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Re: Thread safety clarification
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- To: mass at akuma dot org
- Subject: Re: Thread safety clarification
- From: Loren James Rittle <rittle at latour dot rsch dot comm dot mot dot com>
- Date: Tue, 13 Nov 2001 17:27:37 -0600 (CST)
- Cc: libstdc++ at gcc dot gnu dot org
- References: <200111130033.fAD0XCM52224@latour.rsch.comm.mot.com> <200111130617.fAD6HWq19046@latour.rsch.comm.mot.com> <3BF1461A.email@example.com>
- Reply-to: rittle at labs dot mot dot com
> Are there examples of objects which cannot be accessed from multiple
> threads simultaneously?
Assuming you are speaking about one instantiated object being known to
multiple threads, this is the default assumption you must make as an
application writer using g++ 3.0 and its implementation of the
> For example, can I have multiple threads reading from a
> globally-defined, constant string?
Assuming you mean the C++ string<> not a C-style string, I think the
best answer is don't assume that unless you see it documented as a
special case. BTW, I am fairly sure that it is not documented as a
such a special case. The author of string<> prompted the language you
now read in the general FAQ on threading using g++ 3.0 and its
implementation of the standard library...
I think there is a way to ensure that you don't need user-level locks
when working with read-only string<>. I will sketch it out (and
perhaps Nathan will comment on it). This example is implied from the
application-level lock requirement rules of our implementation. The
trick is that memory is shared for a and b (nested object handle, etc)
but the user doesn't know that thus he isn't required to provide any
application-level locks. If thread_a_code and thread_b_code both use
a, then you need application-level locks around any region accessing a
unless there is a special guarantee for the case (again, I only know
of the ones documented for the STL portion of libstdc++-v3).
const string a = "A really long read-only string...";
const string b = a;
// only use a here
// only use b here
// No other threads use a or b
// start thread a
// start thread b