Use of __attribute__((noreturn)) for the exit function declaration in 'user.h' of the xv6 source code?

Question

In the user.h

https://github.com/mit-pdos/xv6-riscv/blob/a1da53a5a12e21b44a2c79d962a437fa2107627c/user/user.h#L6

exit is only syscall defined this way int exit(int) __attribute__((noreturn)); why this is needed for the exit function declaration?

Answer 1

I don't think the noreturn attribute is strictly required, but it helps.

The noreturn attribute tells the compiler that the exit function will never return. In theory, this allows the compiler to better understand the possible code paths of any code that calls exit, so it can display more accurate warnings and optimize the code better.