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     The getenv function searches the list of environment variables for
     an entry corresponding to <varname>.
 
     Environment variables define the environment in which a process
     executes. (For example, the LIB environment variable defines the
     default search path for libraries to be linked with a program.)
     Because the getenv function is case sensitive, the <varname>
     variable should match the case of the environment variable.
 
     Environment-table entries must not be changed directly. If an
     entry must be changed, use the putenv function. To modify the
     returned value without affecting the environment table, use strdup
     or strcpy to make a copy of the string.
 
     The getenv and putenv functions use the global variable environ to
     access the environment table. The putenv function may change the
     value of environ, thus invalidating the <envp> argument to the
     main function. Therefore, it is safer to use the environ variable
     to access the environment table.
 
     Return Value
 
     The getenv function returns a pointer to the environment-table
     entry containing the current string value of <varname>. The return
     value is NULL if the given variable is not currently defined.
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