FIFOs

FIFOs are sometimes called named pipes. Unnamed pipes can be used only between related processes when a common ancestor has created the pipe. With FIFOs, however, unrelated processes can exchange data.

Once we have used mkfifo or mkfifoat to create a FIFO, we open it using open. Indeed, the normal file I/O functions (e.g., close, read, write, unlink) all work with FIFOs.

When we open a FIFO, the nonblocking flag (O_NONBLOCK) affects what happens.

• In the normal case (without O_NONBLOCK), an open for read-only blocks until some other process opens the FIFO for writing. Similarly, an open for writeonly blocks until some other process opens the FIFO for reading.
• If multiple process block on read, and one process write message into FIFO, will trigger all the reader unblock, but only one process and read content
• If O_NONBLOCK is specified, an open for read-only returns immediately. But an open for write-only returns −1 with errno set to ENXIO if no process has the FIFO open for reading.

There are two uses for FIFOs.

1. FIFOs are used by shell commands to pass data from one shell pipeline to another without creating intermediate temporary files.
2. FIFOs are used as rendezvous points in client–server applications to pass data between the clients and the servers.