Question

Shared Library[*.so in linux and *.dll in windows]

Asked by: sham_hcu

Hello
Following question can be answered in Lnux/Windows Environment.
In linux, I start an executable prog1 which in turn uses  library libc.so.1, so this libc.so.1 gets linked   dynamically into prog1 process address space[virtual address space]. So,  libc.so.1  takes chunk of  process address space of prog1 during link time and libc.so.1 obviously also takes physical address space on RAM.
Now, if i start another executable prog2 which also uses libc.so.1, During link time,  libc.so.1 takes chunk of  process address space of prog2. What about the physical address space of libc.so.1 on RAM with respect to prog2? Is it the same physical address space of libc.so.1 used by prog1 or different?

If it is different, why do we called a library as shared library.

Please correct me, if i said something wrong?

Regards
Sham

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Asked On
2008-03-11 at 05:30:23ID23231439
Tags

Microsoft/Redhat

,

Operating system

Topics

Linux Programming

,

C Programming Language

,

C++ Programming Language

Participating Experts
2
Points
100
Comments
5

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Answers

 

by: evilrixPosted on 2008-03-11 at 05:41:43ID: 21095079

DSO's (Dynamic Shared Objects) are loaded into physical memory once and then reference counted from then on. When the reference count reaches zero it will be unloaded from memory. When a program uses a DSO it's mapped into the process space of the program. Each program will shared the same DSO. This is the same on Linux and Windows; however, how it is implemented is obviously OS specific.

 

by: stefan73Posted on 2008-03-11 at 13:52:36ID: 21100202

Hello sham_hcu,
> Is it the same physical address space of libc.so.1 used by prog1 or different?

That depends. Linkers and loaders basically have 3 kinds of memory chunks (segments): code, data and BSS.

Code is the binary executable part, data is pre-defined data and BSS is zeroed out memory areas.

prog1 might store data in non-stack memory locations (static or global vars), so the data segment is used. This shouldn't be interfering with prog2.

So a .so library should only share the read-only parts of the library (and the OS should lock the respective memory pages against writing -> security issue!), and have individual, non-shared data and BSS segments.

Of course, this is a bit over-simplifying: There's also read-only, non-executable segments, which are used for constant variables, for example. A read-only BSS is also allowed in theory by some memory models (e.g. ELF), but that's not too sensible.

Recommended reading: John R. Levine: "Linkers and Loaders"

Regards,

stefan73

 

by: sham_hcuPosted on 2008-03-12 at 03:04:49ID: 21104310

@evilrix: DSO's (Dynamic Shared Objects) are loaded into physical memory once and then reference counted from then on

Ok, but what would be the layout of DSO on physical space RAM, is it data segment+code segment+stacksegment+... ?

Regards
sham

 

by: evilrixPosted on 2008-03-12 at 03:09:04ID: 21104331

>> what would be the layout of DSO on physical space RAM
Didn't stefan73 answer that for you? To be honest, I'm not really able to expand on that.

 

by: sham_hcuPosted on 2008-03-12 at 04:55:37ID: 31438599

Thanx

20120131-EE-VQP-002

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