Why mkfifo behaves like a LIFO?

Question

If i do.

mkfifo /tmp/a
echo 'one'>/tmp/a

in the while from another terminal

echo 'two'>/tmp/a

and from a third terminal

more /tmp/a

Why i obtain as output of the last command this?

two
one

Answer 1

In

echo 'one' > /tmp/a

The shell does an open(O_WRONLY) on the pipe and then spawns echo which then does the write("one\n").

The open will block until some other process opens the pipe in RD_ONLY or RD_WR though.

And so will the open from your echo two.

So at the moment you do more /tmp/a you've got two processes ready to fire that have not opened the fifo yet let alone written anything to it. Which of those two will be scheduled as soon as more does the open(RD_ONLY) is going to be random.

To avoid blocking, you could do:

exec 3<>  /tmp/a

to unlock the pipe first, and then run your commands which won't block until the pipe is full.

Note however that the above will work on Linux but not on every Unix or Unix-like. The behaviour when opening a pipe in read-write mode is unspecified by POSIX.