[newbie] trying socket as a replacement for nc

Grant Edwards invalid at invalid.invalid
Thu Dec 12 15:16:46 CET 2013


On 2013-12-11, Dan Stromberg <drsalists at gmail.com> wrote:
> On Wed, Dec 11, 2013 at 3:08 PM, Jean Dubois <jeandubois314 at gmail.com>wrote:
>
>> I have an ethernet-rs232 adapter which allows me to connect to a
>> measurement instrument by means of netcat on a linux system.
>> e.g. entering nc 10.128.59.63 7000
>> allows me to enter e.g.
>> *IDN?
>> after which I get an identification string of the measurement instrument
>> back.
>> I thought I could accomplish the same using the python module "socket"
>> and tried out the sample program below which doesn't work however:
>>
>
> Sockets reserve the right to split one socket.send() into multiple
> socket.recv()'s on the other end of the communication, or to aggregate
> multiple socket.send()'s into a single socket.recv() - pretty much any way
> the relevant IP stacks and communications equipment feel like for the sake
> of performance or reliability.

Just to be pedantic: _TCP_ sockets reserver that right.  UDP sockets
do not, and do in fact guarantee that each message is discrete.  [It
appears that the OP is undoubtedly using TCP sockets.]

> The confusing thing about this is, it won't be done on every
> transmission - in fact, it'll probably happen rather seldom unless
> you're on a heavy loaded network or have some MTU issues (see Path
> MTU Discovery, and bear in mind that paths can change during a TCP
> session).  But writing your code assuming it will never happen is a
> bad idea.

And it _will_ fail someday in some odd circumstance when, for example,
some customer is be using it via a dial-up PPP connection, or there is
a satellite link in the path, or there's a flakey router somewhere,
or...

-- 
Grant Edwards               grant.b.edwards        Yow! Those people look
                                  at               exactly like Donnie and
                              gmail.com            Marie Osmond!!



More information about the Python-list mailing list