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Wide characters can frequently characters will contain null bytes if taken from the ASCII character set. As a result, using narrow character functions that rely on null-byte termination can lead to obtuse behavior. Likewise, a narrow character string that is properly null-terminated might not be considered so in a wide character function. Improper use of narrow and wide character strings could result in buffer overflows or data truncation.

Noncompliant Code Example (Using strncpy() instead of wcsncpy())

This example uses strncpy(), which copies , at most , 10 bytes, but will stop copying after it encounters a null byte. Because wide characters can contain null bytes, the code can stop copying prematurely. It is important to recognize that many narrow string functions are byte functions and so can terminate prematurely.

#include <stddef.h>
#include <string.h>
 
void func(void) {
  wchar_t wide_str1[]  = L"0123456789";
  wchar_t wide_str2[] =  L"0000000000";

  strncpy(wide_str2, wide_str1, 10);
}

Noncompliant Code Example (Using wcsncpy() instead of strncpy())

In most implementations, wide characters span multiple narrow characters. The wcsncpy() function copies , at most , 10 wide characters, which is longer than is greater than the number of bytes allocated to narrow_str1. As a result, it will write the first 10 bytes of narrow_str1 into narrow_str2 and then continue padding with L'\0' null wide characters until 10 wide characters have been written.

Note that wcsncpy() does not perform null termination if the source string contains more wide characters than the destination. As a result, it is possible for an attacker to exploit such a vulnerability by passing a maliciously crafted string to wcsncpy(). If the code is intended to copy a certain number of bytes, it can overflow the buffer by writing multiple bytes because wcsncpy() measures copying by wide characters, not by bytes.

#include <wchar_t>t.h>
 
void func(void) {
  char narrow_str1[] = "0123456789";
  char narrow_str2[] = "0000000000";

  wcsncpy(narrow_str2, narrow_str1, 10);
}

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Because these are just warnings, the compiled code can still be run. When run on the i686 Linux platform, both noncompliant code examples began copying information from out of the bounds of the arguments. This behavior indicates a possible buffer overflow vulnerabilityexhibited buffer overflows.

Compliant Solution

This compliant solution uses the appropriate-width function versionsfunctions. Using wcsncpy() for wide character strings and strncpy() for narrow character strings ensures that data is not truncated or overwriting extra memorytruncated and buffer overflow does not occur.

#include <stddef.h>
#include <string.h>
#include <wchar.h>
 
void func(void) {
  wchar_t wide_str1[] = L"0123456789";
  wchar_t wide_str2[] = L"0000000000";
  /* Use of proper-width function. */ 
  wcsncpy(wide_str2, wide_str1, 10);

  char narrow_str1[] = "0123456789";
  char narrow_str2[] = "0000000000";
  /* Use of proper-width function. */ 
  strncpy(narrow_str2, narrow_str1, 10);
}

...

The strlen() function counts the number of characters in a null-terminated byte string preceding the terminating null byte. However, wide characters contain can contain null bytes, particularly when taken from the ASCII character set, as in this example. As a result, the strlen() function will return the number of bytes preceding the first null byte in the string. 

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Modern compilers recognize the difference between a char * and a wchar_t * pointer. As a result, compiling code that violates this rule will generate warnings. It is feasible to have automated software that recognizes functions of improper width and replaces them with functions of proper width (that is, software that uses wcsncpy() when it recognizes that the parameters are of type wchar_t *).

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