Prev: problem expanding rpm macros
Next: what 64 mean in fopen64?
From: Minimiscience on 1 Jul 2008 18:54
There are a couple of minor things about creating a TCP connection in C that
I've been wondering about. They're probably of little consequence, but just to
make sure...

Firstly, after creating the socket, is it strictly necessary to bind it to a
local address before connecting to the remote server? The tutorial from which
I learned the basics of TCP programming [1] states that you should, yet the
example code in my operating system's manpage for getaddrinfo(3) only uses it
when creating a program to listen on a port. If all I'm doing is creating a
client program to send & receive bytes, what effect, if any, does binding the
socket have?

Secondly, when using getaddrinfo(3) to obtain addressing information, what is
the preferred way to specify a preference for TCP rather than UDP? The example
code on my OS simply sets the `ai_socktype' element of `hints' (the third
argument) to `SOCK_STREAM', yet I don't see why setting `ai_protocol' to
`IPPROTO_TCP' wouldn't be a valid alternative. Would it also be acceptable to
use both settings at once?

Just wondering,
-- Minimiscience


[1] http://www.fortunecity.com/skyscraper/arpanet/6/cc.htm

--
There are some things in ascensions zorkmids can't buy.
For everything else, there's Platinum Yendorian Express Card.
From: Barry Margolin on 2 Jul 2008 00:56
In article <g4ecj9$1h6$1(a)chessie.cirr.com>,
Minimiscience <minimiscience+usenet(a)gmail.com> wrote:

> There are a couple of minor things about creating a TCP connection in C that
> I've been wondering about. They're probably of little consequence, but just
> to
> make sure...
>
> Firstly, after creating the socket, is it strictly necessary to bind it to a
> local address before connecting to the remote server? The tutorial from
> which
> I learned the basics of TCP programming [1] states that you should, yet the
> example code in my operating system's manpage for getaddrinfo(3) only uses it
> when creating a program to listen on a port. If all I'm doing is creating a
> client program to send & receive bytes, what effect, if any, does binding the
> socket have?

It's rarely needed, or even appropriate, to call bind() before calling
connect() on a TCP socket. There are some application protocols which
have rules about the source port of a connection, but these are rare,
special cases. Usually you just let the OS select an unused port, which
happens automatically when you call connect() without calling bind().

I took a quick glance at that tutorial, and it's really hard to read and
doesn't explain what it's doing or why.

>
> Secondly, when using getaddrinfo(3) to obtain addressing information, what is
> the preferred way to specify a preference for TCP rather than UDP? The
> example
> code on my OS simply sets the `ai_socktype' element of `hints' (the third
> argument) to `SOCK_STREAM', yet I don't see why setting `ai_protocol' to
> `IPPROTO_TCP' wouldn't be a valid alternative. Would it also be acceptable
> to
> use both settings at once?

I think the intent is to allow the caller to be as generic or specific
as it wishes to be. Using SOCK_STREAM rather than IPPROTO_TCP allows
for future extension to other stream protocols than TCP. But if the
rest of your code assumes TCP, it may be better to use IPPROTO_TCP to
ensure that you don't get something you can't use.

--
Barry Margolin, barmar(a)alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***
From: Rainer Weikusat on 2 Jul 2008 01:54
Minimiscience <minimiscience+usenet(a)gmail.com> writes:
> Firstly, after creating the socket, is it strictly necessary to bind
> it to a local address before connecting to the remote server? The
> tutorial from which I learned the basics of TCP programming [1]
> states that you should,

bind associates an existing socket with a specific protocol address.
Calling connect with an unbound socket causes the kernel to select one
it considers to be appropriate instead.

> Secondly, when using getaddrinfo(3) to obtain addressing information, what is
> the preferred way to specify a preference for TCP rather than UDP?
> The example code on my OS simply sets the `ai_socktype' element of
> `hints' (the third argument) to `SOCK_STREAM', yet I don't see why
> setting `ai_protocol' to `IPPROTO_TCP' wouldn't be a valid
> alternative. Would it also be acceptable to use both settings at
> once?

Presumably (I haven't tested this), you should rather use the latter
if you really want a TCP-address. OTOH, there is little reason to do
that: Applications usually rely on properties a transport protocol
provides (like 'a reliable, full-duplex virtual circuit') and assuming
there was another common protocol providing this except TCP, there
shouldn't be a reason why an application should not be as capable to
use that instead of TCP. That's why I would rather specify the type of
connection I would like to make (eg 'SOCK_STREAM') than a particular
protocol providing such a connection.
 | 
Pages: 1
Prev: problem expanding rpm macros
Next: what 64 mean in fopen64?