How to use length of an array in subtraction - c++

Question

My code below is working perfect when I write number or only len inside of it. However, I want to write(len-3) but it again return as if I write len inside of it. How can I solve it??
I get the length of the array by using:
int len = sizeof(tmp)/sizeof(tmp[0]);
The message variable is a String and received is:
char received = Serial.read();
if (Index<(len-3)){
message=tmp[Index++]=received;
Serial.print(message);
}
My aim is when I write dv35!1234567890*asdfqwertyuiopmnbvcx#asdas the code should return the part of it that is between '!' and '#' then during the if else part the code should get some parts of it. However, the last part should return until 3 chars before the array ends. but it did not work properly. it returns all the parts remaining.(As if I write only len inside of it). the length of the input is not stable so the message part changes depending on the length of the whole array. It should return 3 chars before the end.
Here is my whole code:
char tmp[200] = {0};
String pack;
String command;
String Size;
String messum;
String message;
String checksum;
int len = sizeof(tmp)/sizeof(tmp[0]);
int Index = 0;
bool Seen = false;
void setup(){
Serial.begin(9600);
}
void loop(){
while (Serial.available() > 0){
char received = Serial.read();
if (received == '!'){
Seen = true;
}
else if (received == '#'){
return strdup(tmp);
}
else if (Seen == true){
if(Index<2){
//Serial.print("P");
pack=tmp[Index++] = received;
Serial.print(pack);
}
else if(Index<4){
//Serial.print("C");
command=tmp[Index++] = received;
Serial.print(command);
}
else if(Index<6){
//Serial.print("S");
Size=tmp[Index++] = received;
Serial.print(Size);
}
else if (Index<(len-3)){
Serial.print("M");
message=tmp[Index++]=received;
Serial.print(message);
}
}
}
return NULL;
//input: dv35!1234567890*asdfqwertyuiopmnbvcx#asdas
}

Answer 1

there is no need of most of it.
You define the variable tmp in the global and it has constant length. You do not need to calculate the length of it.
For the char array sizeof(char_array) gives you the length of the array as sizeof(char) is by definition 1
Use the correct types. for the sizes use size_t not int