They're not so much echoes as a continuing necessity. System calls in UNIX-style operating system APIs generally return two integer values, a file descriptor/c

They're not so much echoes as a continuing necessity. System calls in UNIX-style operating system APIs generally return two integer values, a file descriptor/character count/ID/whatever and an error code; and because the twain are largely mutually exclusive the system call mechanism usually (dependent from the architecture of course) has some way of marshalling that into one processor register, be it using alternative return addresses or a flag bit such as the carry flag in the status word. The OS/2 Control Program API and the Windows NT Native API return the error codes explicitly from the HLL wrapper functions, the (for example) open file handle returned from DosOpen() being an integer passed by reference, and their system call marshalling is thus somewhat different. It helps that calling conventions such as APIENTRY32 (http://homepage.ntlworld.com./jonathan.deboynepollard/FGA/function-calling-conventions.html#APIENTRY32 http://homepage.ntlworld.com./jonathan.deboynepollard/FGA/fu...) lend themselves easily to a treat-the-stacked-arguments-as-a-structure system, moreover. I implemented a similar API once, marshalling by-reference system call arguments through processor registers using a pass-by-value-return scheme.