CODEWITHSUNDEEP
pythonselectipcnamed-pipesflush
BrainCore
BrainCore

Reputation: 5412

Named pipe is not flushing in Python

I have a named pipe created via the os.mkfifo() command. I have two different Python processes accessing this named pipe, process A is reading, and process B is writing. Process A uses the select function to determine when there is data available in the fifo/pipe. Despite the fact that process B flushes after each write call, process A's select function does not always return (it keeps blocking as if there is no new data). After looking into this issue extensively, I finally just programmed process B to add 5KB of garbage writes before and after my real call, and likewise process A is programmed to ignore those 5KB. Now everything works fine, and select is always returning appropriately. I came to this hack-ish solution by noticing that process A's select would return if process B were to be killed (after it was writing and flushing, it would sleep on a read pipe). Is there a problem with flush in Python for named pipes?

Upvotes: 4

Views: 4890

Answers (3)

Peter Boothe
Peter Boothe

Reputation: 161

To find out if Python's internal buffering is causing your problems, when running your scripts do "python -u" instead of "python". This will force python in to "unbuffered mode" which will cause all output to be printed instantaneously.

Upvotes: 1

Antoine P.
Antoine P.

Reputation: 4315

What APIs are you using? os.read() and os.write() don't buffer anything.

Upvotes: 1

Ignacio Vazquez-Abrams
Ignacio Vazquez-Abrams

Reputation: 799240

The flush operation is irrelevant for named pipes; the data for named pipes is held strictly in memory, and won't be released until it is read or the FIFO is closed.

Upvotes: 0

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