STR00-C. Represent characters using an appropriate type

Strings are a fundamental concept in software engineering, but they are not a built-in type in C. Null-terminated byte strings (NTBS) consist of a contiguous sequence of characters terminated by and including the first null character and are supported in C as the format used for string literals. The C programming language supports single-byte character strings, multibyte character strings, and wide character strings. Single-byte and multibyte character strings are both described as null-terminated byte strings, which are also referred to as "narrow character strings."

A pointer to a null-terminated byte string points to its initial character. The length of the string is the number of bytes preceding the null character, and the value of the string is the sequence of the values of the contained characters, in order.

A wide string is a contiguous sequence of wide characters (of type wchar_t) terminated by and including the first null wide character. A pointer to a wide string points to its initial (lowest addressed) wide character. The length of a wide string is the number of wide characters preceding the null wide character, and the value of a wide string is the sequence of code values of the contained wide characters, in order.

Null-terminated byte strings are implemented as arrays of characters and are susceptible to the same problems as arrays. As a result, rules and recommendations for arrays should also be applied to null-terminated byte strings.

The C standard uses the following philosophy for choosing character types, though it is not explicitly stated in one place.

signed char and unsigned char

• Suitable for small integer values

"plain" char

• The type of each element of a string literal.
• Used for character data from a limited character set (where signedness has little meaning) as opposed to integer data.

int

• Used for data that can be either EOF (a negative value) or character data interpreted as unsigned char and then converted to int. As a result, returned by fgetc(), getc(), getchar(), and ungetc(). Also, accepted by the character handling functions from <ctype.h>, because they might be passed the result of fgetc(), etc.
• The type of a character constant. Its value is that of a plain char converted to int.

Note that the two different ways a character is used as an int (as an unsigned char + EOF, or as a plain char, converted to int) can lead to confusion. For example, isspace('\200') results in undefined behavior when char is signed.

unsigned char

• Used internally for string comparison functions, even though these operate on character data.  Consequently, the result of a string comparison does not depend on whether plain char is signed.
• Used for situations where the object being manipulated might be of any type, and it is necessary to access all bits of that object, as with fwrite().

Unlike other integer types, unsigned char has the unique property that (quoting from Section 6.2.6.1 of C99):

Values stored in [...] objects of type unsigned char shall be represented using a pure binary notation.

where a pure binary notation is defined as

A positional representation for integers that uses the binary digits 0 and 1, in which the values represented by successive bits are additive, begin with 1, and are multiplied by successive integral powers of 2, except perhaps the bit with the highest position. A byte contains CHAR_BIT bits, and the values of type unsigned char range from 0 to 2 ^{CHAR_BIT - 1} .

That is, objects of type unsigned char may have no padding bits and thus no trap representation. Thus, non-bit field objects of any type may be copied into an array of unsigned char (e.g., via memcpy()) and have their representation examined one byte at a time.

wchar_t

• Wide characters are used for natural-language character data.

Risk Assessment

Understanding how to represent characters and character strings can eliminate many common programming errors that lead to software vulnerabilities.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Other Languages

This rule appears in the C++ Secure Coding Standard as STR00-CPP. Represent characters using an appropriate type.

References

[[ISO/IEC TR 24731-1:2007]]
[[ISO/IEC 9899:1999]] Section 7.1.1, "Definitions of terms," and Section 7.21, "String handling <string.h>"
[[Seacord 05a]] Chapter 2, "Strings"

07. Characters and Strings (STR)      07. Characters and Strings (STR)