The document discusses the linker, which links object files generated by the assembler into executable files. It defines the linker as a system software that combines object files, resolving references between them. Linkers are needed because large programs are separated into multiple files that must be combined into a single executable. There are two types of linking - static linking embeds library code directly into executables while dynamic linking relies on shared libraries present at runtime. The document provides an overview of the compilation process and role of the linker in linking object files and libraries to produce an executable.

1. Introduction to Linker
Prepared by:-
Name - Rahul Dhiman
Rollno - 14508
Sem - 4th sem ( 2nd year)
Branch - Computer Science & Engg.

2. 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.

3. 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.

4. 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

5. 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

6. 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.

7. 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.

8. 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

11. 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.

12. 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 .

13. THANK YOU ALL