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Some implomentations implementations provide a nonportable environment pointer that is valid when main() is called but may be invalidated by operations that modify the environment.

The C Standard, J.5.1 [ISO/IEC 9899:2011], states:

In a hosted environment, the main function receives a third argument, char *envp[], that points to a null-terminated array of pointers to char, each of which points to a string that provides information about the environment for this execution of the program.

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After a call to the POSIX setenv() function or to another function that modifies the environment, the envp pointer may no longer reference the current environment. The Portable Operating System Interface (POSIX^®), Base Specifications, Issue 7 [IEEE Std 1003.1:2013], states:

Unanticipated results may occur if setenv() changes the external variable environ. In particular, if the optional envp argument to main() is present, it is not changed, and thus may point to an obsolete copy of the environment (as may any other copy of environ).

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