likely use several 3rd party  or framework  libraries   It is much easier and faster to just compile against a pre-built library  rather than having to compile all of the 3rd party code into your application   Compiling your code into libraries also encourages good design practices  e g  designing your classes to be used in different types of applications

User · Answer

There are static libraries  LIB  and dynamic libraries  DLL  - but note that  LIB files can be either static libraries  containing object files  or import libraries  containing symbols to allow the linker to link to a DLL    Libraries are used because you may have code that you want to use in many programs  For example if you write a function that counts the number of characters in a string  that function will be useful in lots of programs  Once you get that function working correctly you don t want to have to recompile the code every time you use it  so you put the executable code for that function in a library  and the linker can extract and insert the compiled code into your program  Static libraries are sometimes called  archives  for this reason    Dynamic libraries take this one step further  It seems wasteful to have multiple copies of the library functions taking up space in each of the programs  Why can t they all share one copy of the function  This is what dynamic libraries are for  Rather than building the library code into your program when it is compiled  it can be run by mapping it into your program as it is loaded into memory  Multiple programs running at the same time that use the same functions can all share one copy  saving memory  In fact  you can load dynamic libraries only as needed  depending on the path through your code  No point in having the printer routines taking up memory if you aren t doing any printing  On the other hand  this means you have to have a copy of the dynamic library installed on every machine your program runs on  This creates its own set of problems   As an example  almost every program written in  C  will need functions from a library called the  C runtime library  though few programs will need all of the functions  The C runtime comes in both static and dynamic versions  so you can determine which version your program uses depending on particular needs

User · Answer

One other difference lies in the performance   As the DLL is loaded at runtime by the  exe s   the  exe s  and the DLL work with shared memory concept and hence the performance is low relatively to static linking   On the other hand  a  lib is code that is linked statically at compile time into every process that requests  Hence the  exe s  will have single memory  thus increasing the performance of the process

User · Answer

Another aspect is security  obfuscation   Once a piece of code is extracted from the main application and put in a  separated  Dynamic-Link Library  it is easier to attack  analyse  reverse-engineer  the code  since it has been isolated  When the same piece of code is kept in a LIB Library  it is part of the compiled  linked  target application  and this thus harder to isolate  differentiate  that piece of code from the rest of the target binaries

[dll] DLL and LIB files - what and why?

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