Re: socket connect timeout

[Scott Klement <sk@xxxxxxxxxxxxxxxx>]

Sender: Scott Klement <sk@xxxxxxxxxxxxxxxx>

Hello,

> c                   eval      sock = socket(AF_INET: SOCK_STREAM:
> c                                           IPPROTO_IP)
> c                   if        sock < 0
> c                   eval      msg = 'Error calling socket()!'
> c                   dsply                   msg
> c                   return
> c                   endif


I'm sorry -- but this is very poorly done.  You need to tell the user what 
went wrong, not which API failed.   When a socket can't be created, is it 
because TCP/IP isn't installed?  Is it because the user doesn't have 
enough authority?  Whne a connect() attempt fails, is it because the host 
refused the connection?  Or because it timed out?  Or because the network 
is down?

You need to provide useful error messages.  "socket() failed" or 
"connect() failed" are not useful.

If this is code that you're intending to submit to the FTPAPI project, 
I'll have to reject it if it doesn't provide useful error messages.


> c                   Eval      $connect = false
> c     1             Do        10
> c                   Eval      RC = connect(sock: p_connto: addrlen)
> c                   If        RC < 0
> *c                   eval      msg = 'Not connected...'
> *c                   dsply                   msg
> C                   call      'DELAY'
> C                   Else
> c                   Eval      $connect = true
> c                   eval      msg = 'Connected!'
> c                   dsply                   msg
> C                   leave
> C                   endif
> c                   Enddo
>
> After the above loop I still have a RC that is -1. Any ideas?

Yes... Here's what happens:

a) You call connect() the first time.  Something may be wrong that the 
host isn't available or you provided bad parameters, or whatever.  In that 
case connect() returns -1 to tell you about the error.  If nothing is 
wrong, the connect will be "started" but will complete in the background 
because you're in non-blocking mode. When it's started, it'll return -1 
and errno will be EINPROGRESS to tell you that it's working on it.  In 
either case, you'll always get a -1 from connect().

b) The next time you call connect() will depend on what your first problem 
was.  If the first time was an error, and the connection is not in 
progress, then it'll try to do the same thing as before and produce the 
same error.  If it first time was successful and the connection attempt is 
either in progress or completed, then you'll get an error saying that it's 
in progress or that it's already connected.

Consequently, you'll always get a -1.

The solution to your problems are:

a) ALWAYS ALWAYS ALWAYS check errno to find out what went wrong.  Only 
continue the connection progress if the error is EINPROGRESS.  Don't use 
the -1 by itself.

b) Use the select() API to wait until the socket is writable. Once it's 
writable, you'll know that the connection has been completed.  Get used to 
using the select() API, since non-blocking sockets are pretty much useless 
without it.

c) Once the socket is writable, you might be connected, or you might have 
an error. When you try to use send() or recv() you'll get an error message 
if the connection failed.  Check for that.

d) Consider using signals to implement your timeout, and only switching to 
nonblocking mode AFTER the connection is complete.  I've found that this 
greatly simplifies the code, and makes it easier to do proper error 
reporting.  Non-blocking connects are not fun.
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