How to get hex values from string? - c

Question

I have a function with an input parameter "text" , which consists of a string containing unknown number of 2-character Hexadecimal numbers separated by space. I need to read them and put them in separate array indexes or read them in for loop and work with them one by one. Each of these values represents an encrypted char(the hex values are ascii values of the characters). How could i do that? I think i should use sscanf(); but i can't figure out how to do it.
char* bit_decrypt(const char* text){
int size=strlen(text)/3;
unsigned int hex[size];
char*ptr;
for(int i=0; i<size+1; i++){
hex[i] = strtoul(text, &ptr, 16);
printf("%x ",hex[i]);
}
return NULL;
}
output is: "80 80 80 80 80 80 80 80 80 80 80 80"
should be: "80 9c 95 95 96 11 bc 96 b9 95 9d 10"

Answer 1

You scan always the first value of text, because you forgot to move the input for strtoul right after the end of the previous scan. That's what the **end-parameter of strtoul is good for: it points to the character right after the last digit of a successful scan. Note: if nothing could have been read in, the end-pointer is equal the input pointer, and this indicates a "wrongly formated number" or the end of the string. See the following program illustrating this. Hope it helps.
int main() {
const char* input = "80 9c 95 95 96 11 bc 96 b9 95 9d 10";
const char *current = input;
char *end = NULL;
while (1) {
unsigned long val = strtoul(current, &end, 16);
if (current == end) // invalid input or end of string reached
break;
printf("val: %lX\n", val);
current = end;
}
}

Answer 2

This is also possible solution with the use of strchr, and strtoul(tmp,NULL,16);
In your solution remember that size_t arr_len = (len/3) + 1; since the last token is only 2 bytes long.
Input text tokens are converted to bytes and stored in char array:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void)
{
char text[] = "80 9c 95 95 96 11 bc 96 b9 95 9d 10";
size_t len = strlen(text);
size_t arr_len = (len/3) + 1;
printf("len= %zu arr_len= %zu\n", len, arr_len);
printf("Text:\n%s\n", text);
char array[arr_len];
const char *p1 = text;
char tmp[3];
tmp[2] = 0;
printf("Parsing:\n");
for(size_t i=0; i< arr_len; i++)
{
p1 = strchr(p1,' ');
if(p1)
{
tmp[0] = *(p1-2);
tmp[1] = *(p1-1);
array[i]= (char)strtoul(tmp,NULL,16);
printf("%2x ", (unsigned char) array[i]);
p1++;
if(strlen(p1) == 2 ) // the last char
{
i++;
tmp[0] = *(p1);
tmp[1] = *(p1+1);
array[i]= (char)strtoul(tmp,NULL,16);
printf("%2x", (unsigned char) array[i]);
}
}
}
printf("\nArray content:\n");
for(size_t i=0; i< arr_len; i++)
{
printf("%2x ", (unsigned char) array[i]);
}
return 0;
}
Test:
len= 35 arr_len= 12
Text:
80 9c 95 95 96 11 bc 96 b9 95 9d 10
Parsing:
80 9c 95 95 96 11 bc 96 b9 95 9d 10
Array content:
80 9c 95 95 96 11 bc 96 b9 95 9d 10