Convert comma-delimited Arduino String of hex values to array of bytes

Question

My apologies if this is a naive question. How do I convert a String which has hex values to a byte array that has those hex values?

This:

String s = "0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff";

Needs to be converted to this:

char test[] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };

I used the following procedure. It does convert it, however, each character is saved as a character as opposed to hex value:

unsigned int str_len = s.length() + 1;
char charArray[str_len];
s.toCharArray(charArray, str_len);

Any help will be appreciated. Thank you!

• jsotola: there is no such thing as a hex byte ... it is simply an 8 bit number ... you could use 0xff or 255 because they are the same value
• Amir: @jsotola This is just an example, I need this to be used for all hex values. How do I convert the string to byte array?
• Gabriel Staples: Related: How to convert a hex string to an array of bytes?

Answer 1

You only convert the String object to a char array. It need to further split the char array into substring, and then convert it to number.

To split the char array, you can use strtok() function in c++ to do the job.

There is no str conversion function that can convert "0xff" into uint8_t directly. However, as @rubemnobre mentioned strtol() can be used to convert a c string to a long (in Arduino, it is 4-byte number) and further cast into a uint8_t and store in a byte array.

String s = "0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff";
// Convert String object to char array
unsigned int str_len = s.length()+1;
char charArray[str_len];
s.toCharArray(charArray, str_len);
uint8_t byteArray[7];
int i=0;
// split charArray at each ',' and convert to uint8_t
char *p = strtok(charArray, ",");
while(p != NULL) {
 byteArray[i++] = strtoul(p, NULL, 16);
 p = strtok(NULL, ",");
}
// Print the byteArray
for (i=0; i<sizeof(byteArray)/sizeof(char); i++) {
 Serial.print(byteArray[i]);
 Serial.println(", ");
}
Serial.println();

• Majenko: If you're not strtol's using "endptr" (&ptr in your case) you can just replace it with NULL and the parameter will be ignored. Also if you're casting to uint8_t then strtoul would be a better choice.
• hcheung: @Majenko, thanks and upvoted for your comment, make the code cleaner and simpler. I updated my answer.

Answer 2

There are C functions that can convert text to number. For example, strtol() or atoi(). Here is a little snippet to get you started:

char *str = "FF";
char *ptr;
int result = strtol(str, &ptr, 16); //the 16 is the base the string is in (HEX -> base 16)

result will have 0xFF or 255 by the end. You can use this to achieve what you are trying to do. Take a look at this for functions you can use to manage your String.