1

Reopen voters: flock() doesn't work on named pipes, this is not a dupe.

I was trying to do the following with named pipes:

mkfifo example.txt
( printf "456"; printf "abc"; printf "\n"; ) > example.txt &
( printf "456"; printf "abc"; printf "\n"; ) > example.txt &
( printf "456"; printf "abc"; printf "\n"; ) > example.txt &

If I cat the example.txt sometimes I got: 

456abc
456abc
456abc

Other times I got:

456456456abcabc

abc

I was hoping the writes would be atomic. Is there any way to achieve atomic writes?

Gounou
  • 549
  • 3
  • 5
Chris Stryczynski
  • 5,178
  • 5
  • 40
  • 80
  • 1
    Writes lesser than the page size (nearly always 4096 byte) are atomic. So use `printf "456abc\n"` and it will be atomic. The answer in the other question, is in your case unusable. – peterh Oct 13 '18 at 16:30
  • 1
    The answer in the other question describes using a lockfile, a general solution that is not affected by what the interlocked operations are, whether they be writes to a FIFO or to something else, or indeed whether they even be writes at all. This is, indeed, a duplicate and existing answers at the duplicate address it. – JdeBP Oct 13 '18 at 17:10
  • 2
    Why do you say "`flock()` doesn't work on named pipes"? I just tried it and it seemed to work fine. – Joseph Sible-Reinstate Monica Oct 13 '18 at 17:30
  • 1
    Ditto. I believe that I got the accepted answer from the other question to work with a FIFO. You might get more sympathy / support if you posted either an MCVE that demonstrates that it doesn’t work, or a reference that says that it won’t work. (But I doubt that you’ll find the latter, since at least two of us have gotten it to work.) – G-Man Says 'Reinstate Monica' Oct 13 '18 at 19:13
  • note: your approach to debugging the problem hides some errors .... never repeat the same test data multiple times unless you specifically have repeat the same data ..... use different data in each of the lines (for example: abc123, def456, xyz789) – jsotola Oct 13 '18 at 20:10
  • @G-Man I don't think the OP needs your sympathy (btw, the plea to close voters wasn't by him, but by a later editor), but a proper explanation for that behaviour (which is not caused by the writes not being atomic, because they are), and how to use `flock` with fifos (in his example, he should use `(flock 1; printf ...` -- obvious things like `mkfifo ff; flock ff -c 'echo yes > ff' & cat ff` really do **not** work. –  Oct 13 '18 at 22:52
  • @JosephSible the flock(1) *utility* when used with a file name argument will try to open it with `O_RDONLY` which will lock hard if you're calling it from the fifo writer (it will wait until the writing side of the fifo is opened, which will not happened until the lock is acquired) –  Oct 13 '18 at 23:06
  • Well I haven't gotten this to work, I also disagree that this should be closed. It's not obvious that named pipes work the same way as files (in fact that should just be the answer)... – Chris Stryczynski Oct 14 '18 at 09:19
  • @ChrisStryczynski you can get it to work by adding a `flock 1;` before each `printf` as I already suggested in a comment. –  Oct 15 '18 at 03:21
  • @ChrisStryczynski ... before the first `printf` from each subshell, eg. `( flock 1; printf "456"; printf "abc"; printf "\n"; ) > example.txt &` –  Oct 15 '18 at 03:30

0 Answers0