Convert hex string to number.

str2hex.c

int str2hex(char *str, int len) {
    len = MIN(len, 8);
    int hex = 0;
    int bit = 0;

    for (int i = len - 1; i >= 0; i--) {
        int n;
        char ch = str[i];
        if (ch >= '0' && ch <= '9') {
            n = ch - '0';
        } else if (ch >= 'A' && ch <= 'F') {
            n = 0xa + ch - 'A';
        } else if (ch >= 'a' && ch <= 'f') {
            n = 0xa + ch - 'a';
        } else if (ch == '\0') {
            continue;
        } else {
            break;
        }

        hex |= n << bit;
        bit += 4;
    }

    return hex;
}

Test

char num[] = "24292a";
str2hex(num, sizeof(num));

char a[] = "f6f8fa";
str2hex(a, sizeof(a));

char A[] = "FFFFFF";
str2hex(A, sizeof(A));