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Some functions return a pointer to an object that cannot be modified without causing undefined behavior. These functions include getenv(), setlocale(), localeconv(), asctime(), and strerror(). In such cases, the function call results must be treated as being const-qualified.

The C Standard, 7.2224.4.6, paragraph 4 [ISO/IEC 9899:20112024], defines getenv() as follows:

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Similarly, subclause 7.11.1.1, paragraph 8 [ISO/IEC 9899:20112024], defines setlocale() as follows:

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 And subclause 7.11.2.1, paragraph 8 [ISO/IEC 9899:20112024], defines localeconv() as follows:

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Altering the string returned by setlocale() or the structure returned by localeconv() are undefined behaviors. (See undefined behaviors 120 and 121.) Furthermore, the C Standard imposes no requirements on the contents of the string by setlocale(). Consequently, no assumptions can be made as to the string's internal contents or structure.

Finally, subclause 7.2426.6.23, paragraph 4 4 [ISO/IEC 9899:20112024], states

The The strerror function returns a pointer to the string, the contents of which are locale-specific. The array pointed to shall not be modified by the program, but may be overwritten by . The behavior is undefined if the returned value is used after a subsequent call to the strerror functionthe strerror function, or after the thread which called the function to obtain the returned value has exited.

Altering the string returned by strerror() is undefined behavior. (See undefined behavior 184.)

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