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Re: [Valgrind-users] novice drd user. Errors in std streams
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From: Bart V. A. <bva...@ac...> - 2010-04-06 10:30:35
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On Tue, Apr 6, 2010 at 12:25 PM, Konstantin Serebryany
<kon...@gm...> wrote:
>
>
> On Tue, Apr 6, 2010 at 2:17 PM, Bart Van Assche <bva...@ac...> wrote:
>>
>> On Tue, Apr 6, 2010 at 7:53 AM, Jorge Moraleda <jor...@gm...>
>> wrote:
>> >
>> > Dear all,
>> >
>> > When I compile the following program with:
>> > $ g++ -g -pthread threads.cpp
>> >
>> > // begin program /////////////////////////////////////////////
>> > // file: threads.cpp
>> > #include <pthread.h>
>> > #include <iostream>
>> > #include <sstream>
>> >
>> > void *threadEntry(void *threadid)
>> > {
>> > long tid;
>> > tid = (long)threadid;
>> > for (int i = 0; i<5; i++) {
>> > std::stringstream myStream;
>> > myStream << "this thread is " << tid;
>> > std::string myString(myStream.str());
>> > pthread_yield();
>> > }
>> > pthread_exit(NULL);
>> > }
>> >
>> > int main (int argc, char *argv[])
>> > {
>> > pthread_t threads[2];
>> > int rc;
>> > long t;
>> > for(t=0; t<2; t++) {
>> > rc = pthread_create(&threads[t], NULL, threadEntry, (void *)t);
>> > }
>> > pthread_exit(NULL);
>> > }
>> > // end program /////////////////////////////////////////////
>> >
>> > and run drd on it with:
>> > $ valgrind --tool=drd ./a.out
>> >
>> > I get the following output:
>> >
>> > ==16240== drd, a thread error detector
>> > ==16240== Copyright (C) 2006-2009, and GNU GPL'd, by Bart Van Assche.
>> > ==16240== Using Valgrind-3.5.0-Debian and LibVEX; rerun with -h for
>> > copyright info
>> > ==16240== Command: ./a.out
>> > ==16240==
>> > ==16240== Thread 3:
>> > ==16240== Conflicting load by thread 3 at 0x05132e98 size 1
>> > ==16240== at 0x4ED44C8: std::ostream&
>> > std::ostream::_M_insert<long>(long) (in /usr/lib/libstdc++.so.6.0.13)
>> > ==16240== by 0x400C48: threadEntry(void*) (threads.cpp:12)
>> > ==16240== by 0x4C32870: vgDrd_thread_wrapper
>> > (drd_pthread_intercepts.c:272)
>> > ==16240== by 0x55E8A03: start_thread (pthread_create.c:300)
>> > ==16240== by 0x58DD80C: clone (clone.S:112)
>> > ==16240== Allocation context: BSS section of
>> > /usr/lib/libstdc++.so.6.0.13
>> > ==16240== Other segment start (thread 2)
>> > ==16240== at 0x4C29F2F: pthread_mutex_unlock
>> > (drd_pthread_intercepts.c:633)
>> > ==16240== by 0x4EA387E: std::locale::locale() (in
>> > /usr/lib/libstdc++.so.6.0.13)
>> > ==16240== by 0x4E9FE5F: std::ios_base::_M_init() (in
>> > /usr/lib/libstdc++.so.6.0.13)
>> > ==16240== by 0x4EB4768: std::basic_ios<char, std::char_traits<char>
>> > >::init(std::basic_streambuf<char, std::char_traits<char> >*) (in
>> > /usr/lib/libstdc++.so.6.0.13)
>> > ==16240== by 0x4ED7DEA: std::basic_stringstream<char,
>> > std::char_traits<char>, std::allocator<char>
>> > >::basic_stringstream(std::_Ios_Openmode) (in
>> > /usr/lib/libstdc++.so.6.0.13)
>> > ==16240== by 0x400C21: threadEntry(void*) (threads.cpp:11)
>> > ==16240== by 0x4C32870: vgDrd_thread_wrapper
>> > (drd_pthread_intercepts.c:272)
>> > ==16240== by 0x55E8A03: start_thread (pthread_create.c:300)
>> > ==16240== by 0x58DD80C: clone (clone.S:112)
>> > ==16240== Other segment end (thread 2)
>> > ==16240== at 0x58ACEB7: sched_yield (in /lib/libc-2.10.1.so)
>> > ==16240== by 0x400C63: threadEntry(void*) (threads.cpp:14)
>> > ==16240== by 0x4C32870: vgDrd_thread_wrapper
>> > (drd_pthread_intercepts.c:272)
>> > ==16240== by 0x55E8A03: start_thread (pthread_create.c:300)
>> > ==16240== by 0x58DD80C: clone (clone.S:112)
>> > ==16240==
