问题：QDataStream中写入的数据比QByteArray多了4个byte。

仔细看了下，这个4个byte表示的是QByteArray的数据长度。

转载官方文档：https://doc.qt.io/qt-6/qdatastream.html

QDataStream &QDataStream::readBytes(char *&s, uint &l)

Reads the buffer s from the stream and returns a reference to the stream.

The buffer s is allocated using new []. Destroy it with the delete [] operator.

The l parameter is set to the length of the buffer. If the string read is empty, l is set to 0 and s is set to nullptr.

The serialization format is a quint32 length specifier first, then l bytes of data.

int QDataStream::readRawData(char *s, int len)

Reads at most len bytes from the stream into s and returns the number of bytes read. If an error occurs, this function returns -1.

The buffer s must be preallocated. The data is not decoded.

QDataStream &QDataStream::writeBytes(const char *s, uint len)

Writes the length specifier len and the buffer s to the stream and returns a reference to the stream.

The len is serialized as a quint32, followed by len bytes from s. Note that the data is not encoded.

int QDataStream::writeRawData(const char *s, int len)

Writes len bytes from s to the stream. Returns the number of bytes actually written, or -1 on error. The data is not encoded.

注意到  << 操作符 也是用的writeBytes同样的逻辑

QDataStream &QDataStream::operator<<(const char *s)

This is an overloaded function.

Writes the '\0'-terminated string s to the stream and returns a reference to the stream.

The string is serialized using writeBytes().

所以会多写一个int32在开始。

为了避免这种情况的产生，可以调用对应 W/R RawData 方法。