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POHow do I read a FIFO/named pipe line by line from a C++/Qt Linux app?
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  1. POHow do I read a FIFO/named pipe line by line from a C++/Qt Linux app?
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    copied!<p>How do I read a FIFO/named pipe line by line from a C++/Qt Linux app?</p> <p>Today I can open and read from a fifo from a Qt program, but I can't get the program to read the data line by line. Qt reads the entire file, meaning he waits until the "sender" closes his session.</p> <p>Let's take a example with some shell commands to show what I would like the app to do.</p> <p>First create a fifo</p> <pre><code>mkfifo MyPipe </code></pre> <p>Then we can use cat to read from the fifo</p> <pre><code>cat MyPipe </code></pre> <p>And then we send some data in with another cat</p> <pre><code>cat &gt; MyPipe </code></pre> <p>And then start to type something, and every time you hit enter it arrives at the reader. And then when you close it with Ctrl+D both sides end.</p> <p>Now the sender is easy to create with a QTextStream, you just need to flush when you want to send.</p> <pre><code>QFile file("MyPipe"); if (!file.open(QIODevice::WriteOnly | QIODevice::Text)) return; QTextStream out(&amp;file); for(int i=0; i&lt;3; i++) { out &lt;&lt; "Hello...: " &lt;&lt; i &lt;&lt; "\n"; out.flush(); sleep(2); } file.close(); </code></pre> <p>But then to write a little reader that read line by line is where I'm stuck right now, all my tries with the Qt lib ends up with that I get the data but not until the sender uses file.close() on the fifo. Not when he flush, as occurs when I use cat to read.</p> <p>Like this example:</p> <pre><code>QFile file("MyPipe"); if (!file.open(QIODevice::ReadOnly | QIODevice::Text)) return 0; QTextStream in(&amp;file); QString line; do { line = in.readLine(); qDebug() &lt;&lt; line; } while (!in.atEnd()); file.close(); </code></pre> <p>What am I missing?</p> <p>It just feels like I need to use some kind of isReady or lineAvailable on the stream or something like that, but I can't find anything in the docs that fits...</p> <p>/Thanks</p> <hr> <p><em>Note</em>: </p> <p>If I go with the low level c style and read one char at the time I do get the style Im searching for. But it would be nice to be able to do the same Qt style.</p> <pre><code>FILE *fp; fp=fopen("MyPipe", "r"); char c; while((c=getc(fp)) != EOF) { printf("%c",c); } fclose(fp); </code></pre> <hr> <p><strong>Update</strong>:</p> <p>When I start a debugger the program is hanging on the readLine(), and do not continue until the other party closes the fifo.</p> <p>And I do get the same using ">>"</p> <pre><code> line = in.readLine(); in &gt;&gt; line; </code></pre>
 

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