How the share library be shared by different processes?

I read some documents that share library comiled with -fPIC argument, the .text seqment of the .so will be shared at process fork's dynamic linking stage (eq. the process will map the .so to the same physical address)

i am interested in who (the kernel or ld.so ) and how to accomplish this? maybe i should trace the code, but i dont know where to start it.

Nevertheless, i try to verify the statement.

I decide to check the function address like printf which is in the libc.so that all c program will link. I get the printf virtual address of the process and need to get the physical address. Tried to write a kernel module and pass the address value to kernel, then call virt_to_phys. But it did not work cause the virt_to_phys only 开发者_JAVA技巧works for kmalloc address.

So, process page table look-at might be the solution to find the virtual address map to physical address. Were there any ways to do page table look-at? Or othere ways can fit the verify experiment?

thanks in advance!

The dynamic loader uses mmap(2) with MAP_PRIVATE and appropriate permissions. You can see what it does exactly by running a command from strace -e file,mmap. For instance:

strace -e file,mmap ls 

All the magic comes from mmap(2). mmap(2) creates mappings in the calling process, they are usually backed either by a file or by swap (anonymous mappings). In a file-backed mapping, MAP_PRIVATE means that writes to the memory don't update the file, and cause that page to be backed by swap from that point on (copy-on-write).

The dynamic loader gets the info it needs from ELF's program headers, which you can view with:

readelf -l libfoo.so

From these, the dynamic loader determines what to map as code, read-only data, data and bss (zero-filled segment with zero size in file, non-zero size in memory, and a name only matched in crypticness by Lisp's car and cdr).

So, in fact, code and also data is shared, until a write causes copy-on-write. That is why marking constant data as constant is a potentially important space optimization (see DSO howto).

You can get more info on the mmap(2) manpage, and in Documentation/nommu-mmap.txt (the MMU case, no-MMU is for embedded devices, like ADSL routers and the Nintendo DS).

Shared libraries just a particular use of mapped files.

The address which a file is mapped at in a process's address space has nothing to do with whether it is shared or not.

Pages can be shared even if they are mapped at different addresses.

To find out if pages are being shared, do the following:

1. Find the address that the file(s) are mapped at by examining /proc/pid/maps
2. There is a tool which extracts data from /proc/pid/pagemap - find it and use it. This gives you info as to exactly which page(s) of a mapping are present and what physical location they are at

If two processes have a page mapped in at the same physical address, it is of course, shared.