How to use a static environment

[Craig Howland]

On 04/06/2018 03:27 AM, Giuseppe Modugno wrote:
> 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).
>
Sorry, I forgot to point out to pad, so "GMT0Â Â Â Â Â  " (enough spaces to match the 
length of the CET string).