How can I know whether writing to a named pipe would block?

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I want to write to a named pipe only if it already has a reader. Currently, I'm using timeout to detect whether the attempt to write to the pipe blocked, like so:

#! /usr/bin/env bash
rm -f pipe
mkfifo pipe
sleep 5

timeout 1 bash -c "echo Hello > pipe"

if [[ $? == 0 ]]
then
 echo Somebody got our message
else
 echo Nobody read from pipe, so we didn't send a message
fi

This works. If I tail -f pipe in a separate terminal during the sleep I get one message, and if I don't, I get the other. But is there a better way? Ideally it would be something that doesn't rely on a timeout.

edited 12 hours ago

asked yesterday

MatrixManAtYrService

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up vote
4
down vote

favorite

I want to write to a named pipe only if it already has a reader. Currently, I'm using timeout to detect whether the attempt to write to the pipe blocked, like so:

#! /usr/bin/env bash
rm -f pipe
mkfifo pipe
sleep 5

timeout 1 bash -c "echo Hello > pipe"

if [[ $? == 0 ]]
then
 echo Somebody got our message
else
 echo Nobody read from pipe, so we didn't send a message
fi

edited 12 hours ago

asked yesterday

MatrixManAtYrService

1705

add a commentÂ |Â

up vote
4
down vote

favorite

I want to write to a named pipe only if it already has a reader. Currently, I'm using timeout to detect whether the attempt to write to the pipe blocked, like so:

#! /usr/bin/env bash
rm -f pipe
mkfifo pipe
sleep 5

timeout 1 bash -c "echo Hello > pipe"

if [[ $? == 0 ]]
then
 echo Somebody got our message
else
 echo Nobody read from pipe, so we didn't send a message
fi

edited 12 hours ago

asked yesterday

MatrixManAtYrService

1705

I want to write to a named pipe only if it already has a reader. Currently, I'm using timeout to detect whether the attempt to write to the pipe blocked, like so:

#! /usr/bin/env bash
rm -f pipe
mkfifo pipe
sleep 5

timeout 1 bash -c "echo Hello > pipe"

if [[ $? == 0 ]]
then
 echo Somebody got our message
else
 echo Nobody read from pipe, so we didn't send a message
fi

bash fifo

edited 12 hours ago

asked yesterday

MatrixManAtYrService

1705

edited 12 hours ago

asked yesterday

MatrixManAtYrService

1705

edited 12 hours ago

asked yesterday

MatrixManAtYrService

1705

asked yesterday

MatrixManAtYrService

1705

asked yesterday

MatrixManAtYrService

1705

add a commentÂ |Â

1 Answer
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2
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If you want to write to the pipe, only if there are some process that has opened it for reading, you could open it for writing in non-blocking mode.

With GNU dd:

echo Hello | dd oflag=nonblock of=pipe status=none &&
 echo message has been sent

And you'll get the error message for ENXIO if there was no reader.

Note that the above may still hang if the pipe is full (if there is a reader, but it is not currently reading).

answered 16 hours ago

StÃ©phane Chazelas

287k53531868

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1 Answer
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active

oldest

votes

1 Answer
1

active

oldest

votes

up vote
2
down vote

accepted

If you want to write to the pipe, only if there are some process that has opened it for reading, you could open it for writing in non-blocking mode.

With GNU dd:

echo Hello | dd oflag=nonblock of=pipe status=none &&
 echo message has been sent

And you'll get the error message for ENXIO if there was no reader.

Note that the above may still hang if the pipe is full (if there is a reader, but it is not currently reading).

answered 16 hours ago

StÃ©phane Chazelas

287k53531868

add a commentÂ |Â

up vote
2
down vote

accepted

If you want to write to the pipe, only if there are some process that has opened it for reading, you could open it for writing in non-blocking mode.

With GNU dd:

echo Hello | dd oflag=nonblock of=pipe status=none &&
 echo message has been sent

And you'll get the error message for ENXIO if there was no reader.

Note that the above may still hang if the pipe is full (if there is a reader, but it is not currently reading).

answered 16 hours ago

287k53531868

add a commentÂ |Â

up vote
2
down vote

accepted

If you want to write to the pipe, only if there are some process that has opened it for reading, you could open it for writing in non-blocking mode.

With GNU dd:

echo Hello | dd oflag=nonblock of=pipe status=none &&
 echo message has been sent

And you'll get the error message for ENXIO if there was no reader.

Note that the above may still hang if the pipe is full (if there is a reader, but it is not currently reading).

answered 16 hours ago

287k53531868

If you want to write to the pipe, only if there are some process that has opened it for reading, you could open it for writing in non-blocking mode.

With GNU dd:

echo Hello | dd oflag=nonblock of=pipe status=none &&
 echo message has been sent

And you'll get the error message for ENXIO if there was no reader.

Note that the above may still hang if the pipe is full (if there is a reader, but it is not currently reading).

answered 16 hours ago

287k53531868

answered 16 hours ago

287k53531868

answered 16 hours ago

287k53531868

answered 16 hours ago

287k53531868

add a commentÂ |Â

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