Why doesn't `bc` utility exit with `^C`

Question

I was using the bc utility recently and Ctrl+C cannot be used to exit the program. Ctrl+C's interrupt signal is returned with the message use quit to exit. I can use EOF (Ctrl+D) or quit to exit.

I read thru the difference between ^Z and ^C. Technically, ^C should end the program (and most programs do abort on receiving the interrupt signal). But bc returns a message asking us to use quit instead.

I was wondering is there a specific advantage in this ?

score 6 · Answer 1 · answered Sep 26 '15 at 22:16

I was wondering is there a specific advantage in this?

Yes. If you call a long running function in bc, you can interrupt it using control+c and it will not exit bc itself but the function running inside of bc.

Here is an example of me doing that:

$ bc twins.b 
bc 1.06
Copyright 1991-1994, 1997, 1998, 2000 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'. 

typing 'twins (10)' will print all twin primes less than 10.
twins(10000000)
^C
Runtime error (func=primes, adr=113): interrupted execution
Interruption completed.
print "As you can see, bc is still running\n"
As you can see, bc is still running
5+7
12

If you want to see the twins function, download the bc source code, and look in Examples directory.

Sergiy Kolodyazhnyy · Accepted Answer · 2015-09-26T15:18:24.893

Ctrl+ C sends SIGINT (signal interupt), which makes a process to get interrupted (in other words it kills the process). Some programs can catch this signal and ignore it, which is the case with bc.

The Ctrl+ D character sends EOT (end of transmission) character. Receiving this signal has similar effect as if when a program was reading from file, and reached the end of that file. Effectively, bc is waiting for you to tell it that you have no more input to give.

As for why . . . it's up to the developers to specify how the program exits as well as how it interacts with users

score 1 · Answer 3 · edited Sep 26 '15 at 19:00

A script or an application can catch and prevent events and so intrinsically offer termination methods. Why the developers have done so, you need to ask yourself.

You can see a simple example in the following shell script:

# Call trapint function when recive SIGINT
trap trapint SIGINT

# trapint function:
function trapint {
 echo "*** SIGINT received ***"  # Print a message
 exit 0                          # Exit gracefully
}

Why doesn't `bc` utility exit with `^C`

3 Answers3