Linux Pipes and FIFOs David Ferry, Chris Gill CSE 522S - Advanced Operating Systems Washington University in St. Louis St. Louis, MO 63130

Pipes Pipes are a common way to move data between programs, e.g.: cat filename | grep “search_string” Linux provides pipe() system call that allows inter-process communication Command above relies on shell “pipelines” which do not necessarily use pipe() CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Linux’s Pipes The Linux pipe() system call: Allows anonymous (non- named) communication Is unidirectional Produces / consumes data through file syscalls write() and read() Data stored in kernel via pipefs filesystem See /fs/pipe.c for implementation Process A Process B Write FD Pipe Imp. Read FD CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Pipe() Semantics Pipes are only usable between related processes: pipe() creates a read and write descriptor Process forks Reader deletes write FD, writer deletes read FD Reader reads, writer writes Process A Write FD Read FD fork() Process B Write FD Read FD CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems FIFOs (Named Pipes) Variant to pipes: Handle to FIFO exists as a regular file Read and written like regular file Data is stored in kernel (not disk) Allows non-related processes to communicate Supports multiple readers & writers Must be open at both ends before reading or writing CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Pipe and FIFO Limits Atomicity: I/O is atomic for data quantities less than PIPE_BUF Typical: PIPE_BUF = 4096 bytes Write capacity: Typically 64K Writers block or fail if pipe is full Polling vs. Blocking: Readers may block or fail based on flags set during pipe creation CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems FIFOs vs. Files Even though FIFOs have a handle in the regular file system, they are not files! Files backed by a real filesystem FIFOs not backed Files have no atomicity guarantee FIFOs must be opened for reading and writing before either may occur Files have no practical capacity limit CSE 522S – Advanced Operating Systems

CSE 522S – Advanced Operating Systems Pipe Paradigms Pipes are useful for implementing many design patterns and idioms: Producer / Consumer Client / Server Active Object Process A Pipe Process B Process A Process B FIFO Process D Process C Pipe Process A Process B Pipe CSE 522S – Advanced Operating Systems