In this lesson, you will learn how to verify that your C functions and variables reside exactly where you intended by auditing the linker’s map file.
When you compile and link your project, the linker produces a map file—a comprehensive text report detailing the final memory layout. While the compiler and linker handle the heavy lifting, the map file acts as the "ground truth" to verify that your memory management strategy is actually working.
The Anatomy of a Map File
A typical map file output shows the input sections (from your object files) being grouped into output sections (defined in your linker script). You should look for the Memory Configuration and Linker script and memory map sections.
Consider this snippet from a map file:
.text 0x08000100 0x150
*(.text*)
.text 0x08000100 0x100 main.o
.text 0x08000200 0x50 startup.o
Here, you can explicitly see that main.o starts at address 0x08000100, occupying exactly 0x100 bytes. If you intended for startup.o to be placed at the very beginning of the flash memory, seeing it pushed to 0x08000200 immediately flags an alignment or placement issue in your linker script.
Debugging with Maps
When debugging, the map file is indispensable for two specific tasks:
- Validating Custom Sections: If you use attributes like
__attribute__((section(".my_flash_data")))to place critical data in specific memory regions, verify that the map file lists these symbols in the correct address range. If they appear in the wrong section, the linker script may be missing a wildcard pattern. - Tracking Bloat: If your binary size exceeds your microcontroller's flash capacity, scan the map file for unexpectedly large sections. Often, a large library or a bloated static array will jump out at you, showing exactly which object file is consuming the most memory.
By correlating the addresses in the map file with your MEMORY region definitions, you can confirm that your system is correctly partitioned and that no overlaps or overflows exist before you ever flash your hardware.
Key Takeaway: The linker map file is your final verification tool to confirm that physical memory placement matches your design intent, providing the evidence needed to troubleshoot memory access violations and binary size issues.