[SOLVED] get 2 bytes of __be32 variable (ip address)

hayf · 08-19-2011, 03:19 AM

hi,

I have a __be32 variable that is an IP address.
I need to get the last 2 bytes of that IP and store it in an __be16 variable.
How this can be done?

Thanks,
hayf

rustek · 08-19-2011, 08:28 PM

Hi, you really haven't provided enough information but I'll take a stab.

Using bash, if the variable __be32 = 192.168.12.36

__be16=$(echo $__be32 | cut -f4 -d".")

Then __be16 would be 36

This won't give you the last two bytes but give you the last field.
If __be32 was 192.168.12.136 then __be16 would be 136

I'm just guessing that that is what you really want?

Russ

David the H. · 08-19-2011, 09:27 PM

Even easier:

Code:

__be16="${__be32##*.}"

parameter substitution

But that's again assuming bash or a bash-like shell. We really need to know what programming environment the OP talking about.

rustek · 08-19-2011, 11:41 PM

David's answer gives the last field as well but is much better as it stays within the shell.

Old habits die hard :}

Russ

hayf · 08-21-2011, 07:47 AM

Hi, what i really want is to get the last 2 bytes.
e.g. if the __be32 ip address is 192.168.12.36 i need to get 12.36
i am programming using the C language.

Thanks

Nominal Animal · 08-21-2011, 11:39 AM

For example,

Code:

__be32  addr;
__be16  low;

low = (__be16)( addr & (__be32)0xFFFFU );
    or, on a big-endian architecture
low = ((__be16 *)&addr)[1];
    or, on a little-endian architecture
low = ((__be16 *)&addr)[1];

hayf · 08-22-2011, 09:06 AM

OK that really works

but to get the last 2 bytes, it should be: low = ((__be16 *)&addr)[0];
one last question:
Any idea about how can print a __be16 using printk function? (i need it as 2 bytes ip address e.g. 12.36)
thanks in advance

Nominal Animal · 08-22-2011, 03:52 PM

Quote:

Originally Posted by hayf

OK that is really works

but to get the last 2 bytes, it should be: low = ((__be16 *)&addr)[0];

Only on a little-endian architecture like x86.

(I had a thinko; I thought you meant the data was stored in big-endian order. It is not; only the interpretation of the value is big-endian, i.e. network byte order. Fixed my post.)

Quote:

Originally Posted by hayf

Any idea about how can print a __be16 using printk function? (i need it as 2 bytes ip address e.g. 12.36)

Code:

/* Last two bytes of an IPv4 address */
printk("%d.%d", (addr >> 8U) & 255, addr & 255);

/* A full IPv4 address */
printk("%d.%d.%d.%d",
       (addr >> 24U) & 255,
       (addr >> 16U) & 255,
       (addr >>  8U) & 255,
        addr         & 255);

hayf · 08-23-2011, 03:10 AM

Thank you for the answers

..