How to use a static environment

[Giuseppe Modugno]

Il 05/04/2018 18:28, Craig Howland ha scritto:
> On 04/05/2018 10:49 AM, Giuseppe Modugno wrote:
>> I need to use environment functionality in newlib to have a correct 
>> result from localtime() with the western european timezone (I need TZ 
>> environment variable). I also need to convert UTC struct tm to time_t 
>> with mktime(), so I should delete TZ variable before calling mktime().
>>
>> I know I can call setenv()/unsetenv(), but I would like to avoid 
>> using dynamic allocation (that those functions use). So I'm thinking 
>> to have two static const environments, such as:
>>
>> Â  static const char *const env_with_tz[] = { 
>> "TZ=CET-1CEST,M3.5.0,M10.5.0/3", NULL };
>> Â  static const char *const env_empty[] = { NULL };
>>
>> and change the environment on-the-fly in the following way:
>>
>> Â  extern char **environ;
>> Â  environ = env_with_tz;Â Â Â  Â Â Â  // or environ = env_empty;
>>
>> Is it safe?
>>
>> Of course I need to call tzset() before calling mktime()/localtime() 
>> if the environment has changed.
>>
>>
> Are you trying to avoid the dynamic allocation of set/unset at all, or 
> only when changing the TZ value?  The reason for asking is that you 
> can avoid it in the latter case very easily.  This is because setenv() 
> does not free or allocate if a new string fits in the space taken by 
> the prior value, and just overwrites the value (when the overwrite 
> flag is set).  So you can set TZ=CET-1CEST,M3.5.0,M10.5.0/3, then 
> (rather than unsetting it), set TZ=GMT0.  Then set it back to CET again.

What happens when I set TZ back to CET? setenv() calls again malloc(), 
because "GMT0" (old value) is shorter than "CET..." (new value).