Re: is cin always the keyboard's input?
From: tom_usenet (tom_usenet_at_hotmail.com)
Date: 10/15/03
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Date: Wed, 15 Oct 2003 18:33:15 +0100
On Wed, 15 Oct 2003 16:18:53 +0300, Bob Smith <bobsmith@jippii.fi>
wrote:
>
>
>stephan beal wrote:
>
>> Bob Smith wrote:
>>
>>
>>>question in subject.
>>>does the standard say anything about this or is it platform dependent?
>>>any advice much appeciated
>>>
>>
>> Indeed, the whole point of 'cin' is to keep this detail hidden from you. It
>> is not only platform-dependent, but case-dependent.
>> e.g., when you do any of the following:
>>
>> cat file | myapp
>> myapp < file
>>
>> these are instructing the system to use the file (more specifically, the
>> input stream) as cin.
>>
>>
>> See section 15.17 in:
>> http://www.parashift.com/c++-faq-lite/input-output.html
>
>
>well here is my problem:
>m_reading is a bool value, qDebug(...) is a qt function, qxevent is a
>class which can red from streams.
><snip>
>
> if ( !m_reading ){
> if ( std::cin.rdbuf()->in_avail() ){
in_avail will generally be 0 if you haven't attempted a read operation
- it usually just returns the number of characters buffered by the
underlying streambuf. You need to
a) Set stdin to non-blocking mode (in an OS specific way)
b) Try reading something from cin to fill the buffer.
> m_reading = true;
> qDebug( "reading...");
> QxEvent e;
> while( cin >> e ) {
> m_event_queue.push( e );
> counter++;
> }
Here the stream will be in an error state (eof). All operations on
such a stream will fail. Add here:
if (cin.eof())
{
cin.clear();
}
else
{
//something serious has gone wrong.
qDebug(strerror(errno)); //or whatever.
throw something;
}
> qDebug( "...read!");
> m_reading = false;
> }else{
> qDebug("..no data" );
> }
> }else{
> qDebug("already reading, return to caller");
> }
> qDebug("ready");
></snip>
>
Here's my take on the code:
//set stream to non-blocking
if ( !m_reading ){
m_reading = true;
qDebug( "reading...");
QxEvent e;
while( cin >> e ) {
m_event_queue.push( e );
counter++;
}
if (cin.eof())
{
cin.clear(); //clear error state
}
else
{
throw serious_error();
}
qDebug( "...read!");
m_reading = false;
}else{
qDebug("already reading, return to caller");
}
qDebug("ready");
Alternatively, just set the stream to blocking so that process b waits
for data. You could always put the busy wait in its own thread in b.
Usually you use the posix "select" call to handle this kind of code.
Tom
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