Question

Does ostrstream(char* s, int n) prevent buffer overflow?

Asked by: Monica2003

Hi,

I have the following snippet of code, and I just want to confirm that ostrstream doesn't prevent buffer overflows.  Is this correct?  (Yes, I know I should be using ostringstream but dealing with some legacy code.)

Thanks!

int main()
{
  char buf[5];
 
  ostrstream lStream(buf, 5);
 
  lStream << "abcdefg" << ends;
 
  cout << lStream.str() << endl;
}

                                  
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Asked On
2009-08-17 at 15:20:03ID24659851
Tags

C++

Topic

C++ Programming Language

Participating Experts
2
Points
50
Comments
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Answers

 

by: Infinity08Posted on 2009-08-17 at 15:46:48ID: 25119046

I have quickly checked the standard, but couldn't find a guarantee that buffer overflows are prevented.

That said, trying it out, shows that at least the gcc compiler prevents buffer overflows (ie. it only writes 5 characters into the buffer).

I have never actually needed to use the deprecated strstream classes though, so take my advice as purely theoretical, and based on a test for one specific compiler :)

 

by: efnPosted on 2009-08-17 at 23:11:08ID: 25120451

I'm not positive, but I think ostrstream should prevent buffer overflows.

ostrstream is derived from basic_ostream and initializes the base class with a strstreambuf.  strstrambuf is derived from basic_streambuf.  Section 27.5 of the standard specifies stream buffers in nine pages.  I didn't read it all, but it looks like a stream buffer is responsible for not writing beyond its controlled sequence.  Note that although ostrstream is deprecated, the base classes basic_ostream and basic_streambuf are not.

 

by: Infinity08Posted on 2009-08-18 at 00:29:23ID: 25120726

What isn't clear to me, is that the standard talks about a mechanism for re-allocating the stream buffer to a bigger size to accomodate the extra characters. But whether that is applicable in this specific case, I can't tell for sure. It seems gcc doesn't do this ... But then again, this isn't really something that is completely standardized either.

 

by: Monica2003Posted on 2009-08-18 at 10:38:11ID: 25125614

Hmm..., I ran the above code again, and I'm getting this funny output:  "abcde?;"

I'm using g++ to compile my program.  

 

by: Infinity08Posted on 2009-08-18 at 11:46:57ID: 25126236

>> Hmm..., I ran the above code again, and I'm getting this funny output:  "abcde?;"

Since all 5 characters in the buffer are used up, it's not null terminated (there's no more room for the '\0' terminator). You'll need to account for that when printing the buffer.

 

by: Monica2003Posted on 2009-08-18 at 14:01:58ID: 25127640

I see...  I was expecting the call to "<< ends" to always put the null character in the buffer, even if there is something in there.  Wouldn't that make more sense?

 

by: Infinity08Posted on 2009-08-18 at 14:04:17ID: 25127659

Maybe. But you chose the buffer size to be 5, it's equally reasonable to assume that you knew the limitations, and would provide a large enough buffer.

You can always null-terminate it yourself.

20120131-EE-VQP-002

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