Hex char to int conversion

Question

I'm reading data from UART byte by byte. This is saved to a char. The byte following the start byte gives the number of subsequents bytes incoming (as a hexadecimal). I need to convert this hex number to an integer, to keep count of bytes required to be read.

Presently I'm simply type-casting to int. Here's my code:

char ch;
int bytes_to_read;
while(1){
    serial_read(UART_RX, &ch, sizeof(char));
    if(/*2nd byte*/){
        bytes_to_read = (int)ch;
    }
}

I read about strtol(), but it takes char arrays as input. What's the right way to do this?

Can you provide an example? How is the hex number encoded in the byte? — Joni
Does "as a hexadecimal" mean it's coming as text, in hexadecimal notation? I ask because many people here seem to say "hex" when they mean "binary", i.e. as raw bytes. — unwind
@user3490458: in that case your code is already doing the right thing, i.e. just casting the char to an int - so what's the actual problem ? — Paul R
@user3490458 Do you get this 0x03 really as 4 characters 0, x, 0 and 3 or as one byte with the representation 0x03 and the value 3? Be sure you are clear about the distinction of value and representation. — glglgl

Paul R Paul R · Accepted Answer · 2014-11-10T08:33:58

Your question is unclear, but assuming that ch is a hexadecimal character representing the number of bytes, then you could just use a simple function to convert from hex to int, e.g.

int hex2int(char ch)
{
    if (ch >= '0' && ch <= '9')
        return ch - '0';
    if (ch >= 'A' && ch <= 'F')
        return ch - 'A' + 10;
    if (ch >= 'a' && ch <= 'f')
        return ch - 'a' + 10;
    return -1;
}

and then:

bytes_to_read = hex2int(ch);

ch

bytes_to_read = (int)ch;

Hex char to int conversion

3 Answers