If you have a finite approximation, then you cannot find the absolute error relative to an irrational value: you can only find bounds on the absolute error.
The actual value stored is
>> fprintf('%.999g\n', pi)
Without that contribution, you could still use
and then analyze the IEEE 754 representation given there. This shows you the hex version of the exact bit pattern used. You might want to try it on your own system: if it gives you the same hex then you can rely upon the decimal value that I posted.
If you have the symbolic toolbox, then
which shows the exact value stored.
which shows pi itself rounded to 100 digits.
The symbolic toolbox recognizes the bit pattern for the numeric value stored in pi and typically converts it to the internal symbol representing the irrational number. You can be extra careful with that by using sym('pi') instead of pi