Transcript Inter-process communication: pipes

Linux Pipes and FIFOs
David Ferry, Chris Gill
CSE 522S - Advanced Operating Systems
Washington University in St. Louis
St. Louis, MO 63130
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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
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Linux’s Pipes
The Linux pipe() system call:
Process A
• Allows anonymous (nonnamed) communication
Write FD
• Is unidirectional
Pipe Imp.
• Produces / consumes
Read FD
data through file syscalls
write() and read()
• Data stored in kernel
Process B
via pipefs filesystem
See /fs/pipe.c for implementation
CSE 522S – Advanced Operating Systems
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Pipe() Semantics
Pipes are only usable between related
processes:
1) pipe() creates a read
Write FD
Process A
and write descriptor
Read FD
2) Process forks
fork()
3) Reader deletes write FD,
writer deletes read FD
Write FD
4) Reader reads,
Process B
Read FD
writer writes
CSE 522S – Advanced Operating Systems
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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
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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
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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
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Pipe Paradigms
Pipes are useful for implementing many
design patterns and idioms:
Producer / Consumer
Process A
Pipe
Process B
FIFO
Process D
Process A
Client / Server
Process B
Process C
Active Object
Pipe
Process A
Pipe
CSE 522S – Advanced Operating Systems
Process B
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