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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).

According to the Microsoft Visual Studio 2005/.NET Framework 2.0 help pages:

The getenv function searches the list of environment variables for varname. getenv is not case sensitive in the Windows operating system. getenv and _putenv use the copy of the environment pointed to by the global variable _environ to access the environment. getenv operates only on the data structures accessible to the run-time library and not on the environment "segment" created for the process by the operating system. Therefore, programs that use the envp argument to main or wmain may retrieve invalid information.

When compiled with gcc-3.4.6 and run on Andrew Linux-2.6.16.29, the following code:

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