LinuxQuestions.org - Dynamically linked libraries..

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Dynamically linked libraries..

Hey anyone knows which function loads the dynamically linked libraries required by executables?
Or is there really such function in linux kernel?
I want to k @ runtime which shared library is required by the executable..?

your replies are always welcome!!

If you want to know what shared libraries a program uses, try "ldd".

If you wonder what C functions do that, have a look at "man dlopen". Can't help you with those though.

thanks for the reply..
I want the function in the kernel code which loads the dynamically linked libraries..?

The kernel doesn't load libraries (exception: kernel modules). What you probably mean is dlopen or its internals. Is it correct?

yes you got the point..

If kernel comes to know that the currently executing executable requires some dynamically linked library the what it does?

I did a fair amount of grepping and munging around to get a partial answer to this question. It seems part of the answer is in fs/binfmt_elf.c; load_elf_binary() calls load_elf_interp(), which in turn sets up /lib/ld-linux.so.2 to do the actual work. I suspect that the code for ld-linux is in glibc-2.4.tar.gz which you can get here: http://directory.fsf.org/GNU/glibc.html , but I haven't pinned down where yet.

The reason I know that the interpreter is /lib/ld-linux.so.2 is this:

Code:

$ objdump -p hw | grep -A1 INTERP

  INTERP off    0x00000114 vaddr 0x08048114 paddr 0x08048114 align 2**0

        filesz 0x00000013 memsz 0x00000013 flags r--

$ dd if=hw bs=1 count=19 skip=276 2>/dev/null; echo

/lib/ld-linux.so.2

(0x114 is 276 and 0x13 is 19) You can see load_elf_binary() reading this string from the file analogously to my dd command:

Code:

                if (elf_ppnt->p_type == PT_INTERP) {

/* skip a bit */

                        retval = kernel_read(bprm->file, elf_ppnt->p_offset,

                                          elf_interpreter,

                                          elf_ppnt->p_filesz);

If you strace the executable (mine is just 'hello world'), you'll see the loader mmapping shared libraries:

Code:

$ strace ./hw

execve("./hw", ["./hw"], [/* 40 vars */]) = 0

/* skip a bit... */

open("/lib/tls/libc.so.6", O_RDONLY)    = 3

read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\320O\1"..., 512) = 512

fstat64(3, {st_mode=S_IFREG|0755, st_size=1266800, ...}) = 0

old_mmap(NULL, 1272764, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb7e6a000

/* more... */

So ultimately, the code to load a shared library is just an mmap(), after read()ing the elf header from the library. See 'man 5 elf' for the elf format.

I would feel a lot more confident of everything I just wrote if I had disected ld-linux. I haven't, so take it all with several grains of salt...

of course all that assumes that you're concerned with normal shared libraries which are loaded by the loader at runtime but before your program starts executing. If you mean to load a library later, use dlopen() or such