Shared static library


 
Thread Tools Search this Thread
Top Forums UNIX for Dummies Questions & Answers Shared static library
# 1  
Old 03-26-2012
Shared static library

Hello

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



Thank you
# 2  
Old 03-26-2012
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.
This User Gave Thanks to jim mcnamara For This Post:
# 3  
Old 03-26-2012
Thank you for answer
i have a library libhello.so which contains this function
Quote:
void hello() {printf("hello world");}
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:
Quote:
gcc -shared -ldl -fPIC hello.c libhello.so
LD_PRELOAD=/home/chercheur/libhello.so ./prog
--> 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
# 4  
Old 03-27-2012
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.
This User Gave Thanks to fpmurphy For This Post:
 
Login or Register to Ask a Question

Previous Thread | Next Thread

10 More Discussions You Might Find Interesting

1. AIX

Add shared members from library to same library in a different directory

I'm trying to install libiconv to AIX 7.1 from an rpm off of the perzl site. The rpm appears to install but I get this error message. add shr4.o shared members from /usr/lib/libiconv.a to /opt/freeware/lib/libiconv.a add shr.o shared members from /usr/lib/libiconv.a to ... (5 Replies)
Discussion started by: kneemoe
5 Replies

2. Programming

Shared library with acces to shared memory.

Hello. I am new to this forum and I would like to ask for advice about low level POSIX programming. I have to implement a POSIX compliant C shared library. A file will have some variables and the shared library will have some functions which need those variables. There is one special... (5 Replies)
Discussion started by: iamjag
5 Replies

3. Programming

Even the Static cURL Library Isn't Static

I'm writing a program which uses curl to be run on Linux PCs which will be used by a number of different users. I cannot make the users all install curl on their individual machines, so I have tried to link curl in statically, rather than using libcurl.so. I downloaded the source and created a... (8 Replies)
Discussion started by: BrandonShw
8 Replies

4. Programming

Shared, Static , Dynamic?

if I could compile the same source file as shared/static/dynamic what are the advantages/ disadv of each. PS:by dynamic i am asking about usage of "dlopen". How is it particularly diff from shared libs (2 Replies)
Discussion started by: dragonpoint
2 Replies

5. Programming

Static and Shared Library in Makefile

I am having a devil of a time with a very simple make file. The program needs two shared and one static library. If I link the shared libraries only like below the mysql test app works ... (1 Reply)
Discussion started by: jadsys
1 Replies

6. Linux

Could static library include static library?

I have some static library(libxxx.a libyyy.a). And I want to generate my library(libzzz.a), libzzz.a will use libxxx.a and libyyy.a I wan't my application only use libzzz.a, (means libzzz.a had include libxxx.a, libyyy.a), how can I do that? Thank you. example: I have zzz.c. I do ... (4 Replies)
Discussion started by: freemagic
4 Replies

7. Shell Programming and Scripting

How to change a Makefile from building static library to shared library?

Hi: I have a library that it only offers Makefile for building static library. It built libxxx.a file. How do I in any way build a shared library? (either changin the Makefile or direct script or command to build shared library) Thanks. (1 Reply)
Discussion started by: cpthk
1 Replies

8. Programming

Shared memory for shared library

I am writing a shared library in Linux (but compatible with other UNIXes) and I want to allow multiple instances to share a piece of memory -- 1 byte is enough. What's the "best" way to do this? I want to optimize for speed and portability. Obviously, I'll have to worry about mutual exclusion. (0 Replies)
Discussion started by: otheus
0 Replies

9. UNIX for Advanced & Expert Users

static and shared libraries

can someone explain whether my understanding is correct lets suppose we have a program that uses library x. if x is static then the code of x will be part of our program, so if we're going to have 5 executables of our program, then each executable will have x as part of it. Also, x does not... (2 Replies)
Discussion started by: JamesByars
2 Replies

10. Programming

Shared memory in shared library

I need to create a shared library to access an in memory DB. The DB is not huge, but big enough to make it cumbersome to carry around in every single process using the shared library. Luckily, it is pretty static information, so I don't need to worry much about synchronizing the data between... (12 Replies)
Discussion started by: DreamWarrior
12 Replies
Login or Register to Ask a Question