How to create shared object file on linux ?

Dynamically Linked "Shared Object" Libraries: (.so)

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How to generate a shared object: (Dynamically linked object library file.) Note that this is a two step process.


1. Create object code
2. Create library
3. Optional: create default version using a symbolic link.

Library creation example:

gcc -Wall -fPIC -c *.c
gcc -shared -Wl,-soname,libctest.so.1 -o libctest.so.1.0 *.o
mv libctest.so.1.0 /opt/lib
ln -sf /opt/lib/libctest.so.1.0 /opt/lib/libctest.so
ln -sf /opt/lib/libctest.so.1.0 /opt/lib/libctest.so.1


This creates the library libctest.so.1.0 and symbolic links to it.

Compiler options:

* -Wall: include warnings. See man page for warnings specified.
* -fPIC: Compiler directive to output position independent code, a characteristic required by shared libraries. Also see "-fpic".
* -shared: Produce a shared object which can then be linked with other objects to form an executable.
* -W1: Pass options to linker.
In this example the options to be passed on to the linker are: "-soname libctest.so.1". The name passed with the "-o" option is passed to gcc.
* Option -o: Output of operation. In this case the name of the shared object to be output will be "libctest.so.1.0"

Library Links:

* The link to /opt/lib/libctest.so allows the naming convention for the compile flag -lctest to work.
* The link to /opt/lib/libctest.so.1 allows the run time binding to work.

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Compile main program and link with shared object library:

Compiling for runtime linking with a dynamically linked libctest.so.1.0:

gcc -Wall -I/path/to/include-files -L/path/to/libraries prog.c -lctest -o prog
Use:
gcc -Wall -L/opt/lib prog.c -lctest -o prog


Where the name of the library is libctest.so. (This is why you must create the symbolic links or you will get the error "/usr/bin/ld: cannot find -lctest".)
The libraries will NOT be included in the executable but will be dynamically linked during runtime execution.

List Dependencies:

The shared library dependencies of the executable can be listed with the command: ldd name-of-executable

Example: ldd prog

libctest.so.1 => /opt/lib/libctest.so.1 (0x00002aaaaaaac000)
libc.so.6 => /lib64/tls/libc.so.6 (0x0000003aa4e00000)
/lib64/ld-linux-x86-64.so.2 (0x0000003aa4c00000)


Run Program:

* Set path: export LD_LIBRARY_PATH=/opt/lib:$LD_LIBRARY_PATH
* Run: prog
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Library Info(global sysmbols) using nm command :
[root@kmaiti assembly_test]# nm -D /lib64/libcap.so.2
w _Jv_RegisterClasses
0000003d4e0038b0 A __bss_start
U __ctype_b_loc
U __ctype_tolower_loc
w __cxa_finalize
U __errno_location
U __fxstat
w __gmon_start__
U __lxstat
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