error 017: undefined symbol "ret_memcpy

#pragma unused ret_memcpy

ret_memcpy(source[],index=0,numbytes) {
    new tmp[MAX_STRING];
    new i=0;
    tmp[0]=0;
    if (index>=strlen(source)) return tmp;
    if (numbytes+index>=strlen(source)) numbytes=strlen(source)-index;
    if (numbytes<=0) return tmp;
    for (i=index;i<numbytes+index;i++) {
        tmp[i-index]=source[i];
        if (source[i]==0) return tmp;
    }
    tmp[numbytes]=0;
    return tmp;
}