Thanks for both replies, it makes sense. But where is "data" declared as
an array of chars. In AM.h the type of the struct TOS_Msg member data,
is int8_t.
Mikael
Michael Schippling wrote:
One slight correction...
> " the corresponding
> bytes from m_msg.data are put into the corresponding fields in the
> variable addressed by the pointer body."
>
Implies that values are copied from m_msg.data to someplace, however
this is not the case. The assignment statement:
CountMsg_t* body = (CountMsg_t*)m_msg.data;
simply sets the "body" variable to the address of the m_msg.data array.
You might think that you need to use &m_msg.data to get the address,
but "data" is declared as an array of chars and C treats the array
name (with no index -- data[1]) as a pointer to the first element.
(I think you might get away with using the &, but it's redundant...)
The key fact is that if you write to body->n you will write into
m_msg.data
If the above makes no sense, probably I'm a bad explainer, but look
at the K&R C book...
MS
Sankar Gorthi wrote:
Hi,
CountMsg_t is a structure datatype. body is a pointer variable of
type CountMsg_t.
n is a field in the structure CountMsg_t and so is src.
when you type cast m_msg.data to the CountMsg_t pointer datatype,
depending upon the order in which the values are defined in the
structure definition and the size of the variables, the corresponding
bytes from m_msg.data are put into the corresponding fields in the
variable addressed by the pointer body.
for example:
assume m_msg.data is composed of the following bytes:
48 65 6C 6C 6F
CountMsg_t is defined as follows:
typedef struct CountMsg_t {
uint8_t src;
char n[3];
}CountMsg_t;
then the pointer body will point to the following fields and the
correspondign values:
body->src = 0x48 = 72
body->n[0] = 'e';
body->n[1] = 'l';
body->n[2] = 'l';
body->n[3] = 'o';
Hope that clears stuff up.
Sankar.
On Sat, 18 Nov 2006 06:56:03 -0600, Mikael Ifversen
<[EMAIL PROTECTED]> wrote:
Hi,
Can someone clarify following code for me? It is from the CountSendP
component in boomerang:
CountMsg_t* body = (CountMsg_t*)m_msg.data;
body->n = m_count;
body->src = TOS_LOCAL_ADDRESS;
CountMsg_t is a variable of struct CountMsg.
m_msg is a variable of TOS_Msg.
m_msg.data is referring to the data field of TOS_Msg struct.
body is a pointer to the variable CountMsg_t.
I know that m_msg.data = 29 and m_msg.data is being typecast by
(CountMsg_t*),
but still not sure how to interpret: CountMsg_t* body =
(CountMsg_t*)m_msg.data.
Thanks,
Mikael
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