Writing into stdin with subprocess

Question

I have the following C Program

#include <stdio.h>


int main()
{

    char a[200];
    a[199] = 0;


    printf("Enter some input ->\n");
    scanf("%s"  ,a);

    printf ("\nInput was %s\n", a);

    return 0;
}   

and I'm trying to write some input into it the following way:

from subprocess import *
a = Popen(["my_prog.elf", stdin=PIPE) 
a.stdin.write("MyInput")

yet this doesn't appear to work.... any ideas how to fix this?

** edit **

does anyone have any clue why a.stdin.flush() wont work?

Answer 1

scanfreads one whole line, until it reads the new line character \n, so you have to send this, too:

from subprocess import Popen, PIPE
a = Popen(["my_prog.elf"], stdin=PIPE) 
a.stdin.write("MyInput\n")

flush is only needed, if your output stream (stdin) is buffered (which pipes are not, by default). Even then, you need \n to terminate the line.