Locating STX and ETX in a String in C++ [closed]

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Closed 8 years ago.

Is there a way to detect STX (Start of Text) and ETX (End of Text) character sequences in a message received by recv() in c++? I'm trying fix any partial reads that may happen in TCP so that I can reform the complete message.

Thank you!

EDIT1:

Did the following as per the answer by unwind:

char c = '\x02';
if(memchr((*it).c_str(), c, numberOfCharactersRead) != NULL) 
    cout << "STX found" << endl;

Still I didn't manage to detect the character. Any issue with this implementation? 'it' is an iterator for a vector of the type string.

EDIT2:

This is the complete code for receiving data and checking for the character:

int numberOfCharactersRead = 0;

    do {
        char msgBuffer[1000];
        memset(msgBuffer, 0, 1000);
        numberOfCharactersRead = recv(clientSideSocket, msgBuffer, (1000 - 1), 0);
        if (numberOfCharactersRead < 0) {
            close(clientSideSocket);
        }
        else if (numberOfCharactersRead == 0)
            close(clientSideSocket);
        else {
            char c = '\x02';
            if(memchr(msgBuffer, c, numberOfCharactersRead) != NULL) 
                cout << "STX found" << endl;
            memset(msgBuffer, 0, 1000);
        }

    } while (numberOfCharactersRead > 0); 

So, I'm simply checking for the STX, not yet concatenating the buffered data. However, the check for STX is still failing. Kindly let me know any issue in this approach.

Thanks.

EDIT3:

I got the following hex value for a sample message:

3C 31 34 32 3E 4F 63 74 20 32 35 20 31 31 3A 33 39 3A 31 32 20 6C 6F 63 61 6C 68 6F 73 74 20 5B 20 32 30 31 32 2D 4F 63 74 2D 32 35 20 31 31 3A 33 39 3A 31 32 2C 36 31 33 20 5D 20 20 20 20 49 4E 46 4F 20 7B 20 4D 61 69 6E 2E 6A 61 76 61 20 7D 20 2D 20 74 65 73 74 20 69 6E 66 6F 72 6D 61 74 69 6F 6E 20 6D 65 73 73 61 67 65 20

So the hex value for the STX/ETX is not present. That mean can't use STX and ETX for checking the message formation.

Answer 1

Well, STX and ETX are not "sequences", they are single characters. So you should be able to use a plain memchr() call to search for the characters.

The character values are simply '\x02' for STX and '\x03' for ETX.

Note that you will need to pass in the received size. Since (I assume) there's no guarantee that a partially received message is 0-terminated, you can't use strchr().