UNIX for Dummies Questions & Answers

Hello

Please what does mean shared static library and LD-Preload?



Thank you

There are static libraries - files usually in the /lib directory that end with .a.
There are shared libraries - files usually in the /lib directory that end with .so (or .sl), sometimes called dynamic libraries.

To link means 'Resolve external symbols or functions in your code like printf()'

By default, when you compile C code, you liink against a set of shared libraries.
You have to go out of your way to create an executable that is linked against a static library. Read your compiler documentation on how to do this.

There are a few valid uses for linking statically. Linking against shared libraries, also called dynamic linking, creates an executable that makes much better use of system resources, because eveybody else's executable file shares the same library with your code. MUCH less memory usage. That is why dynamic linking is better.

Got all that?

Next: LD_PRELOAD

This is an envrionment variable that anyone can set. If changes how the UNIX system looks for dynamic libraries. When you first run an executable, you have to find and open the dynamic libraries you linked against so all of the symbols like printf() will be there to use.

You use LD_PRELOAD to force the system to kind of auto-magically link against a library that was NOT one of the original linked libraries. So now the printf() function calls in your executable employ a different library with different code to execute printf.

There are limitations on LD_PRELOAD, to prevent bad guys from doing bad things, but that is beyond the scope of your question.

Thank you for answer
i have a library libhello.so which contains this function
i have a program named prog.c, his executable is prog
in the source code of prog i have not called the fuction hello
i proced like that:
--> not display the message hello world
If it is not possible to make this connection, is there another way to display the message of the function hello?
Thank you

If you have not called hello() something within prog.c, then "hello world" is never going to be outputted. No reason for it to be outputted. Setting LD_PRELOAD will not change that fact.