Converting string hexadecmials to unsigned char (BYTE) in C - c++

Question

I want to convert the hexadecimal string value 0x1B6 to unsigned char - where it will store the value in the format 0x1B, 0x60 We had achieved the scenarios in C++, but C doesn't support std::stringstream.
The following code is C++, how do I achieve similar behavior in C?
char byte[2];
std::string hexa;
std::string str = "0x1B6" // directly assigned the char* value in to string here
int index =0;
unsigned int i;
for(i = 2; i < str.length(); i++) {
hexa = "0x"
if(str[i + 1] !NULL) {
hexa = hexa + str[i] + str[i + 1];
short temp;
std::istringstream(hexa) >> std::hex >> temp;
byte[index] = static_cast<BYTE>(temp);
} else {
hexa = hexa+ str[i] + "0";
short temp;
std::istringstream(hexa) >> std::hex >> temp;
byte[index] = static_cast<BYTE>(temp);
}
}
output:
byte[0] --> 0x1B
byte[1]--> 0x60

Answer 1

I don't think your solution is very efficient. But disregarding that, with C you would use strtol. This is an example of how to achieve something similar:
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
int main(void) {
const char *hex_string = "0x1B60";
long hex_as_long = strtol(hex_string, NULL, 16);
printf("%lx\n", hex_as_long);
// From right to left
for(int i = 0; i < strlen(&hex_string[2]); i += 2) {
printf("%x\n", (hex_as_long >> (i * 4)) & 0xff);
}
printf("---\n");
// From left to right
for(int i = strlen(&hex_string[2]) - 2; i >= 0; i -= 2) {
printf("%x\n", (hex_as_long >> (i * 4)) & 0xff);
}
}
So here we get the full value as a long inside hex_as_long. We then print the whole long with the first print and the individual bytes inside the second for loop. We are shifting multiples of 4 bits because one hex digit (0xf) covers exactly 4 bits of data.
To get the bytes or the long printed to a string rather than to stdout (if that is what you want to achieve), you can use strprintf or strnprintf in a similar way to how printf is used, but with a variable or array as destination.
This solution scans whole bytes (0xff) at a time. If you need to handle one hex digit (0xf) at a time you can divide all the operations by two and mask with 0xf instead of 0xff.