C++ strcat creating infinite loop - c++

Question

Im trying to create an array and each object in the array should have the name Model(i) where is is index, Im doing this so they will have the names indexed in descending order Model5, Model4 ... Im trying to do that using char[] but for some reason in my code the use of strcat inside of for loop make me get stuck on an infinite loop, the second point if someone could help is convert the index in a way that I could concatenate with the name and give to the constructor.
#include <iostream>
#include <stdio.h>
#include <string>
using namespace std;
class CARRO {
public:
CARRO() {};
CARRO(char *modelo, unsigned ano);
char* getModelo();
unsigned getAno();
private:
char modelo[100];
unsigned ano;
};
void swap(int *p, int *q);
int partition(int *v, int start, int end);
int randomizedPartition(int *v, int start, int end);
void qsHelper(int *v, int start, int end);
void quickSort(int *v, int len);
void printList(CARRO *carros, unsigned len);
int main(int argc, char const *argv[]) {
CARRO carros[5];
unsigned len = sizeof(carros)/sizeof(CARRO);
for (int i = 0; i < len; ++i) {
char modelo[] = "Modelo";
char id[] = "I";
strcat(modelo, id);
unsigned ano = 1000 * (i+1);
carros[i] = CARRO(modelo, ano);
cout << carros[i].getModelo() << endl;
}
//printList(carros, len);
return 0;
}
CARRO::CARRO(char *modelo, unsigned ano) {
strcpy(this->modelo, modelo);
this->ano = ano;
}
If I remove the line :
strcat(modelo, id);
The loop works fine. I just cant understand why strcat is somehow generating an infinite loop. the output is this: (with the line strcat)
ModeloI
ModeloI
ModeloI
ModeloI
ModeloI
ModeloI
ModeloI
ModeloI
ModeloI
ModeloI
^CModeloI

Answer 1

The loop works fine. I just cant understand why strcat is somehow generating an infinite loop. the output is this: (with the line strcat)
The loop doesn't work fine! You are trashing memory. Let's make some boxes to represent the stack as might be seen by your program (let's assume the loop has just gone around to 1):
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| char modelo[7] | int i | unsigned len |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| 'M' | 'o' | 'd' | 'e' | 'l' | 'o' | 0 | 1 | 0 | 0 | 0 | 5 | 0 | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
So when you do this:
strcat( modelo, id );
You end up with a buffer overrun by one byte. In my particular example, this writes over the first byte of the variable i, thus causing your loop to continue indefinitely:
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| char modelo[7] | int i |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| 'M' | 'o' | 'd' | 'e' | 'l' | 'o' | 'I' | 0 | 0 | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
^^^^^
Nul-terminator written
Of course, I have laid out your stack like this purely as an example. Your compiler might not keep the stack together like this. There might be extra padding after your array and it might just happen to "work". The variable i might be held in a register and never in memory, or the compiler might have unrolled the loop completely. Your architecture might be big-endian (instead of little-endian as in my example).
The point is, the resulting behaviour is completely undefined. We cannot look at this code and say what will happen, even if you get consistent results on your machine.
So to fix this, you can simply make modelo large enough to store the string "ModeloI" including the terminator, which means making it large enough to store 8 bytes instead of 7:
char modelo[8] = "Modelo";
Then you'll have defined behaviour, regardless of whether the stack is laid out as below or any other way:
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| char modelo[8] | int i |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| 'M' | 'o' | 'd' | 'e' | 'l' | 'o' | 'I' | 0 | 1 | 0 | 0 | 0 |
+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+