5

I've come across various articles and SO questions and I am still confused about something that I use on daily basis, but never realized how confusing it can be. I am experimenting with (named) pipes in Linux.

1st try was simple: figure out how pipe buffers are working:

#1
mkfifo /tmp/mypipe
#2
echo "Hello World" >/tmp/mypipe
ctrl+c
#3
cat /tmp/mypipe

Observation: When I killed echo before cat reads the data nothing was written to pipe (cat keeps running but nothing was read from pipe). I was assuming that when you type producent >named_pipe and you will exit producent then part of data that match pipe buffer size will be written to named_pipe and will remain here until it will be read by consument (now I know that this is not how it works). So what I did next was:

2nd try was to connect consument to other end of pipe:

#1
mkfifo /tmp/mypipe
#2
echo "Hello World" >/tmp/mypipe
#3
cat /tmp/mypipe

Observation: cat command displays the "Hello World" message and both processes ends. The interesting discovery here was that during the #2 step ps -elf does not display the echo command. It seems that echo is waiting until somebody will read from pipe and this is explanation why nothing was printed to pipe in my first attempt.

3rd try was to pipe command that will run "forever" and constantly write to pipe and see what will happened:

#1
mkfifo /tmp/mypipe
#2
yes >/tmp/mypipe
#3
cat /tmp/mypipe

Observation: This worked as expected and cat printed out what yes forwarded to pipe. However I have tried to replace cat with tail -f. When I did this then tail did not print anything until the yes command was killed.

4th try is the big mystery:

# 1#
mkfifo /tmp/mypipe

# 2#
for i in $(seq 1 10000); do echo -n $i"|"> /tmp/mypipe; done

# 3#
for i in $(seq 1 10); do echo "${i}# Read:"; cat /tmp/mypipe && echo ""; done

After this the 3# command start typing something like that:

1# Read:
1|2|3|4|5|6|7|8|9|10|11|12|13|14|15|16|17|18|19|20|21|22|23|24|25|26|27|28|29|30|31|32|33|34|35|36|37|38|39|40|41|42|43|44|45|46|47|48|49|50|51|52|53|54|55|56|57|58|59|60|61|62|63|64|65|66|67|68|69|70|71|72|73|74|75|76|77|78|79|80|81|82|83|84|85|86|87|88|89|90|91|92|93|94|95|96|97|98|99|100|101|102|103|104|105|106|107|108|
2# Read:
109|
3# Read:
110|
4# Read:
111|
5# Read:
112|
6# Read:
113|114|115|
7# Read:
116|
8# Read:
117|
9# Read:
118|119|120|121|122|123|124|125|126|127|128|129|130|131|132|133|134|135|136|137|138|139|140|141|142|143|144|145|146|147|148|149|150|151|152|153|154|155|156|157|158|159|160|161|162|163|164|165|166|167|168|169|170|171|172|173|174|175|176|177|178|179|180|181|182|183|184|185|186|187|188|189|190|191|192|193|194|195|196|197|198|199|200|201|202|203|204|205|206|207|208|209|210|211|212|213|214|215|216|217|218|219|220|221|222|223|224|225|226|227|228|229|230|231|232|233|234|235|236|237|238|239|240|241|242|243|244|245|246|247|248|249|250|251|252|253|254|255|256|257|258|259|260|261|262|263|264|265|266|267|268|269|270|271|272|273|274|275|276|277|278|279|280|281|282|283|284|285|286|287|288|289|290|291|292|293|294|295|
10# Read:
296|297|298|299|300|301|302|303|304|305|306|307|308|309|310|311|312|313|314|315|316|317|318|319|320|321|322|323|324|325|326|327|328|329|

Questions:

1st and 2nd try:

  1. Are the named pipes equivalent to classic | pipes as they are knows e.g. from bash in this particular case?
  2. Does producent always wait for consument? If yes then what is purpose of pipe buffers? Is this behavior known as blocking communication?
  3. How does Linux know when the consument is connected to pipe and thus when the communication can happen? I've tried lsof named_pipe but it gives me nothing, where is this information stored? I have also try following and result was that cat cannot read from pipe.

