Passing narrow string arguments to wide string functions or wide string arguments to narrow string functions can lead to unexpected and undefined behavior. Scaling problems are likely because of the difference in size between wide and narrow characters. (See ARR39-C. Do not add or subtract a scaled integer to a pointer.) Because wide strings are terminated by a null wide character and can contain null bytes, determining the length is also problematic.
char are distinct types, many compilers will produce a warning diagnostic if an inappropriate function is used. (See MSC00-C. Compile cleanly at high warning levels.)
Noncompliant Code Example (Wide Strings with Narrow String Functions)
This noncompliant code example incorrectly uses the
strncpy() function in an attempt to copy up to 10 wide characters. However, because wide characters can contain null bytes, the copy operation may end earlier than anticipated, resulting in the truncation of the wide string.
Noncompliant Code Example (Narrow Strings with Wide String Functions)
This noncompliant code example incorrectly invokes the
wcsncpy() function to copy up to 10 wide characters from
narrow_str2 is a narrow string, it has insufficient memory to store the result of the copy and the copy will result in a buffer overflow.
This compliant solution uses the proper-width functions. Using
wcsncpy() for wide character strings and
strncpy() for narrow character strings ensures that data is not truncated and buffer overflow does not occur.
Noncompliant Code Example (
In this noncompliant code example, the
strlen() function is used to determine the size of a wide character string:
strlen() function determines the number of characters that precede the terminating null character. However, wide characters can contain null bytes, particularly when expressing characters 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 wide string.
This compliant solution correctly calculates the number of bytes required to contain a copy of the wide string, including the terminating null wide character:
Confusing narrow and wide character strings can result in buffer overflows, data truncation, and other defects.
Modern compilers recognize the difference between a
char * and a
wchar_t *, so 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
|Axivion Bauhaus Suite|
|CERT_C-STR38-a||Do not confuse narrow and wide character strings and functions|
|Polyspace Bug Finder|
Pointer implicitly cast to different data type
Allocated memory does not match destination pointer
Function writes to buffer at offset greater than buffer size
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
|[ISO/IEC 9899:2011]||188.8.131.52, "The |