Error while trying to reverse a char array

Question

I'm trying to get better at C++ (I know a little). I'm working on character arrays. I found an exercise where the objective is to reverse a character array (after I convert it from an integer). I'm getting the following error (using VS2005):

Run-Time Check Failure #2 - Stack around the variable 'revBuffer' was corrupted.

When I step through the code, I notice the following:

revBuffer = 0x0012fe40 "100899ÌÌÌÌÌÌÌÌÌÌ998001"

The relevant code is below.

    char buffer[5];
    char revBuffer[5];
    int i;
    int j=5;
    long number = 998001;

    itoa(number, buffer, 10);

    for(i=0; i<strlen(buffer);i++)
    {
        revBuffer[j] = buffer[i];
        j--;
    }

Any help would be great. TIA!

Just trace through your code - look at the values of the indices that'll be used in the first iteration of the for loop. — Cascabel
– Cascabel, Commented May 26, 2010 at 15:22
Not related to your bug, but you should hoist strlen(buffer) out of the for loop expression, so that it doesn't get computed needlessly in every loop. strlen works by scanning the entire string until it sees the null terminator. I think that leaving strlen in the loop would be a form of "premature pessimization" (IMHO). — Emile Cormier
– Emile Cormier, Commented May 26, 2010 at 15:36
This is c not c++, I retagged appropriately. The C++ code would be using std::string and .at() which would have indicated the out_of_range issue. — Matthieu M.
– Matthieu M., Commented May 26, 2010 at 15:37
@Emile: I dunno, buffer is 7 whole bytes (once the bugs are fixed). Count 'em. Count 'em 7 times if you like: I for one am happy to wait while you do that. Agreed though that it's a bad habit in general to turn an O(n) op into an O(n^2) op for no reason. @Matthieu, if you're going to insist on proper use of libraries, then "the" C++ code would use copy_backward, not at(). End of discussion ;-) — Steve Jessop
– Steve Jessop, Commented May 26, 2010 at 16:47

WhirlWind · Accepted Answer · 2010-05-26 15:21:42Z

7

You are overindexing revBuffer. It is size 5, which means you can index it from 0 to 4, but the first index you use in it in your loop is 5.

answered May 26, 2010 at 15:21

WhirlWind

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1 Comment

Matt M Over a year ago

Damn. That was a dumb mistake on my part. All the answers and suggestions were a big help. This site rocks! Thanks!

anon · Accepted Answer · 2010-05-26 15:22:51Z

6

The number you are converting has six digits - the buffer you are using is only big enough to hold 4 plus a null terminator. Make the buffer bigger.

answered May 26, 2010 at 15:22

anon

Comments

pmr · Accepted Answer · 2010-05-26 15:24:08Z

2

While your excercise is helpful you should keep in mind that there also is:

std::reverse(buffer, buffer + 5);

answered May 26, 2010 at 15:24

pmr

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Doug T. · Accepted Answer · 2010-05-26 15:26:47Z

2

In addition to what others have said, It doesn't appear that a null terminator will be placed in the last character of revBuffer even after you fix all your indexing issues. You will need to be sure to do

revBuffer[strlen(buffer)] = '\0';

This is also frought with problems because it assumes that buffer is properly terminated :)

answered May 26, 2010 at 15:26

Doug T.

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JoeG · Accepted Answer · 2010-05-26 15:25:36Z

0

For a start, you're trying to use a buffer that's 5 characters long to store a 6 digit number.

answered May 26, 2010 at 15:25

JoeG

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