    #1
    mkfifo /tmp/mypipe
    #2
    echo 1 >/tmp/mypipe
    #3
    rm /tmp/mypipe
    #4
    mkfifo /tmp/mypipe
    #5
    cat /tmp/mypipe
    
  4. Is typing: producent >/tmp/mypipe the equivalent of typing command | I mean the situation when somebody wants to pipe one command to another but forget to type another command after pipe (ps in this case also did not show first command)?

3rd try:

  1. What is difference between cat and tail -f in this particular case?

4th try:

  1. What is going on here? Why the chunks of read data are not the exact size? I was expecting output as:

    1# Read: 1| 2# Read: 2| 3# Read: 3|

PS: Also I have tried different order of starting commands (reading first and writing after) but the result was the same.

PPS: I hope this is clear but: Producer = process that writes to pipe. Consumer = process that reads from pipe.

Is this possible explain to guy which has mostly scripting knowledge with bit of C? Thank you very much.

EDIT in reply to: Joe Sewell

  1. OK Clear
  2. 2.

I understand that both run in parallel, or in other words, following two are not the same:

find | less

vs

find > /tmp/file && less /tmp/file

My further observation discovers that, when I run following, HDD is not working seems that it is stopped until less command has enough data to display

find | less

When I hit shifg+g (go to the end of file in less) HDD starts immediately to work and data starts outputting. Does this mean that when less command has enough data to display it will somehow tell find to not produce further data? This is what you mean by synchronization? Also the amount of data writes to pipe corresponds to it buffer size? I have also noticed that find changes it state (ps aux - stat column) from S+ to D+ after I hit shift+g in less

S    interruptible sleep (waiting for an event to complete)
D    uninterruptible sleep (usually IO)
+    is in the foreground process group.

┌─[wakatana@~] [63 files, 178Mb]
└──> ps aux | egrep -w 'less|find'
wakatana     6071  0.0  0.0  12736  1088 pts/5    S+   23:15   0:00 find
wakatana     6072  0.0  0.0   7940   928 pts/5    S+   23:15   0:00 less
wakatana     6183  0.0  0.0   7832   892 pts/6    S+   23:20   0:00 egrep --color=auto -w less|find
┌─[wakatana@~] [63 files, 178Mb]
└──> ps aux | egrep -w 'less|find'
wakatana     6071  0.0  0.0  12808  1304 pts/5    D+   23:15   0:00 find
wakatana     6072  0.0  0.0   9556  2508 pts/5    S+   23:15   0:00 less
wakatana     6193  0.0  0.0   7832   892 pts/6    S+   23:21   0:00 egrep --color=auto -w less|find
  • Who sends this signal, consument to producent? If yes then how consument know that he is connected to the pipe which already has producent (e.g. my example with rm pipe)?

  • OK Clear

  • OK Clear

  • I think that the new lines is not the case that confuses me. Based on my previous observations (and you confirmed that: "Yes, both ends wait for each other."). I was expecting this:

  • I. 1st iteration in 1st loop will write to pipe and because nobody is reading it will wait here.

  • II. When 2nd loop is issued then the data which were written by 1st loop in 1st iteration will be read, nothing more was written here so nothing more can be read.

  • III. 2nd loop will wait for next data to be written by 1st loop or (because order no matter) 1st loop will wait until written data will be read by 2nd loop, and so on and so on.

Because of this I was expecting that one write will corresponds to one read. I was also verifying if loop is not running so I modified a bit original command to see if something will be printed to STDOUT even if consument wont be reading, but nothing was printed.

for i in $(seq 1 10000); do
  if [ $(( $i % 5 )) -eq 0 ]; then
    echo $i;
  else
    echo -n $i"|"> /tmp/mypipe;
  fi;
done

"Since the writing process isn't sending any newlines, the reader simply reads until it's told it got "enough.""

  • Who will tell consument that he's got enough?

"In the first case it's probably because the fifo's buffer filled up,"

  • How can I fill buffer if communication is blocked (as i described above)?

"and therefore got flushed through to the reader."

  • What do you mean by this? Sorry for my english.

"While there are ways to make communication asynchronous ..."

