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Statically allocated strings assume a fixed-size character array, meaning that it is impossible to add data after the buffer is filled. Because the static approach discards excess data, actual program data can be lost. Consequently, the resulting string must be fully validated.

Dynamically allocated buffers dynamically resize as additional memory is required. Dynamic approaches scale better and do not discard excess data. The major disadvantage is that, if inputs are not limited, they can exhaust memory on a machine and , consequently , be used in denial-of-service attacks.unmigrated-wiki-markup

Dynamic allocation is often disallowed in safety -critical systems. For example, the MISRA standard requires that "Dynamic dynamic heap memory allocation shall not be used" \ [[MISRA 2004|AA. Bibliography#MISRA 04]\]. Some safety -critical systems can take advantage of dynamic memory allocation during initialization but not during operations. For example, avionics software may dynamically allocate memory while initializing the aircraft , but not during flight.

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There are a number of existing libraries available for managing string data; the library selected depends on the approach adopted for managing null-terminated byte strings. The functions defined by C99, Section 7.21, "String handling <{{string.h}}>" \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] are primarily intended for managing statically allocated strings. However, these functions are problematic because many of them are insufficiently bounded.  Consequently, this standard recommends use of the ISO/IEC TR 24731-1 \[[ISO/IEC TR 24731-1:2007|AA. Bibliography#ISO/IEC TR 24731-1-2007]\] functions for use with statically allocated arrays. (See recommendation [STR07-C. Use C11 Annex K  bounds-checking interfaces for remediation of existing string manipulation code].) These functions provide bounds-checking interfaces to protect against buffer overflows and other runtime constraint violations.

A number of existing libraries are available for managing string data; the library selected depends on the approach adopted for managing null-terminated byte strings. The functions defined by the C Standard, subclause 7.24, are primarily intended for managing statically allocated strings. However, these functions are problematic because many of them are insufficiently bounded. Consequently, this standard recommends using the C11 Annex K functions with statically allocated arrays. (See STR07-C. Use the bounds-checking interfaces for string manipulation.) These functions provide bounds-checking interfaces to protect against buffer overflows and other runtime constraint violations.

ISO/IEC TR 24731 Part II offers another approach, supplying functions that allocate enough memory for their results [ISO/IEC TR 24731-2]. It provides an API that dynamically allocates the results of string functions as needed. Almost all of the APIs in this technical report are also in a current international standard. For example, TR 24731 Part II includes POSIX functions, such as strdup() [ISO/IEC 9945:2003], as well as functions from the Linux Standard Base Core Specification such as asprintf() [ISO/IEC 23360-1:2006 Wiki MarkupISO/IEC TR 24731-2 (in progress) offers another approach, supplying functions that allocate enough memory for their results \[[ISO/IEC TR 24731-2|AA. Bibliography#ISO/IEC ISO/IEC TR 24731-2]\]. ISO/IEC TR 24731-2 provides an API that dynamically allocates the results of string functions, as needed. Almost all of the APIs in this TR are also in a current International Standard. For example, TR 24731-2 includes POSIX functions, such as {{strdup()}} \[[ISO/IEC 9945:2003|AA. Bibliography#ISO/IEC 9945-2003]\], as well as functions from the Linux Standard Base Core Specification such as {{asprintf()}} \[[ISO/IEC 23360-1:2006|AA. Bibliography#ISO/IEC 23360-1-2006]\].

Risk Assessment

Failing to adopt a consistent plan for managing strings within an application can lead to inconsistent decisions, which may make it difficult to ensure system properties, such as adhering to safety requirements.