Differences in the initialization of the EAX register when calling a function in C and C++

Question

There is a curious difference between assemblies of a small program, when compiled as a C-program or as a C++-program (for Linux x86-64).

The code in question:

Answer 1

In C int fun(); can take any number of arguments, so it may even be a varargs function. In C++ however it means it takes no arguments.

The x86-64 sysv abi convention demands that the register AL must contain the number of SSE registers used when invoking a varargs function. You of course pass no argument, so it is zeroed. For convenience the compiler decided to zero the whole eax. Declare your prototype as int fun(void); and the xor shall disappear.