How to check if stdin is empty before clearing anything left in stdin

**mweston379** · 07-04-2019

I have been using the code...
while (getchar() != '\n');
...to clear everything in stdin in my programs. However, I am running into a slight issue on some occasions. There are times in my programs where when I call getchar(), there may or may not be anything in stdin. When it is the case that there is nothing in stdin, what ends up happening is getchar() is called and it is waiting for something in stdin. I then have to enter \n on my keyboard otherwise the program is just stuck on that one line. I am wondering if there is a simple way to first check if stdin is empty before doing while (getchar() != '\n'); in order to avoid this problem?

Not sure if the compiler matters for the answer, but I am using Visual Studio 2017.

**flp1969** · 07-04-2019

I don't have the slightest idea on how to do this on Windows (and I don't care!). To check if the standard input has any data available, in Linux, is simple:

Code:

struct pollfd pfd = { .fd = fileno( stdin ), .events = POLLIN };
if ( poll( &pfd, 1, 0 ) > 0 )
  if ( pfd.revents & POLLIN ) { ... there is data in stdin ... }

On other Unix systems you can use select()...

**GReaper** · 07-04-2019

There is no portable way for you to do this. The thing most programmers do is to always read lines with fgets(), and then parse the lines with sscanf().

Now, this may appear stupid, as many and all resources and tutorials tell you to avoid this, but in Windows, as per the Microsoft Docs, fflush() can be used to clear the input stream.

**rstanley** · 07-04-2019

Originally Posted by GReaper

There is no portable way for you to do this. The thing most programmers do is to always read lines with fgets(), and then parse the lines with sscanf().

Now, this may appear stupid, as many and all resources and tutorials tell you to avoid this, but in Windows, as per the Microsoft Docs, fflush() can be used to clear the input stream.

If in fact, Widoze does allow "fflush(stdin)", DON"T DO IT!

It will make your code non-portable, as with using ANY Non-Standard MS extension to the C Programming Language!!!

Please stick to the C11 C Standard!

**Morgan854** · 07-05-2019

This post is so good to see very helpful for me.

**mweston379** · 07-05-2019

Ok, thanks for the response. Unfortunately I'm only 1 semester in to my CS degree, so none of that code makes sense to me, nor do I know what select is()...Do you know of any simpler ways? If not that's okay, I will come back to your code in the future...my schoool does use Linux at some point.

**mweston379** · 07-05-2019

Originally Posted by flp1969

I don't have the slightest idea on how to do this on Windows (and I don't care!). To check if the standard input has any data available, in Linux, is simple:

Code:

struct pollfd pfd = { .fd = fileno( stdin ), .events = POLLIN };
if ( poll( &pfd, 1, 0 ) > 0 )
  if ( pfd.revents & POLLIN ) { ... there is data in stdin ... }

On other Unix systems you can use select()...

Ok, thanks for the response. Unfortunately I'm only 1 semester in to my CS degree, so none of that code makes sense to me, nor do I know what select is()...Do you know of any simpler ways? If not that's okay, I will come back to your code in the future...my schoool does use Linux at some point.

**flp1969** · 07-05-2019

Originally Posted by mweston379

Ok, thanks for the response. Unfortunately I'm only 1 semester in to my CS degree, so none of that code makes sense to me, nor do I know what select is()...Do you know of any simpler ways? If not that's okay, I will come back to your code in the future...my schoool does use Linux at some point.

If you use Debian (or Ubuntu), install manpages-dev package:

Code:

$ sudo apt install manpages-dev

And, afterwards:

Code:

$ man poll

or

Code:

$ man select

With manpages-dev package you can get almost all functions manuals (at volume 3 - and there are lots of useful info on volume 7 of manuals... you specify the volume by number, as in 'man 3 printf' -- since 'man printf' will get you the command printf [from shell]).

There are lots of useful info on a manual page: Pay attention to DESCRIPTION, RETURN VALUE, ERRORS and SEE ALSO topics -- sometimes there is an EXAMPLES topic (poll has one!).

