Linkers and Loaders

20PT12 – Jonathan Cecil

20PT17 – Naveen Kumar AA
20PT19 – Nikhilesh Shaji

Introduction
Linkers and loaders are crucial components in the software development process,
responsible for linking and loading executable code into memory. They allow software
programs to run on a computer and play an essential role in ensuring that programs run
efficiently and reliably. Linkers combine object files into a single executable program and
resolve external references between them, while loaders allocate memory for the program,
load the executable code into memory, and prepare it for execution. Understanding the
functionality of linkers and loaders is vital for software developers, as it can impact the
performance, stability, and functionality of software programs. In this document, we will
explore the key concepts and processes involved in linkers and loaders, and their significance
in software development.

Linkers

Linkers are software programs that take one or more object files produced by a
compiler and combine them into a single executable program. They are an essential
component in the software development process, responsible for resolving external references
between object files and creating links between them. Linkers can be static or dynamic,
depending on how they link object files. Static linkers combine all necessary object files into
a single executable file, while dynamic linkers allow multiple programs to share a single copy
of a library. Linkers play a crucial role in software development, ensuring that software
programs run efficiently and reliably.
Static Linkers

Static linkers are software programs that combine all necessary object files into a
single executable file. When a program is compiled with a static linker, all library functions
required by the program are included in the executable file. This means that the executable
file can be run independently of the system it was compiled on, as all necessary components
are contained within the file.
Static linking can be useful for creating self-contained executables that can be easily
distributed, as it eliminates the need for external dependencies. However, it can lead to larger
file sizes and slower startup times compared to dynamic linking. This is because all library
functions are included in the executable file, even if they are not used by the program.
Static linking is commonly used in systems programming, where speed and simplicity are
important. It is also used in embedded systems, where the executable file needs to be small
and self-contained. However, in most other scenarios, dynamic linking is preferred as it
allows multiple programs to share a single copy of a library, resulting in smaller file sizes and
faster startup times.

Dynamic Linkers

Dynamic linkers are software programs that allow multiple programs to share a single
copy of a library. When a program is compiled with a dynamic linker, it does not include all
the library functions required by the program. Instead, it contains a reference to the functions,
which are loaded at runtime by the dynamic linker.
Dynamic linking can result in smaller executable files and faster startup times compared to
static linking. This is because the shared libraries are loaded into memory only when they are
needed, and multiple programs can share a single copy of the library.
Dynamic linking is commonly used in modern software development, as it allows for easier
maintenance and updates of shared libraries. It also allows for more efficient use of memory,
as multiple programs can share the same memory space for shared libraries.
Dynamic linking is also used in operating systems, where multiple programs may be running
simultaneously and require access to shared system libraries. In such scenarios, dynamic
linking is essential for efficient memory usage and system performance.
Overall, dynamic linking is a crucial aspect of modern software development, allowing for
efficient use of system resources and easier maintenance of shared libraries.

Advantages and Disadvantages of Static and Dynamic Linkers

A comparison…

Advantages of Static Linking:

Self-contained executables that can be easily distributed
Simple deployment and installation process
No external dependencies required, making the executable file more reliable
No need for additional system resources to load and link libraries during runtime
Suitable for systems programming and embedded systems, where simplicity and speed are
important

Disadvantages of Static Linking:

Larger file sizes, which can slow down startup times
Inefficient use of memory, as all library functions are included in the executable file, even if
they are not used by the program
Difficulty in updating shared libraries and incorporating changes into the executable file

Advantages of Dynamic Linking:

Smaller file sizes, resulting in faster startup times and efficient memory usage
Multiple programs can share a single copy of a library, resulting in less duplication of system
resources
Easier maintenance and updates of shared libraries, as changes can be incorporated without
recompiling the executable file
Suitable for modern software development, where dynamic linking is the preferred option

Disadvantages of Dynamic Linking:

External dependencies required, making the executable file less reliable
Complex deployment and installation process, as shared libraries need to be installed and
managed separately
Slower startup times compared to static linking, as the shared libraries need to be loaded and
linked during runtime
In summary, static linking is suitable for systems programming and embedded systems,
where simplicity and speed are important, while dynamic linking is more suitable for modern
software development, where efficient memory usage and easier maintenance of shared
libraries are preferred. The choice between static and dynamic linking depends on the specific
requirements of the software program and the system it will be running on.

Loaders

In computer programming, a loader is a program that loads executable code from files
or other sources into computer memory. Loaders are an essential component of the operating
system, as they enable programs to be run and executed by the computer.
There are several different types of loaders, including boot loaders, which are responsible for
loading the operating system at startup; link loaders, which link multiple object files into a
single executable file; and dynamic loaders, which load shared libraries into memory at
runtime.
Loaders are responsible for performing several important tasks, including loading executable
code into memory, resolving dependencies between code and libraries, and setting up the
program's execution environment, such as setting up the stack and heap.
One of the main benefits of using a loader is that it allows programs to be loaded and
executed in a dynamic manner, enabling programs to be loaded and executed on demand,
rather than being preloaded into memory. This can help to conserve system resources and
improve overall system performance.

Linking and Loading Process

The linking and loading process can be summarized as follows:
1. The compiler generates object files from source code.
2. The linker combines object files and libraries into a single executable file.
 3. The loader loads the executable file into memory and prepares it for execution.
4. The program is executed.

The need to understand linkers and loaders

Linkers and loaders are essential components of the software development process,
particularly for compiled programming languages like C, C++, and Fortran. They help to
manage the complexity of linking object files and libraries, loading executable files into
memory, and preparing them for execution.

One of the most important functions of linkers is symbol resolution. When a program
is compiled, it may reference symbols that are defined in other object files or libraries. The
linker resolves these symbols by matching them with their corresponding definitions in other
files. This ensures that all symbols are correctly bound to their addresses at runtime, enabling
the program to run as expected.

Loaders are responsible for loading executable files into memory and preparing them
for execution. This involves several tasks, including allocating memory for the program,
resolving dynamic linking, and initializing variables. Once the program is loaded into
memory, the loader transfers control to the program's entry point, which is the first
instruction that is executed when the program starts running.

Understanding the role of linkers and loaders is crucial for software developers. It
allows them to write more efficient and optimized code, particularly when it comes to
managing memory and resolving symbols. Additionally, knowledge of linkers and loaders
can help developers debug programs more effectively, as they can identify issues related to
symbol resolution, memory allocation, and other runtime issues.

In recent years, there has been a growing interest in dynamic linking and loading,
which allows libraries to be loaded at runtime rather than at compile time. This approach
offers several benefits, including smaller executable files and faster load times. However, it
also introduces additional complexity, as dynamic libraries may need to be located and
loaded at runtime, and the correct version of a library must be selected based on its
compatibility with the program.

Conclusion
In conclusion, linkers and loaders play a critical role in software development. They
help to manage the complexity of linking and loading executable files, and enable programs
to run correctly and efficiently. Developers who understand the principles of linking and
loading can write better code and optimize their programs for performance.