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CS 0449 – Shared Object Lab
Background
A Shared Object is an executable file that acts as a shared library of functions. Dynamic linking provides a
way for a process to call a function that is not part of its executable code. The executable code for the
function is located in a .so file, which contains one or more functions that are compiled, linked, and
stored separately from the processes that use them. Shared objects also facilitate the sharing of data
and resources. Multiple applications can simultaneously access the contents of a single copy of a shared
object in memory.
Dynamic linking differs from static linking in that it allows an executable file to include only the
information needed at run time to locate the executable code for a shared object function. In static
linking, the linker gets all of the referenced functions from the static link library and places it with your
code into your executable.
Procedure
1. Login via SSH to unixs.cis.pitt.edu
2. Create a directory in your private directory (mkdir private/lab3). Change directories to it.
3. The first step is to make a library. Let us imagine we are writing our own string library. One
function that we might want to provide is string copy. Edit a file in your favorite text editor, or
pico like:
pico mystr.c
4. Now enter the code necessary to make a string copy function:
void my_strcpy(char *dest, char *src)
{
while(*dest++ = *src++);
}
5. Save the file
6. We now need to compile this into a library. The first step is to invoke the compiler
independently of the linker. We can do this by providing the -c option to gcc. Also, since we do
not know the address in memory where the shared object will be loaded, we need to tell gcc to
create position-independent code via the -fPIC option:
