Substract first occurency from char* - arduino

Question

I have this code on esp32 which get the return of command from an obd adapter
if (receive(buffer, sizeof(buffer)) > 0) {
char *p = buffer;
Serial.print("3 - p:");
Serial.println(p);
Serial.print("31 - size of p : ");
Serial.println(strlen(p));
p = strstr(p, "43 ");
Serial.print("32 - p:");
Serial.println(p);
}
My goal is to substract the first 3 chars "43 " from the buffer
And this is what I get in serial
3 - p:43 01 33 03 01 00 00
>
31 - size of p : 23
32 - p:43 01 33 03 01 00 00
>
33 - size of p : 23
How should I proceed to substract the first "43 " from the buffer ?
Thanks

Answer 1

If you just add an offset to the buffer pointer, so you're in effect skipping three characters, you should be fine:
if (receive(buffer, sizeof(buffer)) > 0) {
char *p = buffer+3;
Serial.print("3 - p:");
Serial.println(p);
}
This should print
3 - p:01 33 03 01 00 00
Is this what you want?

Answer 2

You can use strstr to check if buffer really contains (or starts with) "43 "
If it does, the returned pointer is set to the begin of "43 "
p+3 would point to the characters following "43 ", if this is what you call "subtracting".
If there's no "43 " in buffer, p is NULL, and p+3 points anywhere. Have fun.