Here's an example of 'man poll':
How to check if stdin is empty before clearing anything left in stdin-untitled-jpg

How to check if stdin is empty before clearing anything left in stdin-untitled-jpg

**rstanley** · 07-05-2019

@flp1969

Actually, if the man pages for the Standard Library are not installed during the basic installation of a Debian or Ubuntu Distro, the package, "build-essential", should install all that is needed for C development, including the man pages.

**flp1969** · 07-05-2019

Originally Posted by rstanley

@flp1969

Actually, if the man pages for the Standard Library are not installed during the basic installation of a Debian or Ubuntu Distro, the package, "build-essential", should install all that is needed for C development, including the man pages.

Maybe, but I don't think so:

Code:

$ apt depends build-essential
build-essential
 |Depends: libc6-dev
  Depends: <libc-dev>
    libc6-dev
  Depends: gcc (>= 4:7.2)
  Depends: g++ (>= 4:7.2)
  Depends: make
    make-guile
  Depends: dpkg-dev (>= 1.17.11)

No manpages-dev there! Even if you chek for the dependencies for libc6-dev...

**Click_here** · 07-06-2019

If you are using Windows you can use conio.h - > kbhit()

On Windows it is really easy to see if something is on stdin before trying to read

Code:

#include <conio.h>
... 
if (kbhit()) 
{
  // something on stdin
}

**Click_here** · 07-06-2019

Oh I see - You are not checking for EOF...

Code:

#include <stdio.h> 

int main(void) 
{
 int ch;
 char buf[BUFSIZ];
 puts("Flushing input");
 while ((ch = getchar()) != '\n' && ch != EOF);
...

From here: Flush the input buffer - Cprogramming.com

**rstanley** · 07-06-2019

Originally Posted by flp1969

Maybe, but I don't think so:

Code:

$ apt depends build-essential
build-essential
 |Depends: libc6-dev
  Depends: <libc-dev>
    libc6-dev
  Depends: gcc (>= 4:7.2)
  Depends: g++ (>= 4:7.2)
  Depends: make
    make-guile
  Depends: dpkg-dev (>= 1.17.11)

/usr/share/doc/build-essential/list'

No manpages-dev there! Even if you chek for the dependencies for libc6-dev...

"build-essential" contains a list of "dependencies" that trigger the installation of all that is needed for a basic C & C++ development environment. One of those "dependencies" will trigger the installation of the man-pages.

I have done MANY Debian installations, and I have NEVER needed to manually install the man-pages needed. If a package installed has associated man-pages, it will list the man-pages package as a dependency!

Do a basic installation of Debian or Ubuntu, and then install one package, "build-essential", and watch all the packages that are installed, as dependencies of "build-essential" and subsequent packages.

**rstanley** · 07-06-2019

Originally Posted by Click_here

If you are using Windows you can use conio.h - > kbhit()

On Windows it is really easy to see if something is on stdin before trying to read

Code:

#include <conio.h>
... 
if (kbhit()) 
{
  // something on stdin
}

Once again, I don't recommend the use of conio.h or ANY MS extension to the C Programming Language!

You are locking yourself into a Windows compiler environment. The code CANNOT be ported to Linux or any other non-windows compiler, without replacing the MS specific functions.

Please stick to Standard C, and please advise others to do so as well!!!

**laserlight** · 07-06-2019

As per rstanley's firm insistence, please stick to standard C, and hence ignore flp1969's post #2. Both poll and select are non-standard with respect to C11.

Besides Click_here's mention about EOF that doesn't help differentiate what you're looking to do, the only standard answer you have received thus far is this one:

Originally Posted by GReaper

The thing most programmers do is to always read lines with fgets(), and then parse the lines with sscanf().

Another answer is that you can simply use thst loop only after a read that leaves a newline in the input buffer.

Note that fgets won't necessarily read the entire line as it could be too long, so you might need to use it in a loop until a '\n' is stored by fgets. Some people might suggest the use of POSIX getline instead, but alas, that is non-standard.

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