How Libc shared library loaded in memory and shared amongst processes?

Question

I want to understand how Libc shared library loaded in memory and shared amongst processes? Is there one libc instance loaded in memory and shared amongst all processes or is it each libc instance in memory for each process. I am unclear about how is libc shared between processes.

Thanks Aditya

Answer 1

The one instance of libc is shared among all the processes. See "The Inside Story On Shared Libraries And Dynamic Loading" article:

Specifically, because libraries mostly consist of executable instructions and this code is normally not self-modifying, the operating system can arrange to place library code in read-only memory regions shared among processes (using page-sharing and other virtual memory techniques). So, if hundreds of programs are running and each program includes the same library, the operating system can load a single shared copy of the library’s instructions into physical memory. This reduces memory use and improves system performance.

See also "Dissecting shared libraries" article.