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Allow to read from stdin #3

brendan-rius opened this Issue Apr 19, 2016 · 9 comments


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7 participants

brendan-rius commented Apr 19, 2016

So it is possible to use functions like scanf.

Thanks to @ryukinix for letting me know about this enhancement.


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sdd3 commented Apr 29, 2016

Could you give an example of scanf? I get an unassigned value. It's a great project, much appreciated. If possible it would be useful to have a more extensive example notebook.


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ryukinix commented Apr 29, 2016

Unfortunately that is not implemented yet, @sdd3. Input functions doesn't works yet. The proposal is a behavior for input functions like the default python kernel related behavior, communicating with the jupyter frontend. Like this:

captura de tela de 2016-04-29 10 31 59


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sdd3 commented Apr 29, 2016

Ah, ok. I misunderstood the first post in this thread. Thanks for clearing that up.
Any idea when an input function might be incorporated?


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spoorcc commented Apr 13, 2017

I'm trying to implement this, but I'm a little stuck:
My idea is to override all fgets calls with my_fgets using a macro. This my_fgets in master.c notifies its parent (subprocess.Popen in somehow to send an input_request through stdin_socket. When input_reply is received from the frontend the data is piped to the stdin of subprocess.

I managed to request and receive input from the frontend as such, which works just fine:

 def _raw_input(self, prompt, ident, parent):
          # Flush output before making the request.

          # Send the input request.
          content = dict(prompt=prompt)
          msg = self.session.send(self.stdin_socket, u'input_request', content, parent, ident=ident)

          # Await a response.
          ident,reply = self.session.recv(self.stdin_socket, 0)
              value = reply['content']['value']
              self.log.error("Got bad raw_input reply: %s"%Message(parent))
              value = ''
          return value

    def execute_request(self, stream, ident, parent):
        ''' Wrap execture_request function to force input call '''
        value = self._raw_input('Some info:', ident, parent)
        self.log.warn('Value: {}'.format(value))
        super(CKernel, self).execute_request(stream, ident, parent)


I cannot find or decide on a mechanism to notify from master.c to Python subprocess instance that input is requested:

  • Is there anybody with an opinion or a better suggestion than:
    • Using zmq sockets directly from master.c?
    • Introducing new messaging system between master.c and
    • Piping some predefined message through stderr/stdout?
  • I looked into raising signals SIGTTIN and SIGTTOU (good blog), but they appear to be used from parent-to-child and I want from child to parent.

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spoorcc commented Apr 14, 2017

Ok I'm 1 step further, (for at least a POSIX solution, sorry windows guys 😛 ).

From master.c I raise SIGTTIN to the parent process as such, and in contrary to the name kill the c process will continue just fine.

kill(getppid(), SIGTTIN); 

I have chosen SIGTTIN because people might want to use SIGUSR1 and SIGUSR2 in notebooks.

From within I can catch this as such:

        def sig_forwarder(a,b):
             self.log.warn('Caught SIGTTIN from C')

         signal.signal(signal.SIGTTIN, sig_forwarder)
         p = self.create_jupyter_subprocess([self.master_path,])
         while p.poll() is None:
         signal.signal(signal.SIGTTIN, signal.SIG_DFL)

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matheusmota commented Jul 12, 2017

Is there any alternative for reading inputs using scanf?


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spoorcc commented Jul 26, 2017

I chose fgets to be safe in the implementation I am/was working on.

For reference see:

Sent from my OnePlus ONEPLUS A5000 using FastHub


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masterfish2015 commented Jun 16, 2018

It can be done as following:


int max2i(int x, int y);
char *input_string="25,67";  //<-----input value

int main()
    int a,b,c;
    printf("input 2 integer numbers:");
    sscanf(input_string, "%d,%d", &a, &b);  //<------use sscanf instead of scanf
    printf("max value of (%d,%d) is %d\n", a,b,c);
    return 0;
int max2i(int x,int y)
        return x;
        return y;

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LucaCerina commented Sep 3, 2018

The @masterfish2015 solution didn't work for me with Jupyter 4.4.0 and Notebook 5.5.0, neither from Firefox or Chrome.

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