How to effectively represent and Increase MAC address

[RThaden]

Hi all,

I have a text file with a list of MAC addresses.
Each time, my program is called it reads the last MAC address entry
from the file, increases it by one, writes this new address into a 6
byte binary file and stores the new address in text representation in
the text file. Looks like this

00-0C-F1-B9-A1-11
00-0C-F1-B9-A1-12
00-0C-F1-B9-A1-13
....

My questions are:
1. How do I effectively read the last line of the file.
Right now, I read the whole file until I come to the last line. Not
very elegant but maybe the only quick way to do it?

2. How would you represent the MAC addresses in order to increase them?
I am reading them, ignoring the dashes between the bytes and convert
them with strtol into a 32 bit integer (where I should use 48 bits but
I assume that the first 16 bytes are not changed).
Then I increase this number and convert it back to chars. Sounds
horrible, doesn't it?
Is there a better way?

Thanks in advance,

Rainer Thaden

 

[mlimber]

I have a text file with a list of MAC addresses.
Each time, my program is called it reads the last MAC address entry
from the file, increases it by one, writes this new address into a 6
byte binary file and stores the new address in text representation in
the text file. Looks like this

00-0C-F1-B9-A1-11
00-0C-F1-B9-A1-12
00-0C-F1-B9-A1-13
...

My questions are:
1. How do I effectively read the last line of the file.
Right now, I read the whole file until I come to the last line. Not
very elegant but maybe the only quick way to do it?

Use std::ifstream::seekg().

2. How would you represent the MAC addresses in order to increase them?
I am reading them, ignoring the dashes between the bytes and convert
them with strtol into a 32 bit integer (where I should use 48 bits but
I assume that the first 16 bytes are not changed).
Then I increase this number and convert it back to chars. Sounds
horrible, doesn't it?
Is there a better way?

I generally prefer std::stringstreams to C-style number conversions
(cf.
http://parashift.com/c++-faq-lite/misc-technical-issues.html#faq-39.1
and the FAQ following). You could do a lazy conversion where you
convert the least significant byte, add one, and if and only if there
is a carry, convert the next byte and add one to it. Continue
converting and adding as long as there is a carry.

Cheers! --M

 

[VJ]

Hi all,

I have a text file with a list of MAC addresses.
Each time, my program is called it reads the last MAC address entry
from the file, increases it by one, writes this new address into a 6
byte binary file and stores the new address in text representation in
the text file. Looks like this

00-0C-F1-B9-A1-11
00-0C-F1-B9-A1-12
00-0C-F1-B9-A1-13
...

My questions are:
1. How do I effectively read the last line of the file.
Right now, I read the whole file until I come to the last line. Not
very elegant but maybe the only quick way to do it?

2. How would you represent the MAC addresses in order to increase them?
I am reading them, ignoring the dashes between the bytes and convert
them with strtol into a 32 bit integer (where I should use 48 bits but
I assume that the first 16 bytes are not changed).
Then I increase this number and convert it back to chars. Sounds
horrible, doesn't it?
Is there a better way?

Thanks in advance,

Rainer Thaden

Tried using istream:: seekg?

Here is a link for ifstream:
http://www.cplusplus.com/ref/iostream/ifstream/

 

[Amit]

I am reading them, ignoring the dashes between the bytes and convert

them with strtol into a 32 bit integer (where I should use 48 bits but
I assume that the first 16 bytes are not changed).
Then I increase this number and convert it back to chars. Sounds
horrible, doesn't it?

This way you'll get character not supported by MAC address. How are you
handling that?

 

[RThaden]

Tried using istream:: seekg?

Here is a link for ifstream:
http://www.cplusplus.com/ref/iostream/ifstream/

Yes, I tried. But what do you seek for? The end of the file? And where
to put the position after finding it? Then, you need to make
assumptions over the length of a line. Bad, if you want to allow
comments in lines.

Best regards,

Rainer

 

[RThaden]

This way you'll get character not supported by MAC address. How are you
handling that?

No, I don't

If, e.g., the last byte of the MAC address is bf, then I convert the
chars 'bf' to the number bf, increase it to get c0, save it in a binary
file, convert it back to two chars 'c0' and save that in my text file.
I see no problem here.

Best regards,

Rainer