About C programming questions

[maniac84]

Hi guys,
I've been studying other people's c coding lately and I encountered this declaration:
pRs485Struct->theDataStruct.strData = 1;
What I want to know is, why does it contain this "->" sign?

 

[enjunear]

It's a way to point to a member within a struct.

Read more here: https://stackoverflow.com/questions/2575048/arrow-operator-usage-in-c

Then google "arrow operator c programming" for more information.

 

[jokkebk]

When you have a pointer to a struct, you use the arrow operator. When the variable is a struct (in stack), you use the dot:

struct SomeStruct * ss = ...;
struct SomeStruct ss2;

ss->someField = ...;
ss2.someField = ...;

It also seems your "theDataStruct" is either a struct or union, and not a pointer, as its member "strData" is referenced with a dot.

 

[maniac84]

while (rs485.nRxByte != EOT && strlen(rs485.strRxBuffer) < sizeof(rs485.strRxBuffer)-1 &&
        ulTmpTimer>MS_TIMER)
{
          rs485.nRxByte = serDgetc();

        rs485.strRxBuffer[strlen(rs485.strRxBuffer)] = rs485.nRxByte;
}

I've encounter this dynamic c coding too. It's for receiving serial data.
For this line:
rs485.strRxBuffer[strlen(rs485.strRxBuffer)] = rs485.nRxByte;
if I received 10 bytes of data, can it be put into the buffer to become: rs485.strRxBuffer[0], rs485.strRxBuffer[1], rs485.strRxBuffer[2]..... rs485.strRxBuffer[10]
Because normally what I would write is:
rs485.strRxBuffer[i++] = rs485.nRxByte

 

[ponnus]

Hai,
You can fill the buffer like that.

If the buffer is cleared in the beginning, strlen gives 0. In while loop, each time the buffer is filled by a byte, strlen(rs485.strRxBuffer) also increments. So,next time, the next byte will be stored in the next location.

Also, strlen for a character array gives the length of the string and sizeof operator gives the total size of the array.

 

[maniac84]

Hai,
You can fill the buffer like that.

If the buffer is cleared in the beginning, strlen gives 0. In while loop, each time the buffer is filled by a byte, strlen(rs485.strRxBuffer) also increments. So,next time, the next byte will be stored in the next location.

Also, strlen for a character array gives the length of the string and sizeof operator gives the total size of the array.

Thanks.
By the way, for using serDgetc(), it will know that I have stored it to rs485.nRxByte and move to the next value right?

 

[ponnus]

Hai,
serDgetc() reads a byte, when the while() condition is satisfied.
It stores the byte to rs485.nRxByte.
For the first time strlen gives 0, so rs485.nRxByte will be stored to rs485.strRxBuffer[0].

Second time,strlen gives 1 (buffer contains one character) . So, this time rs485.nRxByte will be stored to rs485.strRxBuffer[1] and so on....

 

[maniac84]

Hai,
serDgetc() reads a byte, when the while() condition is satisfied.
It stores the byte to rs485.nRxByte.
For the first time strlen gives 0, so rs485.nRxByte will be stored to rs485.strRxBuffer[0].

Second time,strlen gives 1 (buffer contains one character) . So, this time rs485.nRxByte will be stored to rs485.strRxBuffer[1] and so on....

So, does this means the speed of this while loop have to be the same as the speed of receiving data of rs485. Am I right?
Or else if the speed of receiving data is faster than this while loop, it wil always get the last character only. Am I right?

 

[ponnus]

If there is speed mismatch, either some bytes will be missed or some bytes will be buffered more than once.