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    <p>As you are using the <a href="http://qt-project.org/doc/qt-4.8/qiodevice.html#read-2" rel="nofollow"><code>QByteArray QIODevice::read(qint64 maxSize)</code></a> function, you may not be detecting errors correctly:</p> <blockquote> <p>This function has no way of reporting errors; returning an empty QByteArray() can mean either that no data was currently available for reading, or that an error occurred.</p> </blockquote> <p>Some things to try:</p> <ul> <li><p>Use the <a href="http://qt-project.org/doc/qt-4.8/qiodevice.html#read" rel="nofollow"><code>qint64 QIODevice::read(char* data, qint64 maxSize)</code></a> which reports errors:</p> <blockquote> <p>If an error occurs ... this function returns -1.</p> </blockquote></li> <li><p>Call <a href="http://qt-project.org/doc/qt-4.8/qiodevice.html#errorString" rel="nofollow"><code>QIODevice::errorString</code></a> and <a href="http://qt-project.org/doc/qt-4.8/qabstractsocket.html#error" rel="nofollow"><code>QAbstractSocket::error</code></a> to find out what is going wrong.</p> <ul> <li>For bonus points, listen to the <code>QAbstractSocket::error</code> error signal.</li> </ul></li> <li><p>If this is a new protocol you are creating, try using <a href="http://qt-project.org/doc/qt-4.8/qdatastream.html" rel="nofollow"><code>QDataStream</code></a> for serialization, this automatically handles length prefixs and is platform independent. Your <code>char_to_int32</code> will break if you mix platforms with different <a href="http://en.wikipedia.org/wiki/Endianness" rel="nofollow">endienness</a>, and may break between different OSs or compilers as <code>int</code> is not guaranteed to be 32 bits (it is defined as <a href="http://en.wikipedia.org/wiki/C_data_types" rel="nofollow">at least 16 bits</a>).</p> <ul> <li>If you can't use <code>QDataStream</code>, at least use the <a href="http://beej.us/guide/bgnet/output/html/multipage/htonsman.html" rel="nofollow">htons, ntohs ...</a> functions.</li> </ul></li> </ul> <p><strong>Edit</strong></p> <p>Here is some example code showing hton/ntoh usage. Note that <code>uint32_t</code> and not <code>int</code> is used as it's guaranteed to be 32 bits. I've also used <code>memcpy</code> rather than pointer casts in the encode/decode to prevent aliasing and alignment problems (I've just done a cast in the test function for brevity).</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;string.h&gt; #include &lt;arpa/inet.h&gt; void encode(uint32_t in, char* out) { /* Host to Network long (32 bits) */ const uint32_t t = htonl(in); memcpy(out, &amp;t, sizeof(t)); } uint32_t decode(char* in) { uint32_t t; memcpy(&amp;t, in, sizeof(t)); /* Network to Host long (32 bits) */ return ntohl(t); } void test(uint32_t v) { char buffer[4]; printf("Host Input: %08x\n", v); encode(v, buffer); printf("Network: %08x\n", *((uint32_t*)buffer)); printf("Host Output: %08x\n\n", decode(buffer)); } int main(int argc, char** argv) { test(0); test(1); test(0x55); test(0x55000000); return 0; } </code></pre>
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    1. COThank you for your answer. Unfortunately, I can't use QDataStream on a server side. How should I use htonl in my case? int32_to_char(htonl(in), char* bytes) or some other way?
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    2. COI've added some example code.
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    3. COThank you! That's really great example. The main problem was solved too: I've tried to use the read(char* data, qint64 maxSize) and it works even without network byte order. Still have no idea what's wrong with QByteArray QIODevice::read(qint64 maxSize).
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