Converting a hex string to a byte array

Question

What is the best way to convert a variable length hex string e.g. \"01A1\" to a byte array containing that data.

i.e converting this:

st         


        

Answer 1

The difficulty in an hex to char conversion is that the hex digits work pairwise, f.ex: 3132 or A0FF. So an even number of hex digits is assumed. However it could be perfectly valid to have an odd number of digits, like: 332 and AFF, which should be understood as 0332 and 0AFF.

I propose an improvement to Niels Keurentjes hex2bin() function. First we count the number of valid hex digits. As we have to count, let's control also the buffer size:

void hex2bin(const char* src, char* target, size_t size_target)
{
    int countdgts=0;    // count hex digits
    for (const char *p=src; *p && isxdigit(*p); p++) 
        countdgts++;                            
    if ((countdgts+1)/2+1>size_target)
        throw exception("Risk of buffer overflow"); 

By the way, to use isxdigit() you'll have to #include .
Once we know how many digits, we can determine if the first one is the higher digit (only pairs) or not (first digit not a pair).

bool ishi = !(countdgts%2);         

Then we can loop digit by digit, combining each pair using bin shift << and bin or, and toggling the 'high' indicator at each iteration:

    for (*target=0; *src; ishi = !ishi)  {    
        char tmp = char2int(*src++);    // hex digit on 4 lower bits
        if (ishi)
            *target = (tmp << 4);   // high:  shift by 4
        else *target++ |= tmp;      // low:  complete previous  
    } 
  *target=0;    // null terminated target (if desired)
}