STR33-C. Guarantee that all strings are null-terminated

Null-terminated byte strings are, by definition, null-terminated. String operations cannot determine the length or end of strings that are not properly null-terminated which can consequently result in buffer overflows and other undefined behavior.

Non-Compliant Code Example

The standard function strncpy() and strncat() do not guarantee that the resulting string is null terminated.  If there is no null character in the first n characters of the source array pointed the result is not be null-terminated as in the following example:

char a[16];
strncpy(a, "0123456789abcdef", sizeof(a));

Compliant Solution 1

The correct solution depends on the original intent. If your intent was to truncate a string but ensure that the
result was a null-terminated string the following solution can be used.

char a[16];
strncpy(a, "0123456789abcdef", sizeof(a)-1);
a[sizeof(a)] = '\0';

Compliant Solution 2

Example using strcpy()

Compliant Solution 3

Example using strncpy_s()

Exception

An exception to this rule applies if the intent of the programmer was to convert a null-terminated byte string to a character array.  To be compliant with this standard, this intent must be made clear statement in comments.

Priority: P12 Level: L1

Failure to properly null terminate null-termianted byte strings can result in buffer overflows and the execution of arbitrary code with the permissions of the vulnerable process by an attacker.

References

• ISO/IEC 9899-1999 Section 7.1.1 Definitions of terms, Section 7.21 String handling <string.h>
• Seacord 05 Chapter 2 Strings
• ISO/IEC TR 24731-2006 Section 6.7.1.3 The strcpy_s function