MIT-6.828 | This is an implementation of MIT

The code is an adaptation of the paper Coroutine prime number sieve by M. Douglas McIlroy to the Xv6 operating system, a re-implementation of Version 6 Unix used for teaching. The technique is from 1968 as an experiment in co-routines via pipes. The paper explains the algorithm and rationale.

The program implements the sieve of Eratosthenes as a kind of production line, in which each worker culls multiples of one prime from a passing stream of integers, and new workers are recruited as primes are discovered.

When Unix came to be, my fascination with coroutines led me to badger its author, KenThompson, to allow writes in one process to go not only to devices but also to matching reads in another process.

...the coroutine sieve has been a standard demo for languages or systems that support interprocess communication. Implementations using Unix processes typically place the three coroutines—source, cull and sink—in distinct executable files.The fact that the whole program can be written as a single source file, in a language that supports neither concurrency nor IPC, is a tribute not only to Unix’s pipes, but also to its clean separation of program initiation into fork for duplicating address spaces and exec for initializing them.

I believe using stdin and stdout is an artifact of its origins in the early days of Unix when piping stdin and stdout between processes was first introduced. It makes a lot more sense in shell.