Unreliable endianness of retrieved data

The method void MPU6050::calcGyroOffsets() contains the following line:

rx = wire->read() << 8 | wire->read();

and similar lines for ry and rz. The same idiom is used in MPU6050::update().

This kind of expression should be avoided, as it does not guarantee the endianness of the retrieved data. This is because, in C++, the order of evaluation of the arguments of the | operator is unspecified.

Presumably, the current version of gcc, when invoked with the options passed by the current version of the Arduino IDE, generates assembly that orders the bytes in the expected way. But this cannot be relied upon, and can very well depend on the compiler, compiler version, compiler options, or even surrounding code. This is the kind of software error that cannot be unveiled by testing, as the code can “just work” even if it is incorrect.

The simplest solution is to issue the calls to wire->read() within separate statements. Unlike the arguments of the | operator, separate statements have a well define order of evaluation:

rx = wire->read() << 8;
rx |= wire->read();

tockn / MPU6050_tockn

Unreliable endianness of retrieved data #42