Question

I need to modify the stuff I have in stringList to be hex, and I need to do it inside MyFunction because WriteI2C needs to take in a hex value. For example, if stringList contained ‘5’ and ‘A’, I would want to pass 0x5A into WriteI2C.

char* stringList[5]; // array of strings (max 5 strings)
char* (*stringListPtr); // pointer to a string

void MyFunction(char* (char* (*stringListPtr))
{
    WriteI2C(a hex value); // ex: WriteI2C(0x5A);
}

Answer 1

Can you use NUL-terminated arrays of char (a. k. a. C strings)? If so:

const char *str = "5A";
int val = strtol(str, NULL, 16);
WriteI2C(val);

If not, you may want to make a NUL-terminated copy, or to reinvent the wheel (note: don’t reinvent the wheel):

int ch2hex(char ch)
{
    if (isdigit(ch)) return ch - '0';
    if (islower(ch)) return ch - 'a' + 10;
    if (isupper(ch)) return ch - 'A' + 10;
    // if this is reached, something very nasty is going on
}

int str2hex(char arr[2])
{
    return (ch2hex(arr[0]) << 4) | ch2hex(arr[1]);
}