Position Independent Code

We all are aware about the concept that the shared library can be used by multiple running processes at same time. But how multiple processes shared a single copy of the shared library?

One way is that,

Assign some customize chunk of the memory to each shared library.

But if you think a little this approach will leads to serious problem. Some of the problem which i can think off is:-

1. Inefficient use of the address space. (Space will be allocated even if we are not using library.)
2. Difficult to manage.(We have to always make sure that none of the space or chunk of memory given to the shared library overlapped).
3. Every time when we do changes inside the library we have to put an extra effort to make shared library fit in its address space. If it does not fit we have to find space for it.
4. When new library is created we have to find a new chunk of memory for it.
5. We cant avoid internal and external fragementation here.

To avoid all these problem modern system use the code Segement. Modern system compile the code system of the shared library or modules so that this can be load and run from anywhere from the memory without modify by the linker . Ofcourse if library is created by us we have to give the path of the place where library is their to the loaders.

With this method single copy of the shared library's code segmment is shared between infinite number of the process.

Note:- Here each process will get its own copy of the read/write data segment

Position Independent Code(PIC) as name suggest is a code which does not depend on position. At these point question arises

Position of What ?

Position of the location of loading the code.

PIC can be defined as a code that can be loaded without worrying about any relocation.

We can use directly use -fPIC option in gcc to create PIC. Shared library must always use this compilation option.

linux>gcc -fPIC source.c -o out        

On x86-64 machine,

their is no special treatment required for the references of symbol in same executable to be PIC.

But reference to the external symbol (function , global variables) requires some special treatment which we will discuss it in next week.