[Balug-talk] getsockname ?

[Vinay S Kulkarni]

Hi Urs,

  Thanks for the response. My confusion was from
another program I ran where I did print out the
sin_port after applying ntohs() to it by in the
unsigned short port variable in my class I saved the
port without ntohs(). Silly as ut may sound, i did
overlook that for quite some time.

Best REgards,
Vinay



--- Urs Thuermann <urs at isnogud.escape.de> wrote:
> >     sin_size = sizeof(my_addr);
> >     if (getsockname(sockfd, (struct sockaddr
> *)&my_addr, &sin_size) == -1) {
> >         perror("getsockname");
> >         exit(1);
> >     }
> >     printf("My TCP port = %d\n",my_addr.sin_port);
> 
> 
> > The run looks like this: 
> > 
> > [root at vin /root]# /home/vinay/server 
> > My TCP port = 62337
> > 
> > 
> > 
> > And lsof gave me this: 
> > 
> > [root at vin /root]# lsof | grep "server.*TCP"
> > server    11037 vinkulkarni    3u  IPv4      47013
>    
> >            TCP *:33267 (LISTEN)
> > [root at altair /root]# 
> > 
> > 
> > If we look at the numbers then we see that the
> digits in both from
> > what the server prints and what lsof tells are the
> same except they
> > are jumbled up. This is purely coincidental. I got
> it only in this
> > run.
> 
> It is coincidental that 62337 and 33267 have the the
> decimal digits
> just jumbled up.  But look at the hexadecimal
> represantations of these
> numbers:
> 
> 	62337 = 0xF381
> 	33267 = 0x81F3
> 
> And this is *not* coincidental.  The correct port
> number is 33267 = 0x81F3.
> When using system calls involving a struct
> sockaddr_in, the port number
> is always be stored in memory in network byte order,
> which is big
> endian.  That means the most significant byte is
> stored at the lower
> memory address, followed the the less significant
> bytes.  That is in
> memory the port number 0x81F3 is stored as
> 
> 	0x81 0xF3
> 
> Intel x86 however are little endian, placing the
> least significant
> byte at the lower memory address.  When the CPU
> reads the 2-byte value
> from memory, it uses 0x81 as the least significant
> byte and 0xF3 as
> the most significant byte, yielding 0xF381 = 62337. 
> To get the
> correct port number, you have to swap the two bytes
> by using ntohs().
> Change your printf() as follows:
> 
> 	printf("My TCP port = %d\n",
> ntohs(my_addr.sin_port));
> 
> > The same code compiled on the ultra-10 that I work
> on runs just
> > fine.
> 
> This is because Sparcs are big endian.  That means
> that network byte
> order and host byte order are the same.  The
> routines ntohs(),
> htons(), ntohl(), and htonl() are NOPs, i.e. the do
> nothing, while on
> Intel x86, which is little endian, they swap the
> byte order.
> 
> To make your program portable, so that it works on
> litte endian and
> big endian machines you should always use ntohs(),
> etc.
> 
> 
> urs


=====
Vinay Kulkarni

The past is history. The future is a mystery. But this moment is a gift. That's why it's called the 'present'.

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