Can a Program Run Without the main() Function in Embedded Systems?

When developing embedded systems, the traditional main() function, familiar to most C and C++ programmers, is not always necessary. In embedded programming, especially in environments with custom bootloaders or specialized startup sequences, the entry point of a program can be defined in various ways. This flexibility allows embedded systems to handle tasks in ways that diverge from standard desktop or server applications. In this blog, we will explore how programs can run without a main() function in embedded systems, focusing on examples with linker scripts and startup code.

Understanding the Role of main() in Embedded Systems

In standard programming, the main() function serves as the entry point of an application. It is where execution begins after the operating system loads the program. In embedded systems, however, the program execution model is often different due to the lack of a traditional operating system and the presence of specialized hardware.

1. Startup Code and Linker Scripts

In embedded systems, especially in microcontrollers and processors, the startup code and linker scripts define how and where the program begins execution. Let’s break down these components and how they can be used to run a program without a main() function.

What is Startup Code?

Startup code is executed immediately after a reset or power-up. It is responsible for initializing hardware, setting up the stack, and preparing the system before the main application code runs.

Consider a simple example for an ARM Cortex-M microcontroller:

In the above example:

• The _start label is the entry point, which initializes the system and then jumps to the main function.
• Even though main is used here, the startup code could be configured to jump to any other function or address if needed.

What is Linker Scripts?

Linker scripts define the memory layout and entry points of the program. They can specify where the code and data are placed in memory and the entry point of the application.

Here’s a simplified example of a linker script for an ARM Cortex-M microcontroller:

In the above script:

• The ENTRY(Reset_Handler) directive sets the entry point of the program to Reset_Handler.
• Reset_Handler would be defined in the startup code or assembly file.

Summary:

While the main() function is a fundamental concept in standard programming, embedded systems often use different methods to manage program execution. The main() function may be absent or not used in the traditional sense. Instead, embedded systems rely on:

• Startup code and linker scripts to define the entry point.
• Bare-metal programming to control hardware directly.
• RTOS for managing tasks and threads.
• Bootloaders for managing firmware and application loading.

Understanding these variations is crucial for embedded systems developers to effectively design and implement software for a wide range of applications and environments.

Happy Reading!!!