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Linking and Loading 1CS502 Spring 2006

Linking & Loading

CS-502 Operating Systems

{This topic is not covered in Tannenbaum.Silbershatz et al devote about 2 pages to it in 8.1}

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What happens to your program

after it is compiled, but before it

can be run?

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Executable files

The OS expects executable files to have aspecific format

Header info

Code locations Data locations

Code & data

Symbol Table

List of names of things defined in your program andwhere they are defined

List of names of things defined elsewhere that areused by your program, and where they are used.

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Example

#include

int main () {

printf (hello,

world\n)

}

Symbol defined in

your program and

used elsewhere

main

Symbol defined

elsewhere and used by

your program

printf

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Example

#include

extern int errno;

int main () {

printf (hello,

world\n)

}

Symbol defined in

your program and

used elsewhere

main

Symbol defined

elsewhere and used by

your program

printf

errno

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Two-step operation (in most systems)

Linking: Combining a set of programs, includinglibrary routines, to create a loadable image

a) Resolving symbols defined within the set

b) Listing symbols needing to be resolved by loader

Loading: Copying the loadable image intomemory, connecting it with any other programs

already loaded, and updating addresses as needed (In Unix) interpreting file to initialize the processaddress space

(in all systems) kernel image is special (own format)

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From source code to a process

Binding is the process of mappingnames to addresses

Most compilers produce relocatableobject code

Addresses relative tozero

The linker combines multiple object

files and library modules into a singleexecutable file

Addresses also relative tozero

The Loader reads the executable file

Allocates memory

Maps addresses within file to physicalmemory addresses

Resolves names of dynamic libraryitems

Source

(.c, .cc)

Object

(.o)

Executable

In-memory Image

Compiler

Linker

Loader

Other Objects

(.o)

Dynamic libraries(.dll)

Static libraries

(.a)

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Static Linking and Loading

Printf.c

Printf.o

StaticLibrary

gcc

ar

Linke

r

Memory

HelloWorld.c

gcc

HelloWorld.o

Loader

a.Out

(or name of

your command)

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Classic Unix

Linker is inside ofgcc command

Loader is part ofexec system call

Executable image contains all object and library

modules needed by program Entire image is loaded at once

Every image contains copy of common library

routines Every loaded program contain duplicate copy of

library routines

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Linking and Loading 10

CS502 Spring 2006

Dynamic Loading

Routine is not loaded until it is called

Better memory-space utilization; unusedroutine is never loaded.

Useful when large amounts of code areneeded to handle infrequently occurringcases.

Silbershatz says [incorrectly] No special support from the operating system is

required

Must be implemented through program design

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CS502 Spring 2006

Linker-assisted Dynamic Loading

Programmer marks modules as dynamicto linker

For function call to a dynamic function

Call is indirect through a link table Each link table initialized with address small stub of

code to locate and load module.

When loaded, loader replaces link table entry with

address of loaded function When unloaded, loader replaces table entry withstub address

Static data cannot be made dynamic

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CS502 Spring 2006

Shared Libraries

Observationeveryone links to standardlibraries (libc.a, etc.)

Consume space in

every executable image every process memory at runtime

Would it be possible to share the commonlibraries?

Automatically load at runtime?

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CS502 Spring 2006

Shared libraries (continued)

Libraries designated as shared .so, .dll, etc.

Supported by corresponding .a libraries containingsymbol information

Linker sets up symbols to be resolved atruntime

Loader: Is library already in memory?

If so, map into new process space map, an operation to be defined

If not, load and then map

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CS502 Spring 2006

Run-time Linking/LoadingPrintf.c

Printf.o

Shared

Library

gcc

arLinker

Memory

gcc

Loader

Save disk space.Startup faster.

Might not need all.

Run-time

Loader

HelloWorld.c

HelloWorld.o

a.Out

(or name of

your command)

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CS502 Spring 2006

Dynamic Linking

Complete linking postponed until execution time.

Stub used to locate the appropriate memory-

resident library routine.

Stub replaces itself with the address of the routine,and executes the routine.

Operating system needs to check if routine is in

processes memory address space.

Dynamic linking is particularly useful for

libraries.

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CS502 Spring 2006

Dynamic Shared Libraries

Static shared libraries requires address

space pre-allocation

Dynamic shared librariesaddress binding

at runtime

Code must be position independent

At runtime, references are resolved as

Library_relative_address + library_base_address

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CS502 Spring 2006

Overlays(primarily of historical interest)

Keep in memory only those instructions and data

that are needed at any given time.

Needed when process is larger than amount of

memory allocated to it. Can be implemented by user

no special support needed from operating system, but

programming design of overlay structure is complex

Can be done with OS helpthink about Unix exec

system call

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CS502 Spring 2006

Linking - Summary

Linkerkey part of OSnot in kernel

Combines object files and libraries into a

standard format that the OS loader can

interpretResolves references and does static relocation

of addresses

Creates information for loader to completebinding process

Supports dynamic shared libraries

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