CODEWITHSUNDEEP
c++csocketsudp
Haris
Haris

Reputation: 14043

UDP receive data as unsigned char

I am a trying to receive some data from network using UDP and parse it.

Here is the code, 

char recvline[1024];
int n=recvfrom(sockfd,recvline,1024,0,NULL,NULL);

for(int i=0;i<n;i++)
   cout << hex <<static_cast<short int>(recvline[i])<<" ";

Printed the output, 

19 ffb0 0 0 ff88 d 38 19 48 38 0 0 2 1 3 1 ff8f ff82 5 40 20 16 6 6 22 36 6 2c 0 0 0 0 0 0 0 0

But I am expecting the output like, 

19 b0 0 0 88 d 38 19 48 38 0 0 2 1 3 1 8f 82 5 40 20 16 6 6 22 36 6 2c 0 0 0 0 0 0 0 0

The ff shouldn't be there on printed output.

Actually I have to parse this data based on each character,

Like,

parseCommand(recvline);

and the parse code looks,

void parseCommand( char *msg){

  int commId=*(msg+1);

  switch(commId){
    case 0xb0 : //do some operation
                break;

    case 0x20 : //do another operation
                break;
   }
}

And while debugging I am getting commId=-80 on watch.

Note: In Linux I am getting successful output with the code, note that I have used unsigned char instead char for the read buffer. 

unsigned char recvline[1024];
int n=recvfrom(sockfd,recvline,1024,0,NULL,NULL);

Where as in Windows recvfrom() not allowing the second argument as unsigned it giving build error, so I chose char

Upvotes: 1

Views: 1206

Answers (2)

MikeMB
MikeMB

Reputation: 21126

As you store your data in a signed data type conversions/promotion to bigger data types will first sign extend the value (filling the high order bits with the value of the MSB) even if it then gets converted to unsigned datatypes.

  • One solution is to define recvline as uint8_t[] in the first place an cast it to char* when passing it to the recvfrom function. That way, you only have to cast it once and you are using the same code in your windows and linux version. Also uint8_t[] is (at least to me) a clear indication that you are using the array as raw memory instead of a string of some kind.

  • Another possibility is to simply perform a bitwise And: (recvline[i] & 0xff). Thanks to automatic integral promotion this doesn't even require a cast.

Personal Note:
It is really annoying that the C and C++ standards don't provide a separate type for raw memory (yet), but with any luck well get a byte type in a future standard revision.

Upvotes: 3

Smeeheey
Smeeheey

Reputation: 10316

Looks like you might be getting the correct values, but your cast to short int during printing sign-extends your char value, causing ff to be propogated to the top byte if the top bit of your char is 1 (i.e. it is negative). You should first cast it to unsigned type, then extend to int, so you need 2 casts:

cout << hex << static_cast<short int>(static_cast<uint8_t>(recvline[i]))<<" ";

I have tested this and it behaves as expected.

In response to your extension: the data read is fine, it is a matter of how you interpret it. To parse correctly you should:

  uint8_t commId= static_cast<uint8_t>(*(msg+1));

  switch(commId){
    case 0xb0 : //do some operation
                break;

    case 0x20 : //do another operation
                break;
   }

Upvotes: 3

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