Getting reg value

My code is below:
```
auto src_opnd = instr_get_src(inst, 0);
if (!opnd_is_reg(src_opnd))
break;
reg_id_t reg_id = opnd_get_reg(src_opnd);
auto res = reg_get_value(reg_id, &drcontext);
```
The functions is executed by `dr_insert_clean_call`.
I want to get a register value at this point, the code prints same result for same register even register's value is different at different point.

Am I making a mistake? I want to get a register value at some point.