About returning more than one value in C/C++/Assembly

Question

I have read some questions about returning more than one value such as What is the reason behind having only one return value in C++ and Java?, Returning multiple values fro

Answer 1

Yes, this is sometimes done. If you read the Wikipedia page on x86 calling conventions under cdecl:

There are some variations in the interpretation of cdecl, particularly in how to return values. As a result, x86 programs compiled for different operating system platforms and/or by different compilers can be incompatible, even if they both use the "cdecl" convention and do not call out to the underlying environment. Some compilers return simple data structures with a length of 2 registers or less in the register pair EAX:EDX, and larger structures and class objects requiring special treatment by the exception handler (e.g., a defined constructor, destructor, or assignment) are returned in memory. To pass "in memory", the caller allocates memory and passes a pointer to it as a hidden first parameter; the callee populates the memory and returns the pointer, popping the hidden pointer when returning.

^{(emphasis mine)}

Ultimately, it comes down to calling convention. It's possible for your compiler to optimize your code to use whatever registers it wants, but when your code interacts with other code (like the operating system), it needs to follow the standard calling conventions, which typically uses 1 register for returning values.