If you open up src/stdlib/builtin/lv_mem_core_builtin.c in lvgl you will see all of the different memory functions. You need to create your own version of all of those functions. Then set LV_USE_STDLIB_MALLOC to LV_STDLIB_CUSTOM make sure in the C course file that contain your versions of the functions that you include src/stdlib/lv_mem.h as that is where the function prototypes are