Linkers

Introduction to Linker

Prepared by:-Name - Rahul DhimanRollno - 14508Sem - 4th sem ( 2nd year)Branch - Computer Science & Engg.

WHAT IS LINKER ? Linker is a system software. Linker is also referred as link editor. It links the object (.obj) files generated by the assembler. It combines those object files into single excecutable file , library

file, or another object file.

WHY WE NEED LINKERS ?

Often in a large program , you will separate out the code into multiple files to keep related functions together.

Each of these multiple files can be compiled into object code. But our final goal is to create a single executable file. There must be a way to combine each of these object files into a

single executable .We call this linking. Linking is process that is carried out by the linker.

USE OF LINKER IN COMPILATION:-

A.c B.c

A.obj B.obj

Linker

Executable File(.exe)

Linker’s use occurs at the very end of the compilation right after the assembler.

Here A.c and B.c are the two source code files for project (say) .

After compilation of these two files ,the object files obtained are A.obj and B.obj.

Now to get a single executable file ,we pass these object files to linker or link editor.

Library

TYPES OF LINKING:-

Linker takes the object code generated by the compiler/assembler ,and links it against a c library (and /or libgcc.a or whatever link library you provide).This can be done in two ways :-

a) Static linking b) Dynamic linking

STATIC LINKING:-

When linking statically , linker invocation is done during the build process i.e just after the compiler/assembler run.

linker takes the object code and checks it for unresolved references if any. Linker checks if it can resolve these unresolved refrences from the

available libraries (including user created libraries also). It adds the binary code from these libraries to the executable. After

this executable is complete. In this linking ,executable file is quiet large both on disk and

memory because the code from the libraries is duplicated over and over.

DYNAMIC LINKING:-

When linking dynamically, linker is invoked during the loading of an executable.

It also checks the object code for unresolved references. The unresolved references are resolved against the libraries

currently present in the system. In dynamic linking , the on-disk executable file is smaller in size

and allows for in-memory space saving strategies such as shared libraries.

Executable depends on the presence of libraries it references . If a system doesn’t have those libraries executable can’t run.

OVERVIEW OF MAKING A EXECUTABLE FILE:-

First compiler takes the source code files of your project written in c / c++ etc.

Compiler compiles these multiple into their corresponding assembly language files followed by extension (.asm).

Assembler converts the assembly language code file into binary code file to which we referred as object file.

After the assembler linker comes and does it’s work in two steps :- a ) Symbol addressing b ) Resolve addressing After linker has done it’s work , a new file with extension

(.exe) is created which is the executable file of your project.

Compiler

Source code files

Assembler

Linker

Executable file

LINKER USED IN OPERATING SYSTEM:-

In computing , a dynamic linker is the part of an operating system.

It loads and links the shared libraries needed by an executable when it is executed.

Dynamic linker functionality and implementation depends upon two factors : -

Specific operating system ( like windows , linux , Mac etc. ) Executable format

In Ubuntu operating system linker used is a standalone linker and also referred via “ld”. On Unix-like systems, the linker is typically invoked with the ld command.

HOW TO SAVE INTERMEDIATE FILES IN BETWEEN COMPILATION OF A PROGRAM ?

Suppose you have created a C program say “hello.c”. When you compile this program in Ubuntu using gcc compiler by

typing “gcc hello.c” . Without saving it’s object code ,assembly code and binary code

files , it’ll directly create a executable file (ELF for GCC). To save those intermediate files you can use the command “gcc –Wall –save-temps hello.c –o hello”. Then you can check that the intermediate files are created in the

same directory .

THANK YOU ALL