Re: how to create a network buffer ?

James Kanze <>
1 May 2007 01:59:36 -0700
On Apr 30, 6:17 pm, aki <> wrote:

i am writing codes for implementing network protocols.
i have written a method which is receiving a packet from network.
i have assumed that the packet i will receive will be of type char*.
i need to test my method and
for that i want to create a network buffer which will contain a
packet of format as->
 example of Configuration req packet
                        uint8_t code;
                        uint8_t identifier;
                        uint16_t length;
                        option n

for eg option1 can be.... AuthProtocol with fields as
                            uint8_t type;
                            uint8_t length;
                            uint16_t authpro;

i was tryin to do like this.. but it has a limitation....
supposing packet contains header and only one option..
it will need 8 byte aas a whole..

char *buffer=new char(8);// allocating buffer with required memory

No. Here, you've allocated a single char, and initialized it
with the value 8. what you probably want is:

    std::vector< char > buffer( 8 ) ;

Although for various reasons, I find it easier to work with an
std::vector< unsigned char > for this sort of thing (unless the
protocol is text based).

If you're generating a buffer for output, just declaring:

    std::vector< unsigned char > buffer ;

and using push_back to insert the values is generally
sufficient. In this case, something like:

    buffer.push_back( code ) ;
    buffer.push_back( identifier ) ;
    size_t lengthOffset - buffer.size() ;
    buffer.push_back( 0 ) ; // Save space for length...
    buffer.push_back( 0 ) ;
    insertOptions( buffer ) ;
    assert( buffer.size() < 1 << 16 ) ;
    buffer[ lengthOffset ] = buffer.size() >> 8 ;
    buffer[ lengthOffset + 1 ] = buffer.size() ;

(This assumes Internet format for unsigned integers.)

memset(buffer,1,1); // setting code field as 1

You don't need memset to set a single byte.

memset(buffer+1,4,1); // setting identifier as 4
memset(buffer+2,25,2); // here i am not able to put length in 2 byte
field as memset works with only one byte ...

Memset cannot be used for anything other than a stream of bytes.

Forget it (and memcpy) for generating buffers for protocols.
Just use a vector< unsigned char >, as above, then, to pass it
to the socket interface:

    reintepret_cast< char* >( &buffer[ 0 ] ) ;

(Generally, reintepret_cast's are not a very good idea, but this
is the exception which confirms the rule.)

James Kanze (Gabi Software) email:
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