reading from stdin

hello

I can't figure out how to make a program which won't get paused reading from the stdin.
Here is an example:

Code:

---

string input_line;

while (true) {

  if ("stdin is not empty") {
      cin >> input_line;
  }

  ... (some code to execute continuously)
}

---

I'm going to execute the program under linux so:

Code:

---

$ tail -f file.txt | ./program

---

thanks in advance for help

>if ("stdin is not empty") {
There's really no portable way to do this without blocking for input if the stream really is empty. But if you search for kbhit, you can probably find some suitable implementation-dependent solutions.

Hi Prelude

I've looked about kbhit()
That function (wich seems being working only under borland-family compilers) concerns the key management, that is not what I'm searching for.
A web research sends me around keyboard-input related functions, which I already know.

My program needs to do his own stuff until it will check that the stdin is not empty.

Thanks for the answer anyway :)

edit: I don't need it to be portable, at the moment, I'm working under linux

Then I don't understand your problem. Can you describe it in a different way?

ok

Imagine a server software that manages, say, a chat network,
running in background.
If I want to send some messages to the server (like restart or shutdown, etc.)
I should do that through a temporary file, which the program will read everytime I add a new line to it.

Using for example "cin.get ()" will pause the server, until I send it a new message.

So I'd need something like "cin.empty ()" to check if the server is receiving any message from the stdin or not.

Hope it is clearer now. ;)

I see. Well it's not portable, so I can't guarantee that it'll work for you, but try in_avail:

Code:

---

if ( cin.rdbuf()->in_avail() > 0 ) {
  cin >> input_line;
}

---

What about cin.peek()? I'm not sure if peek() blocks or just returns EOF if nothing is in the buffer?

>What about cin.peek()?
peek is built around sgetc, which calls underflow if it can't read anything, which isn't guaranteed not to block.

in_avail should work, as c++ reference says:
http://www.cplusplus.com/reference/i.../in_avail.html

but my test program didn't work...
any suggestion about?

edit: looking for peek ()

Code:

---

// test.cpp
int main ()
{
        string line;

        cout << "Input: ";
        cin >> line;

        cout << "in_avail: " << cin.rdbuf ()->in_avail () << endl;

        return 0;
}

---

shell output:

Code:

---

$ g++ -o test test.cpp
$ ./test
Input: hello
in_avail: 0

---

On linux you could use select().
You probably have to set stdin to unbuffered mode.

checked out: peek () blocks the program.

Quote:

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You probably have to set stdin to unbuffered mode.

---

what does it mean? :confused:

Quote:

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Originally Posted by carlorfeo

in_avail should work, as c++ reference says:
http://www.cplusplus.com/reference/i.../in_avail.html

---

It says that in_avail calls showmanyc(). That function can check if underflow is possible, and return the number of available characters. However there is no requirement that cin implements this functionality; the default behavior is simply to return 0.

You should be able to do this using "select()", perhaps something like this:

Code:

---

bool checkifdata(FILE *f)
{
  int fno = fileno(f);
  fd_set fdset;
  FD_ZERO(fdset);
  FD_SET(fno, fdset);
 
  return select(fno+1, fdset, NULL, NULL, NULL);
}

---

Sorry if this is a late reply, but I got sidetracked.

--
Mats

Quote:

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Originally Posted by King Mir

It says that in_avail calls showmanyc(). That function can check if underflow is possible, and return the number of available characters. However there is no requirement that cin implements this functionality; the default behavior is simply to return 0.

---

Ok thanks, I don't know what "underflow" means exactly, but I figured out what the matter is.

-----------------------------
select() seems to be the right way, checking it out...

Quote:

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Originally Posted by carlorfeo

what does [unbuffered] mean?

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matsp and other linux people can explain that. I haven't worked on unix for quite a few years. And I'm just guessing that stdin would have to be unbuffered for what you want. That may not be the case.