Dynamic Function Loading on STM32 Microcontrollers: A Practical Guide

Introduction

The project implements a dynamic loading library for microcontrollers (e.g., STM32). It allows an ELF‑format image to be loaded from external storage into RAM at runtime, providing functionality similar to Windows DLLs or Linux shared objects.

Repository

https://gitee.com/wzh1845462801/dynamic_loader

Software Architecture

/common/dl_extern_lib.h

– defines the base address of the host‑side function vector table and related macros. /common/dl_stdio_lib.h – maps stdio.h functions to vector‑table indices for the loaded application. /common/dl_stdlib_lib.h – maps stdlib.h functions to vector‑table indices. /common/dl_time_lib.h – maps time.h functions to vector‑table indices. /rel_axf_project_template/app/dl_stdio_lib.c – redirects stdio calls for the example application. /rel_axf_project_template/app/dl_stdlib_lib.c – redirects stdlib calls for the example application. /rel_axf_project_template/app/dl_time_lib.c – redirects time calls for the example application. /src/dl_arch.c – performs code and data relocation and cache flushing; implementation is architecture‑specific (tested on Cortex‑M7). /src/dl_elf.h – ELF format decoder. /src/dl_lib.c – core implementation of the dynamic loader. /src/dl_lib.h – public API exposed to applications. /src/dl_port.c – host‑side low‑level functions that must be ported to the target platform (e.g., file I/O, memory allocation). /src/dl_port.h – declarations and macros for the porting layer. /src/dl_vector.c – host‑side function declarations used by the loaded application.

Installation

Confirm that the target MCU is supported. The library has been validated on an STM32H743 (Cortex‑M7). It should work on other Cortex‑M cores, but chips lacking proper relocation instructions or cache control may require code adjustments.

If the MCU uses an instruction/data cache, enable cache handling by setting DL_CACHE_USE 1 in /src/dl_port.h.

Port the low‑level functions in /src/dl_port.c to the actual storage driver. The reference implementation uses FatFs; replace the file‑open/read/write calls with the driver used in your project.

Build the example project located in rel_axf_project_template. The build produces a relocatable ELF file named dll_generate.axf in rel_axf_project_template/Objects/.

Verify the generated ELF with a tool such as fromelf (or llvm-objdump) to ensure it is relocatable and contains the expected sections before attempting dynamic loading.

Usage

Declare a handler variable of type DL_Handler in the host application.

Load the ELF image ( dll_generate.axf) into RAM using your storage driver.

Include dl_lib.h and call dl_load_lib("dll_generate.axf", &handler). Check that the return value equals DL_NO_ERR.

After a successful load, obtain the address of a function exported by the loaded module with dl_get_func(handler, "function_name").

If needed, retrieve the module’s entry point (the dl_main function) via dl_get_entry(handler).

When the dynamically loaded code has finished executing, release resources with dl_destroy_lib(handler).

Illustrations