UDP receive data as unsigned char

Question

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

What happens when you cast to `uint8_t` instead of `short int` ? — Arunmu, Jun 07 '16 at 08:34
Why are you casting to short int when the subject line of your post says you want unsigned char? — Richard Critten, Jun 07 '16 at 08:46
@Haris You give below a try `std::cout << hex << setfill('0') << setw(2) << recvline[i] << " "`; — Arunmu, Jun 07 '16 at 08:52
@Arunmu That doesn't matter. The representation is done by the I/O manipulators. — πάντα ῥεῖ, Jun 07 '16 at 08:52
@Arunmu Your proposal won't work. See [here](http://stackoverflow.com/questions/19562103/uint8-t-cant-be-printed-with-cout/19562163#19562163) please. — πάντα ῥεῖ, Jun 07 '16 at 08:53
@πάνταῥεῖ I see, thanks. For my last comment, it will print leading '0'. I guess OP doesnt want that. — Arunmu, Jun 07 '16 at 08:53
@Arunmu The problem is that `char` will be printed as character, not as a number. — πάντα ῥεῖ, Jun 07 '16 at 08:55
@Haris did you check return value from recvfrom()? You need to make sure the receive did not fail. — Erik Alapää, Jun 07 '16 at 09:26

Smeeheey · Accepted Answer · 2016-06-07T08:54:41.353

3

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;
   }

edited Jun 07 '16 at 08:54

answered Jun 07 '16 at 08:40

Smeeheey

9,906
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39

`cout << hex <(recvline[i])<<" ";` giving me the same result. Actually my ultimate purpose is parse received data. – Haris Jun 07 '16 at 08:43
Thanks for the answer, that work fine, I also tried in this way and seems working `unsigned char* msg1 = reinterpret_cast(msg); int commId=*(msg1+1);` – Haris Jun 07 '16 at 09:18

MikeMB · Answer 2 · 2016-06-07T09:41:03.630

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.

UDP receive data as unsigned char

2 Answers2