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

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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