Windows的rand_s线程安全吗？

Question

Just as in title. Is suspect it is, but I couldn't find it anywhere explicitly stated. And for this property I wouldn't like to rely on speculations.

Answer 1

If you use the multithreaded version of the CRT, all functions are thread safe, because any thread-specific information is stored in TLS . rand_s actually doesn't use state information in the first place, since it just calls an OS API, so question of thread-safety doesn't arise for rand_s. rand(), however depends on a seed value to generate a random number.

Answer 2

Chris said: rand() is not thread-safe because its internal state is static, but rand_s() should be thread-safe, however.

Jeff added however that with the multithreaded version of MSVCRT, rand() 's state is held in thread-local storage, so it's okay still.

Answer 3

Visual Studio comes with the source to the runtime library. While some of it can be rather painful to wade through, rand_s() is pretty simple.

All rand_s() does is call SystemFunction036() in ADVAPI32.DLL to get the random value. Anything in ADVAPI32.DLL should be thread-safe.

For its part, rand_s() gets the pointer to that function in a thread-safe manner.

Answer 4

I don't know if rand_s is thread-safe, but it seems like it probably is, since it seems to make a round-trip to the OS for entropy. (as long as you link to the VC++ multi-thread CRT, all bets are off if you link to the single-thread one)

If it's supported by windows CRT, you can try a call to rand_r which is the posix reentrant version of rand. OR even better boost::random, if you're already using boost.

considering how pervasive multi-threading will be soon, no one should be using rand() anymore in new code - always try to use rand_r/rand_s/boost/various platform-dependent secure rands/etc.

Answer 5

我想不出为什么rand_s（）甚至rand（）不是线程安全的任何原因。