>> > ==16240== Conflicting load by thread 3 at 0x05132eb9 size 1
>> > ==16240== at 0x4ED44D3: std::ostream&
>> > std::ostream::_M_insert<long>(long) (in /usr/lib/libstdc++.so.6.0.13)
>> > ==16240== by 0x400C48: threadEntry(void*) (threads.cpp:12)
>> > ==16240== by 0x4C32870: vgDrd_thread_wrapper
>> > (drd_pthread_intercepts.c:272)
>> > ==16240== by 0x55E8A03: start_thread (pthread_create.c:300)
>> > ==16240== by 0x58DD80C: clone (clone.S:112)
>> > ==16240== Allocation context: BSS section of
>> > /usr/lib/libstdc++.so.6.0.13
>> > ==16240== Other segment start (thread 2)
>> > ==16240== at 0x4C29F2F: pthread_mutex_unlock
>> > (drd_pthread_intercepts.c:633)
>> > ==16240== by 0x4EA387E: std::locale::locale() (in
>> > /usr/lib/libstdc++.so.6.0.13)
>> > ==16240== by 0x4E9FE5F: std::ios_base::_M_init() (in
>> > /usr/lib/libstdc++.so.6.0.13)
>> > ==16240== by 0x4EB4768: std::basic_ios<char, std::char_traits<char>
>> > >::init(std::basic_streambuf<char, std::char_traits<char> >*) (in
>> > /usr/lib/libstdc++.so.6.0.13)
>> > ==16240== by 0x4ED7DEA: std::basic_stringstream<char,
>> > std::char_traits<char>, std::allocator<char>
>> > >::basic_stringstream(std::_Ios_Openmode) (in
>> > /usr/lib/libstdc++.so.6.0.13)
>> > ==16240== by 0x400C21: threadEntry(void*) (threads.cpp:11)
>> > ==16240== by 0x4C32870: vgDrd_thread_wrapper
>> > (drd_pthread_intercepts.c:272)
>> > ==16240== by 0x55E8A03: start_thread (pthread_create.c:300)
>> > ==16240== by 0x58DD80C: clone (clone.S:112)
>> > ==16240== Other segment end (thread 2)
>> > ==16240== at 0x58ACEB7: sched_yield (in /lib/libc-2.10.1.so)
>> > ==16240== by 0x400C63: threadEntry(void*) (threads.cpp:14)
>> > ==16240== by 0x4C32870: vgDrd_thread_wrapper
>> > (drd_pthread_intercepts.c:272)
>> > ==16240== by 0x55E8A03: start_thread (pthread_create.c:300)
>> > ==16240== by 0x58DD80C: clone (clone.S:112)
>> > ==16240==
>> > ==16240==
>> > ==16240== For counts of detected and suppressed errors, rerun with: -v
>> > ==16240== ERROR SUMMARY: 6 errors from 2 contexts (suppressed: 208 from
>> > 140)
>> >
>> > My question is: Are the errors reported real multi threading errors?
>> > If not, how can I tell drd to suppress them? (In my real world program,
>> > I
>> > have thousands of errors in hundreds of contexts, and I am trying to
>> > find the real ones.)
>> > If yes, can you help me understand what is wrong? Both threads only
>> > use local variables. I have been reading about the global object
>> > "locale" used by streams for localization, but I have read that it is
>> > protected by a global mutex.
>>
>> Hello Jorge,
>>
>> Unfortunately not all libraries have been designed with data-race
>> detection tools in mind. Several libraries contain code that triggers
>> benign data races. Examples are the I/O code in libstdc++ and in libc.
>>
>> You can either create a suppression pattern to suppress the above
>> races, or even simpler, add the following code in main() before thread
>> creation starts:
>>
>> std::ostringstream dummy;
>> dummy << 0;
>>
>> The above code will make sure that locale initialization, which is
>> triggered by sending an number to an I/O stream, will happen before
>> any threads are created and hence no races will be reported anymore on
>> locale initialization.
>
> This will not hide the race on _ZNSs4_Rep20_S_empty_rep_storageE.
> And suppressing errors in string guts may hide real races.
The above indeed won't hide races on
_ZNSs4_Rep20_S_empty_rep_storageE. But I've never claimed that the
above code would hide these races. And Jorge didn't report any such
races in his original e-mail. So your reply seems off-topic to me.
Bart.
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