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Some 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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