https://*.com/questions/28708234/how-to-fix-defined-in-discarded-section-linker-error

 

void * memcpy(void *dest, const void *src, sizte_t n) __attribute__((used));
void * memcpy(void *dest, const void *src, size_t n) {
    uint8_t *d = (uint8_t *)dest;
    uint8_t *s = (uint8_t *)src;
    while(n--) {
    *d++ = *s++;
    }
    return dest;
}