[shared_ptr] why segfault?

[Xu, Peng]

greetings,

my test code is as follows:
"
int k = 128;
std::vector<boost::shared_ptr<int> > v(k);
for(unsigned int i = 0; i < k; ++i)
{
v[i].reset(&k);
// v[i].reset(new int(0));
}

int t = rand() % k + 1;
std::cout << "element " << t << " points to integer value " << *(v[t].get()) << std::endl;
}
"

It's intended to use shared_ptr in an atypical way(the normal one was commented out). The output is:
"
element 68 points to integer value 128
Segmentation fault
"

Attached debugger and got this stack:
"
Program received signal SIGSEGV, Segmentation fault.
0x0000002a95c16ed0 in _int_free () from /lib64/tls/libc.so.6
(gdb) bt
#0 0x0000002a95c16ed0 in _int_free () from /lib64/tls/libc.so.6
#1 0x0000002a95c17317 in free () from /lib64/tls/libc.so.6
#2 0x0000002a95862cee in operator delete () from /usr/lib64/libstdc++.so.5
#3 0x0000000000403ac7 in boost::checked_delete<int> (x=0x7fbfffe9c8) at checked_delete.hpp:34
#4 0x00000000004035b4 in boost::detail::sp_counted_impl_p<int>::dispose (this=0x509060) at sp_counted_impl.hpp:78
#5 0x0000000000402cf8 in boost::detail::sp_counted_base::release (this=0x509060) at sp_counted_base_gcc_x86.hpp:146
#6 0x0000000000402dee in _ZN5boost6detail12shared_countD9Ev (this=0x508058) at shared_count.hpp:199
#7 0x0000000000402e0d in boost::detail::shared_count::~shared_count (this=0x508058) at shared_count.hpp:65536
#8 0x0000000000402831 in destr_detour25 () at shared_ptr.hpp:65536
#9 0x0000000000402855 in boost::shared_ptr<int>::~shared_ptr (this=0x508050) at sp_counted_base_gcc_x86.hpp:110
#10 0x0000000000403822 in std::_Destroy<boost::shared_ptr<int> > (__pointer=0x508050) at stl_construct.h:125
#11 0x00000000004037f6 in std::__destroy_aux<boost::shared_ptr<int>*> (__first=0x508050, __last=0x509050) at stl_construct.h:101
#12 0x00000000004037c7 in std::_Destroy<boost::shared_ptr<int>*> (__first=0x508050, __last=0x509050) at stl_construct.h:143
#13 0x0000000000401d3f in _ZNSt6vectorIN5boost10shared_ptrIiEESaIS2_EED9Ev (this=0x7fbfffe900) at stl_vector.h:297
#14 0x0000000000401da1 in std::vector<boost::shared_ptr<int>, std::allocator<boost::shared_ptr<int> > >::~vector (this=0x7fbfffe900) at stl_vector.h:65536
#15 0x000000000040160a in destr_detour44 () at shared_ptr.cpp:42
#16 0x0000002a95bcaacd in __libc_start_main () from /lib64/tls/libc.so.6
#17 0x000000000040108a in _start () at start.S:96
"

I expected some problems with the code, but the segfault surprised me. Can anybody tell me why?

Thanks a lot,

[Olaf van der Spek]

Post by Xu, Peng
int t = rand() % k + 1;

What is that + 1 doing there?
Arrays and vectors start at index 0.

[Joe Gottman]

Post by Xu, Peng
greetings,
"
int k = 128;
std::vector<boost::shared_ptr<int> > v(k);
for(unsigned int i = 0; i < k; ++i)
{
v[i].reset(&k);
// v[i].reset(new int(0));
}

v[i].reset(&k) assumes that &k was allocated via a call to new.
Since it wasn't, you will eventually get a segfault when the shared
pointer at v[i] attempts to delete &k. To get around this, use the
version of reset() that takes a pointer and a deleter object.

struct null_deleter {
void operator()(const void *) {} // Do-nothing operator()
};

int k = 128;
std::vector<boost::shared_ptr<int> > v(k);
for (unsigned int i = 0; i < k; ++i) {
v[i].reset(&k, null_deleter());
}


Joe Gottman

[Xu, Peng]

-----Original Message-----
From: boost-users-***@lists.boost.org
[mailto:boost-users-***@lists.boost.org] On Behalf Of Joe Gottman
Sent: Tuesday, March 20, 2007 6:10 PM
To: boost-***@lists.boost.org
Subject: Re: [Boost-users] [shared_ptr] why segfault?

Post by Xu, Peng
greetings,
"
int k = 128;
std::vector<boost::shared_ptr<int> > v(k);
for(unsigned int i = 0; i < k; ++i)
{
v[i].reset(&k);
// v[i].reset(new int(0));
}

v[i].reset(&k) assumes that &k was allocated via a call to new.
Since it wasn't, you will eventually get a segfault when the shared
pointer at v[i] attempts to delete &k. To get around this, use the
version of reset() that takes a pointer and a deleter object.

struct null_deleter {
void operator()(const void *) {} // Do-nothing operator()
};

int k = 128;
std::vector<boost::shared_ptr<int> > v(k);
for (unsigned int i = 0; i < k; ++i) {
v[i].reset(&k, null_deleter());
}


Joe Gottman


That helps. Thanks a lot!

[Xu, Peng]

I checked out boost HEAD recently and tried to use asio library. In
folder PATH/boost/libs/asio, I did a "firefox index.html". It cannot
find PATH/boost/libs/asio/doc/html/index.html. There's even no html
folder in asio/doc. Is it just me?

p.s., I certainly can find online documents at asio.sf.net, but it's not
as convenient as having local files.

[Darren Garvey]

Hi Xu,

Post by Xu, Peng
I checked out boost HEAD recently and tried to use asio library. In
folder PATH/boost/libs/asio, I did a "firefox index.html". It cannot
find PATH/boost/libs/asio/doc/html/index.html. There's even no html
folder in asio/doc. Is it just me?

It's not just you. Chris explained why the boost docs are temporarily gone
here: http://article.gmane.org/gmane.comp.lib.boost.asio.user/448

HTH,
Darren

[Xu, Peng]

Thanks Darren for filling me in.



________________________________

From: boost-users-***@lists.boost.org
[mailto:boost-users-***@lists.boost.org] On Behalf Of Darren Garvey
Sent: Thursday, March 22, 2007 5:23 PM
To: boost-***@lists.boost.org
Subject: Re: [Boost-users] [asio] broken link?



Hi Xu,

On 22/03/07, Xu, Peng <***@intel.com> wrote:

I checked out boost HEAD recently and tried to use asio library. In
folder PATH/boost/libs/asio, I did a "firefox index.html". It cannot
find PATH/boost/libs/asio/doc/html/index.html. There's even no html
folder in asio/doc. Is it just me?


It's not just you. Chris explained why the boost docs are temporarily
gone here: http://article.gmane.org/gmane.comp.lib.boost.asio.user/448


HTH,
Darren

[Christopher Kohlhoff]

Actually, if you're working off boost CVS, and you have boostbook set up, you
can build the docs yourself. Just run "bjam --v2" from the libs/asio/doc
directory.

Cheers,
Chris