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The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program but may be overwritten by a subsequent call to the getenv function.

This gives an implementation the latitude, for example, to return a pointer to a statically allocated buffer of static storage duration.  Consequently, do not to store this pointer because it may be overwritten by a subsequent call to the getenv() function or invalidated as a result of changes made to the environment list through calls to the POSIX functions putenv() or  setenv(), or other implementation-specific means. Storing the pointer for later use can result in a dangling pointer or a pointer to incorrect data. This string should be referenced immediately and discarded; if later use is anticipated, the string should be copied so the copy can be safely referenced as needed.

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The localeconv(), setlocale(), and strerror() functions have similar caveatsrestrictions. Do not access the objects returned by any of these functions after a subsequent call.

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Annex K provides the getenv_s()function for getting a value from the current environment. However, note that according to the standard, getenv_s() can still have data races with other threads of execution that modify the environment list.

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