CODEWITHSUNDEEP
cunixiooutputsignals
kimsh
kimsh

Reputation: 11

How does a Unix program like bc cleanly handle input/ouput stream when started as a background job?

I noticed that the Unix bc program does not print out it's usual prompt (the three symbols ">>> ") when being started as a background process (like if you execute it as "bc &"). This is confusing to me because from my limited knowledge of Unix, starting a program as a background job will keep it running until as soon as it tries to read from stdin, at which point it will receive a signal to stop itself.

But running bc as a background job ("bc &") will not cause it to at least print out the ">>> " prompt before stopping itself which tells me that the program handles that somehow. I am curious as to how it does this. When I wrote a naive program that only tries to emulate the input/output interaction, it still prints out ">>> " before being suspended which doesn't look very clean at all and the behavior gets even more bizarre on certain shells.

I tried looking through the Unix bc source code and I was able to trace the code to parts where it is printing out the ">>> " prompt, but how it was handling not printing out the prompt when started as a background process was beyond me. And I know that obviously you would never start an input/output interactive program in the background as that goes against intended functionality and common sense, but I am more interested in the concepts behind it like if this was implemented with signal handling and/or if this is some more advanced input/output stream buffering or some other Unix concept that I am not familiar with.

Upvotes: 1

Views: 52

Answers (1)

n. m. could be an AI
n. m. could be an AI

Reputation: 120079

The first thing your version of bc does is call the tcsetattr function. This function, when called from a background process, causes the SIGTTOU signal to be sent to the process, which by default causes the process to stop.

Any program that manipulates terminal attributes (vim, bash, anything that uses readline or curses, ...) will probably behave exactly the same way.

Upvotes: 1

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