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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.

However, modifying the environment by using the {{setenv()}} or {{putenv()}} functions, or by any other means, may cause the environment memory to be reallocated, with the result that {{envp}} now references an incorrect location. For example, \[[Austin Group 08|AA. C References#Austin Group 08]\] says the following.

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).

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

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