Re: Memory Lay out - shared library

["Hareesh Nagarajan" <hnagar2@xxxxxxxxx>]

On 4/7/06, shivaligupta <shivaligupta308@xxxxxxxxx> wrote:

Where in memory is shared library loaded?
i guess it should be in the process virtual address space only but if
that is the case how another process which wants to use the same library
comes to know that the library has already been loaded and how is it
able to use that?

Read the following link:
 http://www.tldp.org/HOWTO/Program-Library-HOWTO/shared-libraries.html

But, to help you out a little, check this out:

$ cat x.c
#include <stdio.h>

main()
{
        printf("hit any key...");
        getchar();
}

$ gcc x.c -ox
$ ./x

$ readelf -a x | grep getchar
0804960c  00000107 R_386_JUMP_SLOT   00000000   getchar
     1: 00000000   221 FUNC    GLOBAL DEFAULT  UND getchar@xxxxxxxxx (2)
    82: 00000000   221 FUNC    GLOBAL DEFAULT  UND getchar@@GLIBC_2.0

$ readelf -a x | grep printf
08049614  00000307 R_386_JUMP_SLOT   00000000   printf
     3: 00000000    57 FUNC    GLOBAL DEFAULT  UND printf@xxxxxxxxx (2)
    97: 00000000    57 FUNC    GLOBAL DEFAULT  UND printf@@GLIBC_2.0

The addresses of printf and getchar are undefined and will be resolved at run time.

$readelf -a ./x

[snip]
0x00000001 (NEEDED)                     Shared library: [libc.so.6]
[snip]

With this hint, ld figures out it needs to load the shared library. Let us run program ./x in one terminal and check its process map from another.

hareesh@salty:~$ pmap `pgrep ./x`
11680:   ./x
08048000      4K r-x--  /x
08049000      4K rw---  /x
40000000     88K r-x--  /ld-2.3.2.so <--- Loader
40016000      4K rw---  /ld-2.3.2.so
40017000     12K rw---    [ anon ]
40022000   1192K r-x--  /libc-2.3.2.so <--- glibc (code section; perhaps)
4014c000     36K rw---  /libc-2.3.2.so <--- glibc (data section; perhaps)
40155000     12K rw---    [ anon ]
bfffe000      8K rw---    [ stack ]
ffffe000      4K -----    [ anon ]
 total     1364K

-- 
c u

./hareesh