Question

Dynamic and static linking.

Asked by: shajithchandran

hi,
I want to creat dll in linux.

Suppose that one C code file has a function (eg square()).
I want to use this code through linking.

Now when i write some application i will do this

#include<square.h>

int main()
{

square();
...
}

That is i want to learn how to do dynamic and static linking. I have no prior knowledge so please explain in detail.
 I want to know every thing which is required to do this.

Thanks
shaj

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Asked On
2003-08-26 at 21:58:10ID20721083
Topic

C Programming Language

Participating Experts
5
Points
500
Comments
13

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Answers

 

by: sunnycoderPosted on 2003-08-26 at 22:26:51ID: 9229924

 

by: pankajtiwaryPosted on 2003-08-27 at 01:25:11ID: 9230504

Well dlls in windows are equivalent to your so (shared objects) in linux. Static objects have .o extensions. To produce the statically linked library (or rather object) you can compile the .c file (which contains the definition of square()) with a -c flag.

prompt> cc -c square.c

This will produce an statically linked library file square.o

now compile the main file(lats say main.c) the same way.

cc -c main.c

which will produce main.o. Now you can link the two object files (main.o and square.o) to produce the executable binary. Or alternatively you can directly do

cc main.c square.o to produce the a.out executable.

For a full reading...you can refer http://www-106.ibm.com/developerworks/library/l-shobj/

 

by: reckPosted on 2003-08-27 at 07:51:00ID: 9232875

First of all, dll's are called so file under linux. These are dynamic libraries. The static libraries are called .a files.

I'll explain creating and using .a files first since this is easier.
.a files are actually a collection of object (.o) files.

Suppose you have a .c file which contains the square() routine. You want to make a static library containing this routine.
You just do:

gcc -c square.c                  --> This will give you an object file (square.o)

To create your static library, you do:

ar -r square.a square.o            --> This will give you a static library file (square.a)

The -r option says to insert new files into the archive, which is square.a.

Suppose you want to use the routine in anotherfile, say test.c, you don't need to include the square.h file.

First you compile the c file:

gcc -c test.c

Then you link the file, with the static library

gcc -o test test.o square.a

This is all there is to creating and using static libraries...

Creating and using .so files is a bit more complicated.

First you compile your source files:

gcc -c test.c                  --> This gives you the object file test.o
gcc -c square.c                  --> This gives you the object file square.o

Then you create your dynamic library:

gcc -shared -o libsquare.so square.o      --> This gives you the dynamic library libsquare.so

To use it you don't include the .h file, but you link your program against your library:

gcc -o test test.o -L. -lsquare

The -o option specifies the output file (executable).
The -L option specifies the path to the shared library (In this case the current directory)
The -l option specifies the shared library (the lib and .so are added to square to get libsquare.so)

Now when you execute your program, you may receive the message

error while loading shared libraries, cannot open shared object file: No such file or directory.

This is because the linker cannot find the library. You can either use the environment variable LD_LIBRARY_PATH:

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/path/to/shared/object/file      --> This adds the path to the variable

Or you can put the path in /etc/ld.so.conf and run ldconfig.

I hope this is of assistance to you...

 

by: 1cePosted on 2003-08-27 at 13:30:40ID: 9235560

reck: neat info, can you talk abit about how to go about loading the .so in a program also? like the equivalent of LoadLibrary and GetProcAddress

 

by: grg99Posted on 2003-08-27 at 14:19:23ID: 9235894

Before you get all excited about .so files and the like, pls thing about the downside:

The user has to have the .so file
It has to be the correct version of the .so file.
It can't be just any xxx.so file with the same name, it has to be the matching one.
They have to set their search path to see the .so file.

Any common run-time (like C library) .so files have to match the version that your
code was compiled with, and they have to match the OS API version.
Sometimes you luck out and their system has the proper one. Sometimes not.


------

These arent insuperable roadblocks, but they *often* slow down your users,
atking an hour or two out of their day before they corral all the right .so files,
set the right paths, repeat this for all their logins.

At the very least, there will be a lot of cursing.

----

So you might want to distribute your program as one pre-linked binary.
I know, not very sophisticated, but at least the program has a chance of running
right off the bat, without a lot of .so file chasing.

 

by: shajithchandranPosted on 2003-08-27 at 22:12:27ID: 9237795

hi reck,
Really good info.

>>To use it you don't include the .h file
Why is .h not required. We use printf function for that we have to use stdio.h otherwise it will give undefined  symbol.
Is libc a static or dynamic lib?

What should be done so that we have to include square.h.
How to  u can add more fucntion to the same library.
that today i creat a library for square function.
Tommorrow i want to have squareroot also in the same library. How to do that.

hi grg99
>>It has to be the correct version of the .so file.
What is this version problem. Can u explain a little more on it. Can u tell me the internal of it. How is the version matched. Do the exe file has it. Who does the matching.

now when i run a.out which has links to some library function what happens . How is it get loaded. Can explain in steps in detail.

In short i want to know how a exe file is run. In and out about it.

 

by: shajithchandranPosted on 2003-08-27 at 22:17:20ID: 9237810

will ar -r will add new file to the existing lib?

 

by: shajithchandranPosted on 2003-08-27 at 22:26:53ID: 9237849

yes i able to add more fucntions to lib by using ar -r.
Now let me try dynamic lib.

 

by: shajithchandranPosted on 2003-08-27 at 22:46:16ID: 9237910

Yes even dynamic lib is working properly. Reck u r absolutly correct.
What abt .h problem. Why is stdio.h requried for printf while square.h is not required??
what is the difference b/w libc and our lib.

 

by: shajithchandranPosted on 2003-08-27 at 22:54:48ID: 9237938

Can u put some light on these things also
ldconfig ,dlopen and dlsym.

 

by: shajithchandranPosted on 2003-09-01 at 01:27:00ID: 9260310

No one to answer!!!

 

by: sunnycoderPosted on 2003-09-01 at 01:30:40ID: 9260327

      ldconfig creates the necessary links and cache (for use by the run-time linker, ld.so) to the most recent shared libraries found in the directo­ries specified on the command line, in the file /etc/ld.so.conf, and in the trusted directories (/usr/lib and /lib).  ldconfig checks the header and  file names of the libraries it encounters when determining which versions should have their links updated.  ldconfig ignores symbolic links when scanning for libraries.

for more info

man 8 ldconfig

man 3 dlopen

man 3 dlsym

 

by: reckPosted on 2003-09-01 at 02:45:46ID: 9260623

Sorry bout not replying, but I was working on a deadline...I must read some docs, cause I don't know of the top of my head why you include the .h file for stdio.h, but not for your own library...

20120131-EE-VQP-002

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