Size of strcat Destination Array

Question

Take the following program:

#include <iostream>
#include <cstring>

using namespace std;

int main()
{
    char a[8] = "Hello, ";
    char b[7] = "world!";

    strcat(a, b);

    cout << a;

    return 0;
}

Notice that a and b have the same size as their assigned strings.

The documentation states that for strcat(a, b) to work, a needs to be large enough to contain the concatenated resulting string.

Nevertheless, cout << a displays "Hello, world!". Am I entering undefined behavior?

Answer 1

"Am I entering undefined behavior?"

Yes. A region at the end of a[] has been written over. It worked, this time, but might have belonged to something else.

Here I use a struct to control memory layout, and demonstrate it:

#include <iostream>
#include <cstring>

using namespace std;

int main()
{
    struct S {
        char a[8];
        char b[5];
        char c[7];
    };

    S s;
    strcpy( s.a , "Hello, " );
    strcpy( s.b , "Foo!" );
    strcpy( s.c , "world!" );


    strcat(s.a, s.c);

    cout << s.a << endl;
    cout << s.b << endl;

    cin.get();

    return 0;
}

This outputs:

Hello, world!
orld!

Instead of:

Hello, world!
Foo!

The strcat() has stomped all over b[].

Please note that in a real life example such bugs may be far more subtle, and lead you to wondering why perfectly innocent function calls 250 lines later crash and burn horribly. ;-)

EDIT: May I also recommend that you use strcat_s, instead? Or, even better, std::strings:

#include <string>
#include <iostream>

using namespace std;

int main()
{
    string a = "Hello, ";
    string b = "world!";
    a = a + b;
    cout << a;
}

Answer 2

Am I entering undefined behavior?

Yes.

---

^{If the documentation says "a needs to be large enough to contain the concatenated resulting string", why don't you simply believe it? What is there to doubt?}