  • Can you please briefly describe what is the difference between asynchronous and synchronous in this case?

closed as unclear what you're asking by jasonwryan, Anthon, Michael Homer, cuonglm, Stephen Kitt Mar 12 '15 at 10:06

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

5

To answer your list of questions by number:

  1. Named pipes, a.k.a. fifos, are essentially equivalent to unnamed pipes generated by the shell. The big difference is that synchronization between the two ends is intuitive with the shell version, while named pipes as you seem to be using them require a bit of knowledge about what the shell is doing for you.

  2. Yes, both ends wait for each other. The purpose of fifos, like shell pipes, is to pass output from one process to the input of another. They are not temporary files. I suspect that's where you're getting confused. In the case of a shell command like cat somefile.txt | less, both commands are running simultaneously as forked processes, with the pipe serving to synchronize the two. This can be modified, if memory serves, in C, but not as easily with shell commands.

  3. Processes can receive signals when the other end of a pipe gets a connection, but more often than not the whole intent, as noted above, is to keep the two processes synchronized. The writer sends something, and it knows it can continue when the write operation completes.

  4. bash and tcsh will not let you pipe "into oblivion." The command isn't even run.

  5. tail -f has to read the entire stream until it gets an EOF, in this case on stdin, before it can display anything. In your experiment the end never appeared. cat, on the other hand, can start working on its input immediately.

  6. Since the writing process isn't sending any newlines, the reader simply reads until it's told it got "enough." In the first case it's probably because the fifo's buffer filled up, and therefore got flushed through to the reader. The subsequent output is probably similar, and will probably vary based on system timing.

Let me address another bit of confusion here. The shell handles redirection before a command is run. That means that you wouldn't see cat in a list of processes, because bash is stuck waiting for the other end of the fifo to connect before running cat or whatever writer is involved. Likewise it won't execute the reading command until the writer is connected.

I think the biggest misunderstanding you have here is that named pipes are not temporary files. Neither are unnamed pipes. While there are ways to make communication asynchronous, it looks like you'd be better off creating actual temporary files under /tmp, unless you do want both processes running simultaneously.

3
find | less

When I hit shift+g (go to the end of file in less) HDD starts immediately to work and data starts outputting. Does this mean that when less command has enough data to display it will somehow tell find to not produce further data?

No. Pipes work on read()s and write()s. less tells find nothing - but the system kernel which owns that pipe will not flush it as long as there is a read()ing process associated with it. So find write()s until it can write() no more because the pipe buffer fills up and the kernel does not accept another write(). So find is blocked. When you skip to the end of the pipe input less read()s the pipe and drains some - (all) - of that pipe buffer so find can now write() again - and so it does. It resumes its search of your disk and attempts to once more fill that buffer or complete its work.

The same is true of named pipes - which are the exact same thing - except that the kernel consents to associating both ends of the pipe with a pseudo-file - a link - somewhere in the filesystem. When you:

mkfifo pipe
echo > pipe &
rm pipe
mkfifo pipe
cat pipe

...you're working with two different pipes. The first was associated with a file-system name that was unlink()ed by rm - but echo still waits for a read()er on its pipe - though it is highly unlikely any process will be able to find the other end now. In fact, you can essentially make a named pipe anonymous in this way while still using it - which is usually best practice for security reasons.

Try:

   mkfifo pipe
   exec 3<>pipe 4<>pipe
   cat  <&3     >out.log &
   rm pipe
   echo read this, cat\! >&4
  • Three questions: 1. whole communication management between processes through pipes is kernel business? 2. Pipe buffer is used when both clients are connected to pipe (e.g. piping to oblivion)? 3. Pipe has it's own capacity and when producent is writing to pipe and consument is not reading then pipe buffer is full and kernel will block producent until consument will read the buffer? – Wakan Tanka Mar 26 '15 at 17:45
  • @WakanTanka - For 1, probably yes. And the same is true for all other files. Basically anything with an inode is the kernel's sole property. The pipe buffer is different though. What you're thinking about is probably the lib c actually - most progs will withhold write()s until they have buffered enough output to make some few kb at least of data per write() - the lib C does that, and it saves on system calls. The kernel also limits the pipe buffer though - pipes are only so big and so the kernel will only guarante atomic writes() of so much data at a time. I think 2|3 are the same q? – mikeserv Mar 26 '15 at 17:52

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