Re: Calling _beginthreadex() during creation of global object (or before main() is called)
On Sun, 24 Dec 2006 20:46:24 -0000, "Paul" <vhr@newsgroups.nospam> wrote:
As I understand, threads begin to run after main() is entered (or perhaps
just before).
They are scheduled to run as soon as they're created, assuming they weren't
created in the suspended state.
If this understanding is correct, are there any pitfalls to be
expected if _beginthreadex() is called during a creation of a global
(namespace-level) object to create a running (not suspended) thread? I am
not counting on it to begin to run before its time but may this cause a
problem? (The code written as per above seems to have caused problems in
some environments with the thread creation by _beginthreadex() failing the
test for a valid handle; unfortunately I do not have any specific error
codes to hand.)
You must not do that for globals in DLLs, because their construction occurs
during the execution of DllMain, and DllMain forbids thread creation, among
other things. Globals in EXEs don't have this problem, but I don't
recommend starting threads from their ctors. It's a very bad idea to allow
secondary threads to continue to run as the program shuts down following
return from main() or calling exit(), and avoiding this means joining with
all the threads before you exit. Since main() (or more generally, code
executed from main) has to deal with the threads' termination, it can't
escape dealing with the threads, so it might as well coordinate their
start-up. This way, both program start-up and termination are
single-threaded, and I find this model easier to reason about. On the
start-up side, it addresses the things Bruno talked about, namely allowing
you to handle thread creation errors and guaranteeing all EXE globals are
constructed once the thread begins running.
In case of an error, _beginthreadex() sets errno with a set of codes more
limited than that GetLastError() may return. Will it make sense to obtain a
code from GetLastError() in the case of _beginthreadex()?
You don't know that the last Windows API _beginthreadex called was
CreateThread, so it doesn't make sense to call GetLastError.
--
Doug Harrison
Visual C++ MVP