problem with array of struct

[Markus]

Hi!
I've the follwoing in my C/C++-Program:

There's a struct defined as following:

struct t_datensatz {
char flag;
char ID;
char Tel[29];
};

..
..

int main void() {
t_datensatz ds[2];

ds[0].flag='1';
ds[0].ID='2';
strcpy(ds[0].Tel,"TelegrammmmmmmmmmmmmmmmmmmmmA");
cout << "OUT: " << ds[0].flag << ds[0].ID << ds[0].Tel << endl;

ds[1].flag='3';
ds[1].ID='4';
strcpy(ds[1].Tel,"TelegrammmmmmmmmmmmmmmmmmmmmB");
cout << "OUT: " << ds[1].flag << ds[1].ID << ds[1].Tel << endl;

ds[2].flag='5';
ds[2].ID='6';
strcpy(ds[2].Tel,"TelegrammmmmmmmmmmmmmmmmmmmmC");
cout << "OUT: " << ds[2].flag << ds[2].ID << ds[2].Tel << endl;

// This doesn't work, why??
for(int i=0;i<=2;i++)
cout << "OUT: " << ds[i].flag << ds[i].ID << " " <<
ds[i].Tel <<
endl;

}

Produced Output:

OUT: 12TelegrammmmmmmmmmmmmmmmmmmmmA
OUT: 34TelegrammmmmmmmmmmmmmmmmmmmmB
OUT: 56TelegrammmmmmmmmmmmmmmmmmmmmC
OUT: 12 TelegrammmmmmmmmmmmmmmmmmmmmA34Telegrammmmmmmmmmmm mmmmmmmmmB56TelegrammmmmmmmmmmmmmmmmmmmmC
OUT: 34 TelegrammmmmmmmmmmmmmmmmmmmmB56Telegrammmmmmmmmmmm mmmmmmmmmC
OUT: 56 TelegrammmmmmmmmmmmmmmmmmmmmC

Ok, the first three lines are fine. But why does the for-statement not
work as it is intented to??? It should produce exactly the same output
as the "manual" cout-statements.
I guess the problem is "ds[i].Tel". What's wrong in that little
program?

Tnx,
Markus

[Noah Roberts]

Markus wrote:

Hi!
I've the follwoing in my C/C++-Program:

There's a struct defined as following:

struct t_datensatz {
char flag;
char ID;
char Tel[29];
};

.
.

int main void() {
t_datensatz ds[2];
Holds 2 items.
ds[0].flag='1';
ds[0].ID='2';
strcpy(ds[0].Tel,"TelegrammmmmmmmmmmmmmmmmmmmmA");
cout << "OUT: " << ds[0].flag << ds[0].ID << ds[0].Tel << endl;

ds[1].flag='3';
ds[1].ID='4';
strcpy(ds[1].Tel,"TelegrammmmmmmmmmmmmmmmmmmmmB");
cout << "OUT: " << ds[1].flag << ds[1].ID << ds[1].Tel << endl;

The following results in overflow of ds. ds[2].flag='5';
ds[2].ID='6';
strcpy(ds[2].Tel,"TelegrammmmmmmmmmmmmmmmmmmmmC");
cout << "OUT: " << ds[2].flag << ds[2].ID << ds[2].Tel << endl;

// This doesn't work, why??
for(int i=0;i<=2;i++)
cout << "OUT: " << ds[i].flag << ds[i].ID << " " <<
ds[i].Tel <<
endl;
Your for loop is of course also doing 3 instead of 2...overflow...
}

Produced Output:

OUT: 12TelegrammmmmmmmmmmmmmmmmmmmmA
OUT: 34TelegrammmmmmmmmmmmmmmmmmmmmB
OUT: 56TelegrammmmmmmmmmmmmmmmmmmmmC
OUT: 12 TelegrammmmmmmmmmmmmmmmmmmmmA34Telegrammmmmmmmmmmm mmmmmmmmmB56TelegrammmmmmmmmmmmmmmmmmmmmC
OUT: 34 TelegrammmmmmmmmmmmmmmmmmmmmB56Telegrammmmmmmmmmmm mmmmmmmmmC
OUT: 56 TelegrammmmmmmmmmmmmmmmmmmmmC

That is odd results, but the output of your program is undefined so...

Ok, the first three lines are fine. But why does the for-statement not
work as it is intented to??? It should produce exactly the same output
as the "manual" cout-statements.
I guess the problem is "ds[i].Tel". What's wrong in that little
program?
buffer overflow at the least. I don't see anything else wrong.
Tnx,
Markus

--
Noah Roberts
- "If you are not outraged, you are not paying attention."

[Rolf Magnus]

Markus wrote:

Hi!
I've the follwoing in my C/C++-Program:

There's a struct defined as following:

struct t_datensatz {
char flag;
char ID;
char Tel[29];
};

.
.

int main void() {
The above makes me suspicious. Is that really the exact code you tried?
t_datensatz ds[2];

ds[0].flag='1';
ds[0].ID='2';
strcpy(ds[0].Tel,"TelegrammmmmmmmmmmmmmmmmmmmmA");
You're writing one beyond the end of ds[0].Tel. It can hold 29 elements,
but you're feeding 30 to it. Don't forget the '\0' character that is
automatically appended to string literals.
cout << "OUT: " << ds[0].flag << ds[0].ID << ds[0].Tel << endl;

ds[1].flag='3';
ds[1].ID='4';
strcpy(ds[1].Tel,"TelegrammmmmmmmmmmmmmmmmmmmmB");
cout << "OUT: " << ds[1].flag << ds[1].ID << ds[1].Tel << endl;

ds[2].flag='5';
ds[2].ID='6';
strcpy(ds[2].Tel,"TelegrammmmmmmmmmmmmmmmmmmmmC");
cout << "OUT: " << ds[2].flag << ds[2].ID << ds[2].Tel << endl;
Again, you're writing one beyond an array, this time it's the array ds,
which can hold 2 elements, but you try to write 3 of them into it.
// This doesn't work, why??
for(int i=0;i<=2;i++) ^^
Always think twice if the end condition for the loop counter contains a
<=. Your array only can hold 2 elements, from ds[0] to ds[1], so your
loop counter must stop before 2, i.e. it must be 'i<2'.
cout << "OUT: " << ds[i].flag << ds[i].ID << " " <<
ds[i].Tel <<
endl;

}

[Default User]

Markus wrote:

Hi!
I've the follwoing in my C/C++-Program:

There's a struct defined as following:

struct t_datensatz {
char flag;
char ID;
char Tel[29];
};

Why are you using char buffers instead of std::string? Especially fixed
width buffers, which often waste space and are subject to buffer
overflow. As others have pointed out, that's what your problem is.


Brian Rodenborn