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Null-terminated byte strings (NTBS) must contain a null-termination character at or before the address of the last element of the array before they can be safely passed as arguments to standard string-handling functions, such as {{strcpy()}} or {{strlen()}}.  This is because these functions, as well as other string-handling functions defined by C99 \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\], depend on the existence of a null-termination character to determine the length of a string.  Similarly, NTBS must be null terminated before iterating on a character array where the termination condition of the loop depends on the existence of a null-termination character within the memory allocated for the string, as in the following example:

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.

In the second noncompliant code example, {{memset()}} is used to clear the destination buffer; unfortunately, the third argument incorrectly specifies the size of the destination array \[[Schwarz 05|AA. C References#Schwarz 05]\].

The {{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.

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.1.1, "Definitions of terms," Section 7.20.3.4 "The realloc function," and 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"
\[[MITRE 07|AA. C References#MITRE 07]\] [CWE ID 119|http://cwe.mitre.org/data/definitions/119.html], "Failure to Constrain Operations within the Bounds of an Allocated Memory Buffer," [CWE ID 170|http://cwe.mitre.org/data/definitions/170.html], "Improper Null Termination"
\[[Schwarz 05|AA. C References#Schwarz 05]\]
\[[Seacord 05a|AA. C References#Seacord 05]\] Chapter 2, "Strings"
\[[Viega 05|AA. C References#Viega 05]\] Section 5.2.14, "Miscalculated NULL termination"