Re: Read access to std::map from multiple threads

[mathieu]

On Jun 18, 9:15 pm, Francis Litterio <em...@not.available> wrote:
> mathieu wrote:
> >   I am currently having some very random seg faults. The few times
> > where it seg fault this is somewhere during a read access to a
> > std::map. Those accesses are concurrent (SMP machine). Is there
> > something I need to do even for read only access ?
>
> I assume that _something_ writes to the memory that is being read.
> Given that, you'll need to synchronize all the readers and writers using
> a mutex.

Hi Fran,

  I was not searching at the right spot. The fact the result from a
find in a std::map was trashed was (I believe) because initilization
was done too late (from a thread). I'll try to initialize the map
earlier.

Thanks
-Mathieu

Just for reference, here was the backtrace that lead me to believe
std::map was not thread safe:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x40804950 (LWP 8374)]
0x00002b78be3fd104 in std::_Rb_tree_rotate_left () from /usr/lib/
libstdc++.so.6
(gdb) bt
#0  0x00002b78be3fd104 in std::_Rb_tree_rotate_left () from /usr/lib/
libstdc++.so.6
#1  0x00002b78be3fd28c in std::_Rb_tree_insert_and_rebalance () from /
usr/lib/libstdc++.so.6
#2  0x00002b78ba63b9ce in std::_Rb_tree<gdcm::Tag, std::pair<gdcm::Tag
const, gdcm::DictEntry>, std::_Select1st<std::pair<gdcm::Tag const,
gdcm::DictEntry> >, std::less<gdcm::Tag>,
std::allocator<std::pair<gdcm::Tag const, gdcm::DictEntry> >
>::_M_insert (this=0x646c90, __x=0x0, __p=0x2aaab802df40,
__v=@0x40802ba0) at /usr/lib/gcc/x86_64-linux-gnu/4.1.2/../../../../
include/c++/4.1.2/bits/stl_tree.h:821
#3  0x00002b78ba63bbb1 in std::_Rb_tree<gdcm::Tag, std::pair<gdcm::Tag
const, gdcm::DictEntry>, std::_Select1st<std::pair<gdcm::Tag const,
gdcm::DictEntry> >, std::less<gdcm::Tag>,
std::allocator<std::pair<gdcm::Tag const, gdcm::DictEntry> >
>::insert_unique (this=0x646c90, __v=@0x40802ba0) at /usr/lib/gcc/
x86_64-linux-gnu/4.1.2/../../../../include/c++/4.1.2/bits/stl_tree.h:
931
#4  0x00002b78ba63bc3f in std::map<gdcm::Tag, gdcm::DictEntry,
std::less<gdcm::Tag>, std::allocator<std::pair<gdcm::Tag const,
gdcm::DictEntry> > >::insert (this=0x646c90, __x=@0x40802ba0)
    at /usr/lib/gcc/x86_64-linux-gnu/4.1.2/../../../../include/c++/
4.1.2/bits/stl_map.h:396
#5  0x00002b78ba63bca3 in gdcm::Dict::AddDictEntry (this=0x646c90,
tag=@0x40802c40, de=@0x40802c20) at /home/mmalaterre/Projects/gdcm/
branches/gdcm-2-0/Source/DataDictionary/gdcmDict.h:69