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The C standard says getenv() has the following behavior [ISO/IEC 9899:2011]:

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.

Consequently, it is best not to store this pointer, as 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 putenv(), setenv(), or other 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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CERT C++ Secure Coding Standard: ENV00-CPP. Do not store the pointer to the string returned by getenv()

ISO/IEC 9899:2011 Section  Section 7.22.4, "Communication with the environment," and Section 7.22.4.6, "The getenv function"

ISO/IEC TR 17961 (Draft) Using an object overwritten by getenv, localeconv, setlocale, and strerror [libuse]

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[MSDN] _dupenv_s() and _wdupenv_s(), getenv_s(), _wgetenv_s()
[Open Group 2004] Chapter 8 , and "Environment Variables," strdup
[Viega 2003] Section 3.6, "Using Environment Variables Securelyenvironment variables securely"

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