C++ “std::cin >> ” is not pausing to read input from Emacs shell

Question

The following program is from https://www.cs.bu.edu/teaching/tool/emacs/programming/

#include <iostream>

int main()
{
  int howmany;

  std::cout << "Enter a number: ";
  std::cin >> howmany;
  std::cout << "howmany=" << howmany << std::endl;

  return 0;
}

It compiled and ran in Emacs shell:

SPC c C  (compile)
SPC !    (shell-command)
Shell command: ./total

The program did not stop and prompt for input. The program just blows by the "std::cin >> howmany;" and assigns 0 to howmany. I did not enter any input. For one second the mini buffer said:

Enter a number: 0

The *Messages* buffer says:

user-error: Minibuffer window is not active
Enter a number: howmany=0

The executable works fine when run from Bash shell:

$ ./total
Enter a number: 2 
howmany=2

Why does std::cin work in Bash shell, but not in Emacs shell?

https://www.gnu.org/software/emacs/manual/html_node/efaq-w32/Subprocess-buffering.html says:

having an explicit flag to control their buffering behavior,
typically -i for interactive, or by a special environment variable. 

Would that fix it? Where does the -i flag go?

I am using Emacs 25.2.1 with Evil Spacemacs and Ivy on Linux.

UPDATE_1

The interactive program works when shell-commend is called with asynchronously option:

SPC SPC (counsel-M-x) async-shell-command

Enter a number: 2
howmany=2

From Emacs help describe-function shell-command:

shell-command is an interactive compiled Lisp function in ‘simple.el’.

...

If COMMAND ends in ‘&’, execute it asynchronously.
The output appears in the buffer ‘*Async Shell Command*’.
That buffer is in shell mode.  You can also use
‘async-shell-command’ that automatically adds ‘&’.

Otherwise, COMMAND is executed synchronously.  The output appears in
the buffer ‘*Shell Command Output*’.

Why would interactive program not work on a synchronous system? https://en.wikipedia.org/wiki/Asynchronous_system

I am thinking of binding async-shell-command to SPC !. Is there a disadvantage to always running async-shell-command instead of the default shell-command?

Answer 1

async-shell-command works with interactive programs. async-shell does not.

Emacs has OS command-line interfaces and terminal emulators. It's not always obvious which one to use. Here is a comparison: http://ergoemacs.org/emacs/emacs_shell_vs_term_vs_ansi-term_vs_eshell.html

When to use a dedicated terminal: http://ergoemacs.org/emacs/emacs_shell_vs_terminal.html