In yesterday's entry, I talked about how Unix's errno is so limited partly because of how the early Unix kernels didn't return multiple values from system calls. It's worth noting that this isn't a limitation in operating system kernels and typical system call interfaces; instead, it's a limitation imposed by C. If anything, it's natural to return multiple values from system calls.

Typically, system call interfaces use CPU registers because it's much easier (and usually faster) for the kernel to access (user) CPU register values than it is to read or write things from and to user process memory. If you can pass system call arguments in registers, you do so, and similarly for returning results. Most CPU architectures have more than one register that you could put system call results into, so it's generally not particularly hard to say that your OS returns results in the following N CPU registers (quite possibly the registers that are also used for passing arguments).

Using multiple CPU registers for system call return values was even used by Research Unix on the PDP-11, for certain system calls. This is most visible in versions that are old enough to document the PDP-11 assembly versions of system calls; see, for example, the V4 pipe(2) system call, which returns the two ends of the pipe in r0 and r1. Early Unix put errno error codes and non-error results in the same place not because it had no choice but because it was easier that way.

(Because I looked it up, V7 returned a second value in r1 in pipe(), getuid(), getgid(), getpid(), and wait(). All of the other system calls seem to have only used r0; if r1 was unused by a particular call, the generic trap handling code preserved it over the system call.)

I don't know if there's any common operating system today with a system call ABI that routinely returns multiple values, but I suspect not. I also suspect that if you were designing an OS and a system call ABI today and were targeting it for a modern language that directly supported multiple return values, you would probably put multiple return values in your system call ABI. Ideally, including one for an error code, to avoid anything like errno's limitations; in fact it would probably be the first return value, to cope with any system calls that had no ordinary return value and simply returned success or some failure.