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# 1  
Old 07-24-2002
Shared Library

hello all
I want to work in shared libraries how can i work in
Linux Environment ?
# 2  
Old 07-24-2002
Lightbulb shared libraries

from "Unix Programming FAQ (v1.37)"

6.3 How to create shared libraries / dlls?
==========================================

The precise method for creating shared libraries varies between different
systems. There are two main parts to the process; firstly the objects to be
included in the shared library must be compiled, usually with options to
indicate that the code is to be position-independent; secondly, these
objects are linked together to form the library.

Here's a trivial example that should illustrate the idea:

/* file shrobj.c */

const char *myfunc()
{
return "Hello World";
}

/* end shrobj.c */

/* file hello.c */

#include <stdio.h>
extern const char *myfunc();

main()
{
printf("%s\n", myfunc());
return 0;
}

/* end hello.c */

$ gcc -fpic -c shrobj.c
$ gcc -shared -o libshared.so shrobj.o
$ gcc hello.c libshared.so
$ ./a.out
Hello World

By far the best method if you want the library and build procedure to be
anything approaching portable is to use GNU Libtool. This is a small suite
of utilities which know about the platform-dependent aspects of building
shared libraries; you can distribute the necessary bits with your program,
so that when the installer configures the package, he or she can decide
what libraries to build. Libtool works fine on systems which do not
support shared libraries. It also knows how to hook into GNU Autoconf and
GNU Automake (if you use those tools to manage your program's build
procedure).

If you don't want to use Libtool, then for compilers other than gcc, you
should change the compiler options as follows:

AIX 3.2 using xlc (unverified)
Drop the `-fpic', and use `-bM:SRE -bE:libshared.exp' instead of
`-shared'. You also need to create a file `libshared.exp' containing
the list of symbols to export, in this case `myfunc'. In addition,
use `-e _nostart' when linking the library (on newer versions of AIX,
I believe this changes to `-bnoentry').

SCO OpenServer 5 using the SCO Development System (unverified)
Shared libraries are only available on OS5 if you compile to ELF
format, which requires the `-belf' option. Use `-Kpic' instead of
`-fpic', and `cc -belf -G' for the link step.

Solaris using SparcWorks compilers
Use `-pic' instead of `-fpic', and use `ld -G' instead of `gcc
-shared'.

(Submission of additional entries for the above table is encouraged.)

Other issues to watch out for:

* AIX and (I believe) Digital Unix don't require the -fpic option,
because all code is position independent.

* AIX normally requires that you create an `export file', which is a list
of symbols to be exported from the shared library. Some versions of the
linker (possibly only the SLHS linker, svld?) have an option to export
all symbols.

* If you want to refer to your shared library using the conventional
`-l' parameter to the linker, you will have to understand how shared
libraries are searched for at runtime on your system. The most common
method is by using the `LD_LIBRARY_PATH' environment variable, but
there is usually an additional option to specify this at link time.

* Most implementations record the expected runtime location of the shared
library internally. Thus, moving a library from one directory to
another may prevent it from working. Many systems have an option to
the linker to specify the expected runtime location (the `-R' linker
option on Solaris, for example, or the `LD_RUN_PATH' environment
variable).

* ELF and a.out implementations may have a linker option `-Bsymbolic'
which causes internal references within the library to be resolved.
Otherwise, on these systems, all symbol resolution is deferred to the
final link, and individual routines in the main program can override
ones in the library.

==========================================
# 3  
Old 08-03-2002
How do I link a shared library against a static library?

Hi,

I have a question about shared library, how do I link it against a static library?

For example.

I need to create a static library mystatic.a, which includes my1.o, which uses a function in a shared library libmyshared.so.

I tried to link:

$ gcc -static -o mystatic.a my1.o libmyshared.so

does it work?

Thanks

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