Combine const char with string into another const char

Question

I've got two const char [] PROGMEM variables that I need to concatenate into another const char [] PROGMEM. I'm totally new to C and couldn't understand how to use strncpy on a previous question.

const char data_one[] PROGMEM = {0x00,0x01,0x02};
const char data_combined[] PROGMEM = data_one + "{0x03,0x04,0x05}";

Does this code compile? Also do you understand the keyword `const` and its semantics? — Ed Heal, Jun 10 '17 at 17:55
@EdHeal It compiles as part of a [larger script](https://raw.githubusercontent.com/Wi-PWN/Wi-PWN/master/arduino/Wi-PWN/data.h) — Sam Denty, Jun 10 '17 at 18:01
How about `const char data_one_and_two[] PROGMEM = {0x00,0x01,0x02, 0x03,0x04,0x05}";`? Can you use pointers into that for the purposes where you need the subsets? — Yunnosch, Jun 10 '17 at 18:25
@Yunnosch `data_one[]` contains over 100,000+ characters and due to limited storage space, it can only be declared once. — Sam Denty, Jun 10 '17 at 18:27
Arduino is **not** C! It is not a scripting language either! — too honest for this site, Jun 10 '17 at 18:33
"Full script is 100,000+ characters tried to edit again" - **what???** — too honest for this site, Jun 10 '17 at 18:34

score 1 · Answer 1 · edited Jun 10 '17 at 17:59

1

Using memcpy (or strcpy or similar) will not be possible since the destination is constant and read-only. You also cannot use something like you show in your sample code.

The only solution I can think of is also one I really don't recommend, since it will make the code somewhat obfuscated and hard to read and maintain, and that is using pre-processor macros.

edited Jun 10 '17 at 17:59

Ed Heal

59,252
17
87
127

answered Jun 10 '17 at 17:58

Some programmer dude

400,186
35
402
621

1

There's nothing wrong with macros, when they are used correctly. – Sam Varshavchik Jun 10 '17 at 17:59
1

@SamVarshavchik True, but in this case I really don't see any way to use macros without it becoming a maintenance nightmare. – Some programmer dude Jun 10 '17 at 18:04
`#define DATA_ONE ...` and `#define DATA_TWO ...`, then use `DATA_ONE` and `DATA_TWO` in two places, each. Doesn't look like it's particularly difficult to maintain. – Sam Varshavchik Jun 10 '17 at 18:12
@SamVarshavchik Except when you need to modify e.g. `DATA_ONE` without modifying the combined use together with `DATA_TWO`. And a few other similar cases. – Some programmer dude Jun 10 '17 at 18:14
It's obvious to me that the intent is for the combined use to always be consistent. – Sam Varshavchik Jun 10 '17 at 20:21

Combine const char with string into another const char

1 Answers1