Pipes

The backend can send messages to the user by sending the message to a print supervisor by way of a pipe. This mechanism provides a one-way communication path between the printer backend and the print supervisor.

The print supervisor must open an unnamed pipe and obtain two file descriptors, one for read operations and one for write operations. The print supervisor must export the write end in the PIO_IPCWRITEFD environment variable before calling the printer backend with the _fork and _exec subroutines. If the PIO_IPCWRITEFD environment variable is set, the printer backend writes any messages to the write end of the pipe.

The print supervisor typically calls the _select subroutine to poll the read side of the pipe for incoming messages. In addition to checking for exit status of the printer backend using the _waitpid subroutine, the print supervisor polls for I/O on the pipe. The print supervisor sets up a signal handler for the SIGCHLD signal and performs a block read on the pipe. The signal handler examines the exit status of the printer backend and performs any action necessary. When no unread messages remain on the pipe, the print supervisor closes the pipe and proceeds to other cleanup work.