Troubleshooting Reversed Linker Placement Order In Map Files After Compilation

Hey guys! Have you ever encountered a situation where the linker placement order in your map file seems reversed after compiling your code? It's a common head-scratcher, especially when you're working with embedded systems and memory mapping. In this article, we're diving deep into this issue, focusing on scenarios involving C, linkers (specifically the ARM linker), FreeRTOS, and linker errors. We'll explore how linker sections are defined, retained, and ultimately placed in memory, and what might cause unexpected reversals in the map file.

Defining Linker Sections and Memory Regions

Let's kick things off by understanding how linker sections and memory regions are defined. In the context of embedded systems, particularly when using ARM architecture, you often need precise control over where your code and data are placed in memory. This is where linker scripts come into play. Linker scripts are configuration files that instruct the linker how to combine object files, libraries, and other input sections into a final executable image. They also define the memory map of your target system, specifying the available memory regions and their attributes.

Memory Regions

Memory regions are defined using the MEMORY directive in the linker script. This directive allows you to specify the name, origin (starting address), length, and attributes of each memory region. For example, in the provided scenario, a memory region named TEST_SECTION is defined:

MEMORY
{
 TEST_SECTION (RWIX) : ORIGIN = 0x41C00000 , LENGTH = 0x00007800
}

Here, TEST_SECTION is the name of the region, RWIX indicates the memory attributes (Read, Write, Execute, and possibly additional attributes depending on the linker), ORIGIN sets the starting address to 0x41C00000, and LENGTH specifies the size of the region as 0x00007800 bytes. Defining memory regions is the foundation for controlling memory layout, ensuring that different parts of your application reside in specific memory areas.

Linker Sections

Within these memory regions, you place linker sections. Linker sections are named blocks of code or data that the linker combines from your object files. Common sections include .text (for code), .data (for initialized data), .bss (for uninitialized data), and custom sections you define yourself. Placing sections into specific memory regions is done using the SECTIONS directive in the linker script. This directive allows you to map input sections (from your compiled code) to output sections within the defined memory regions.

For instance, you might have code in your C files that you want to place in TEST_SECTION. You would achieve this by creating a custom section and instructing the linker to place it in the desired memory region. This level of control is crucial in embedded systems where memory is often limited and specific regions might have special purposes (e.g., fast RAM, flash memory).

Understanding the interplay between memory regions and linker sections is vital for managing memory effectively and avoiding issues like reversed placement order. Now, let's delve deeper into how sections are retained and how this relates to the problem at hand.

Retaining Linker Sections

In the world of embedded systems development, especially when working with linkers like the ARM linker, retaining specific sections is crucial for ensuring that essential code and data are not inadvertently discarded during the linking process. This is where the --retain linker option comes into play. Understanding how to use --retain effectively is key to resolving issues where the linker placement order appears reversed in the map file.

The --retain option is a directive you pass to the linker, instructing it to keep a particular section or symbol in the final output image, even if the linker doesn't detect any direct references to it. This is particularly important for sections containing interrupt vectors, initialization routines, or other critical code that might not be explicitly called from the main application flow but needs to be present in the final image. Without --retain, the linker's garbage collection mechanism might remove these sections, leading to unexpected behavior or system crashes.

How --retain Works

The --retain option essentially tells the linker to treat the specified section or symbol as if it were being actively used by the application. This prevents the linker from optimizing it away during the linking process. The syntax for using --retain typically involves specifying the section or symbol name using a wildcard pattern. For example, --retain=".my_custom_section" would instruct the linker to retain the section named .my_custom_section. The asterisk wildcard can be used to match multiple sections or symbols, such as --retain="*(.interrupt_vector*)" to retain all sections starting with .interrupt_vector. In the context provided, `--retain=\