Checking available input on stdin

04-22-05, 11:21 AM

Lionel B

I know this has probably come up frequently, but couldn't find a satisfactory reference... I have some code which needs
to read from stdin but must not block waiting for input if there is no input pending on stdin. I have some code which
does the job (at least on my system... I wonder about the portability, though) but it's an ugly C hack; in particular I
am not happy about mixing C and C++ style I/O.

Simplified version:

--- BEGIN CODE: test.cpp ---

#include <cstdlib>
#include <iostream>
#include <sstream>

bool inavail(FILE* stream)
{
long int nchars;
if (fseek(stream,0,SEEK_END)!=0) return false;
if ((nchars=ftell(stream))<0) return false;
if (fseek(stream,0,SEEK_SET)!=0) return false;
return (nchars>0);
}

using namespace std;

int main()
{
ostringstream oss;
if (inavail(stdin)) oss << cin.rdbuf();

cout << '[' << oss.str() << ']';

return EXIT_SUCCESS;
}

--- END CODE ---

--- test.txt ---
foo
bar
-------------------

"Correct" results:

$ test < test.txt <RET>
[foo
bar]

$ test <RET>
[]

(no user intervention required; if the call to inavail() is omitted, then the second run waits for user input and EOF)

I guess I would be happy with a C++ equivalent of my inavail() function... the name is a giveaway: couldn't seem to get
anything useful out of streambuf::in_avail().

Any hints much appreciated,

04-22-05, 10:04 PM

David Harmon

On Fri, 22 Apr 2005 11:21:50 +0100 in comp.lang.c++, "Lionel B"
<me> wrote,
>I know this has probably come up frequently, but
>couldn't find a satisfactory reference...

That is because there is no satisfactory answer.

04-23-05, 09:38 AM

Lionel B

David Harmon wrote:
> On Fri, 22 Apr 2005 11:21:50 +0100 in comp.lang.c++, "Lionel B"
> <me> wrote,
>
>>I know this has probably come up frequently, but
>>couldn't find a satisfactory reference...
>> That is because there is no satisfactory answer.

Possibly not... nevertheless I would be happy to replicate the
functionality of my (essentially C) inavail() function in a more C++
style - perhaps using streambuf. On the grounds of "Anything You Can Do
In C You Can Do In C++ (tm)" this ought to be possible...

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