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System Programming: Communication through pipes Raymond Namyst Dept. of Computer Science University of Bordeaux, France https://gforgeron.gitlab.io/progsys/ 1
System Programming:Communication through pipes
Raymond NamystDept. of Computer Science
University of Bordeaux, France
https://gforgeron.gitlab.io/progsys/
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The concept of pipe
• Major mechanism used by the shell• ls | grep pattern• ./prog | cat -n | less• Etc.
• Some operating systems (MS-DOS) implement pipes using files• Example: ls | grep pattern
• The output of “ls” is redirected to a temporary file• The system waits for the termination of “ls”• “grep pattern” is executed, with its input redirected from the file
• No parallelism• Max file size limit can be reached
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The concept of pipe
• In Unix systems, pipes are special objects allocated in the kernel• FIFO ordering• Fixed capacity
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The concept of pipe
• In Unix systems, pipes are special objects allocated in the kernel• FIFO ordering• Fixed capacity
• The “pipe” syscallint pipe(int fildes[2]);
• Creates a pipe and returns two file descriptors• One for reading (fildes[0]) and one for writing (fildes[1])
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fildes[1] fildes[0]
The concept of pipe
• In Unix systems, pipes are special objects allocated in the kernel• FIFO ordering• Fixed capacity
• The “pipe” syscallint pipe(int fildes[2]);
• Creates a pipe and returns two file descriptors• One for reading (fildes[0]) and one for writing (fildes[1])
• Reading and writing are done using usual read/write• No lseek
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write read
int tube[2];pipe (tube);
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process
filedescriptor
table
0
12
3
4
5
6
7
…
3tube
Opened files table
Mode: RD_ONLYOffset: 0Ref counter: 1File info:
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Mode: WR_ONLYOffset: 0Ref counter: 1File info: write read
int tube[2];pipe (tube);
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process
filedescriptor
table
0
12
3
4
5
6
7
…
3tube 4
write read
Simplified view
Trying out our first pipe
• pipe.c
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Pipe are intended to allow communication between processes• But pipes created with “pipe” are anonymous
• No way to share their name• Look at mkfifo if named pipes are really what you need
• Fortunately, pipes are inherited when forking new processes…
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int tube[2];pipe (tube); fork();
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parent
filedescriptor
table
0
12
3
4
5
6
7
…
3tube 4
child
0
12
3
4
5
6
7
…
3tube 4
write read
Sending characters through a pipe
• Hand-made implementation of• Line numbering + to upper case
• pipe-n-fork.c
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father child
file
Touppercat -n
stdout
The concept of pipe
• When both sides of a pipe are opened• read is blocking if the pipe is empty• write is blocking if the pipe is full
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The concept of pipe
• When both sides of a pipe are opened• read is blocking if the pipe is empty• write is blocking if the pipe is full
• Pipe closed on the write side• read returns 0 (end of file)
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The concept of pipe
• When both sides of a pipe are opened• read is blocking if the pipe is empty• write is blocking if the pipe is full
• Pipe closed on the write side• read returns 0 (end of file)
• Pipe closed on the read side• write raises an exception (“Broken pipe”)
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Sending characters through a pipe
• Redirections to only use STDIN/STDOUT • pipe-n-redir.c
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father child
file
Touppercat -n
stdout
Sending characters through a pipe
• Redirections to only use STDIN/STDOUT • Exec to use legacy
• cat -n• tr a-z A-Z
• pipe-n-exec.c
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father child
file
tr a-z A-Zcat -n
stdout
Sending characters through a pipe
• Problem:• Child still output characters after
shell prompt
• One solution would be to swap roles of father & child…
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father child
file
tr a-z A-Zcat -n
stdout
Sending characters through a pipe
• Problem:• Child still output characters after
shell prompt
• But the shell acts differently: it creates two childs!
• ultimate-pipe.c
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Child 1 Child 2
file
tr a-z A-Zcat -n
stdoutFather
fork fork
Setting up a chain of processes
• chain.c
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P1 PN-1
Cmd N-1Cmd 1
stdout
P0
Cmd 0
stdin
…
Atomicity of read/write
• What happens if multiple processes simultaneously write into (or read from) the same pipe?• Atomic if size < PIPE_BUF
• (typically 512 bytes)
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writeread
writeread
read
Additional resourcesavailable on
http://gforgeron.gitlab.io/progsys/
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