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Re: bash sockets: printf \x0a does TCP fragmentation


From: Chet Ramey
Subject: Re: bash sockets: printf \x0a does TCP fragmentation
Date: Tue, 25 Sep 2018 09:48:28 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 9/25/18 8:25 AM, Greg Wooledge wrote:
> On Tue, Sep 25, 2018 at 05:15:02AM -0700, L A Walsh wrote:
>> On 9/24/2018 6:05 AM, Greg Wooledge wrote:
>>> On Sat, Sep 22, 2018 at 11:50:17AM +0200, address@hidden wrote:
>>>> On 9/22/18 7:30 AM, Bob Proulx wrote:
>>>>> address@hidden wrote:
>>>>>> printf -- "$data" >&5 2>/dev/null
>>>>> What happens if $data contains % format strings?  What happens if the
>>>>> format contains a sequence such as \c?  This looks problematic.  This
>>>>> is not a safe programming proctice.
>>>
>>> Looking ONLY at this one line, there is an obvious bug, which Bob has
>>> pointed out.  It should be
>>>
>>> printf %s "$data" >&5 2>/dev/null
>> ----
>>    This brings to mind a consideration:
>> As %s says to print a string of data (presumably not
>> including a NUL byte), then what happens if "$data" is
>> a paragraph of text with embedded newlines.  In that case,
>> it sounds like bash might break apart the single printf
>> output into smaller packets rather than transmitting the
>> entirety of "$data" in 1 write (presuming it is less than
>> the maximum data size for a network packet).
> 
> Yes, I'm sure it does.  In fact, bash's printf and echo builtins are
> already known to use multiple calls to write() even when sockets and
> newlines are not involved.

Yes, bash does line buffering and provides a buffer for stdout and
stderr. This has been noted here previously and isn't specific to
printf or echo.


-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://tiswww.cwru.edu/~chet/



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