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Re: runaway zselect

cool... thanks!

On Sun, 17 May 2009, Bart Schaefer wrote:

On May 17,  4:43pm, Atom Smasher wrote:
} i would expect zselect to return an error and stop the while loop when
} "w" stops producing output.

That's not how the select() system call works. The descriptor is still open on the reading end of the pipe even though the descriptor on the writing end of the pipe has been closed. That means from select()'s point of view, the descriptor is available for reading, even though the only thing that can be read from it is the end-of-file condition.

If this were not the case, zselect would fail as soon as the writer exits, even if all output from the writer had not yet been consumed. (Remember that there is OS-level buffering involved, so the writing end may be closed before the pipe is "empty".)

What you need to fix this is to test the result of "read" as well. Either:

   while zselect -r 0 && read -r foo
	print -r -- $foo
   done <<( w )

Or equivalently:

   while zselect -r 0
	read -r foo || break
	print -r -- $foo
   done <<( w )


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		August 2003

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