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Upon return, functions should guarantee that any objects object returned by the function, or any modified value referenced by a pointer argument, is a valid object of function return type or argument type. Failure to do so can result in  Otherwise, type errors can occur in the program.

A good example is the null-terminated byte string type in C. If a string lacks the terminating null character, the program may be tricked into accessing storage after the string as legitimate data. This may cause a program to process  A program may, as a result, process a string that it should not process, which might be a security flaw in itself. It may also cause the program to abort, which might be a denial-of-service attack. Note also the emphasis is not to produce unterminated strings.

This The emphasis of this recommendation is to avoid producing unterminated strings; it does not address processing of already existing unterminated strings. However, by preventing the creation of unterminated strings, the need to process them is greatly lessened.

Noncompliant Code Example

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The standard {{strncpy()}} function does not guarantee that the resulting string is null terminated \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]. If there is no null character in the first {{n}} characters of the {{source}} array, the result may not be null -terminated. If there is no null character in the first n characters of the source array, the result may not be null-terminated.

Code Block
bgColor #FFcccc lang c
char *source; char a[NTBS_SIZE]; /* ... */ if (source) { errno_t errchar* b = strncpy(a, source, 5); // if (err != 0) { /* Handle error */ } } b == a } else { /* handleHandle NULLnull string condition */ }

Compliant Solution (strncpy_s()

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, C11 Annex K)

The C11 Annex K  Wiki MarkupThe {{strncpy_s()}} function copies up to {{n}} characters from the source array to a destination array \[[TR 24731|AA. C References#ISO/IEC TR 24731-1-2007]\]. If no null character was copied from the source array, then the {{n{}}}th position in the destination array is set to a null character, guaranteeing that the resulting string is null-terminated.

char *source;
char a[NTBS_SIZE];
/* ... */
if (source) {
  errno_t err = strncpy_s(a, sizeof(a), source, 5);
  if (err != 0) {
    /* Handle error */
  }
}
else {
  /* handleHandle NULLnull string condition */
}

Risk Assessment

Failure to do so can to enforce type safety can result in type errors in the program.

Automated Detection

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 API07-CPP. Enforce type safety.

References

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.21, "String handling <string.h>"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "CJM String Termination"
\[[ISO/IEC TR 24731-1:2007|AA. C References#ISO/IEC TR 24731-1-2007]\] Section 6.7.1.4, "The strncpy_s function"

Related Guidelines

Key here (explains table format and definitions)

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Image Added Image Added API09-C. Compatible values should have the same typeAPI04-C. Provide a consistent and usable error checking mechanism      13. Application Programming Interfaces (API)      API08-C. Protect header prototypes from misinterpretation