---
title: "Buffer Overflow | Microsoft Docs"
ms.custom: ""
ms.date: "11/04/2016"
ms.reviewer: ""
ms.suite: ""
ms.technology:  
  - "cpp-windows"
ms.tgt_pltfrm: ""
ms.topic: "article"
dev_langs: 
  - "C++"
helpviewer_keywords: 
  - "buffers [C++], character sizes"
  - "buffer overflows [C++]"
  - "MBCS [C++], buffer overflow"
ms.assetid: f2b7e40a-f02b-46d8-a449-51d26fc0c663
caps.latest.revision: 8
author: "ghogen"
ms.author: "ghogen"
manager: "ghogen"
translation.priority.ht: 
  - "cs-cz"
  - "de-de"
  - "es-es"
  - "fr-fr"
  - "it-it"
  - "ja-jp"
  - "ko-kr"
  - "pl-pl"
  - "pt-br"
  - "ru-ru"
  - "tr-tr"
  - "zh-cn"
  - "zh-tw"
---
# Buffer Overflow
Varying character sizes can cause problems when you put characters into a buffer. Consider the following code, which copies characters from a string, `sz`, into a buffer, `rgch`:  
  
```  
cb = 0;  
while( cb < sizeof( rgch ) )  
    rgch[ cb++ ] = *sz++;  
```  
  
 The question is: Was the last byte copied a lead byte? The following does not solve the problem because it can potentially overflow the buffer:  
  
```  
cb = 0;  
while( cb < sizeof( rgch ) )  
{  
    _mbccpy( rgch + cb, sz );  
    cb += _mbclen( sz );  
    sz = _mbsinc( sz );  
}  
```  
  
 The `_mbccpy` call attempts to do the correct thing — copy the full character, whether it is 1 or 2 bytes. But it does not take into account that the last character copied might not fit the buffer if the character is 2 bytes wide. The correct solution is:  
  
```  
cb = 0;  
while( (cb + _mbclen( sz )) <= sizeof( rgch ) )  
{  
    _mbccpy( rgch + cb, sz );  
    cb += _mbclen( sz );  
    sz = _mbsinc( sz );  
}  
```  
  
 This code tests for possible buffer overflow in the loop test, using `_mbclen` to test the size of the current character pointed to by `sz`. By making a call to the `_mbsnbcpy` function, you can replace the code in the `while` loop with a single line of code. For example:  
  
```  
_mbsnbcpy( rgch, sz, sizeof( rgch ) );  
```  
  
## See Also  
 [MBCS Programming Tips](../text/mbcs-programming-tips.md)