Code segment sharing between two processes

Question

Suppose we run two processes back to back say :-

$ grep abc abc.txt ==> pid 100
$ grep def def.txt ==> pid 101

I read in the book "Beginning Linux programming" chapter# 11 that the code section of the processes would be shared, as it is read only. Is it so? I think if grep is compiled as shared library only then the code section would be shared.

One more question, in case of shared libraries how does the OS knows that the library has already been loaded or not? Suppose if 2 processes are simultaneously calling a shared library function then how does the virtual address of two processes be converted to physical address pointing the same location in RAM?

Answer 1

The OS doesn't load files into memory anymore. Instead, files are memory mapped. This means an inode and an offset of a file on disk will be connected to a page in memory. This makes it pretty simple to find out if some part of a file has already been loaded. Also, you can keep only part of a file in RAM (after setup, you don't need the setup code anymore, so you can "forget" about it and reuse those pages for something more useful).

Answer 2

The libraries and executables are not loaded, but mapped into memory with mmap(2). Basically, when you mmap() something with MAP_SHARED flag, others who map the same file will get the same memory pages.