I need to change locale in the thread to parse a double with strtod() correctly, I'm using setlocale() for this (C++). Is it thread safe?
Update: Another problem. When I invoke setlocale() in my main() function it doesn't affect in other routines deeper. Why??? There is a lot of code, so it's problematic to write the chunk.
You need to consult the documentation for whatever implementation you're using. C++ doesn't currently specify anything about threads so it comes down to the implementation (which you haven't yet told us).
For example, my Linux manpage for setlocale has the snippet:
This string may be allocated in static storage.
which doesn't absolutely indicate that it's thread-unsafe but I'd be very wary. It's likely that calling it with NULL (i.e., querying) would be thread-safe but as soon as you have a thread modifying it, all bets are off.
Probably the safest thing to do (assuming it isn't thread-safe) would be to protect all calls to setlocale with a mutex and have a special function to format your numbers along the lines of:
claim mutex
curr = setlocale to specific value
format number to string
setlocale to curr
release mutex
return string