Re: Built-in printf Sits Awkwardly with UDP.

[Steven W. Orr]

On 7/18/2011 10:14 AM, Ralph Corderoy wrote:
> Hi,
>
> (I'm not subscribed to the list so please keep me CC'd.)
>
> Consider
>
>      printf 'foo bar\n'>/dev/udp/localhost/4242
>
> with bash's printf built-in, it works fine.  One packet is sent.
> However, make that two lines
>
>      printf 'foo\nbar\n'>/dev/udp/localhost/4242
>
> and two packets are sent, one per line.  If the aim is to send a string
> of data in one packet this causes problems.
>
> Using /usr/bin/printf from coreutils gives one write(2), so that's a
> possible workaround.
>
> Is this happening because the built-in printf is using putchar(3) in the
> PC() macro and stdio thinks file descriptor 1 is still to a tty so it's
> persisting in line buffering?  It would seem nicer if fewer write(2)s
> were done when stdout isn't a tty, and not just for UDP use.
>
> Cheers, Ralph.

Ooh! Can I try to answer this one?!

Bash is always running in unbuffered mode, so every time a line is written, it 
automatically gets flushed. So... The only way to write in fully buffered mode 
with control over when things get flushed is to do it from a program that is 
not a bash script. So, yes. You need to do something outside of bash to get 
your output to the device.

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