Using std::atomic to implement a lockfree structure, Dtor crashes - c++

Question

I'm refering to cppreference sample of std::atomic, trying to add a Dtor function for stack:
#include<atomic>
template<class T>
struct node{
T data;
node* next;
node(const T&data):data(data),next(nullptr){}
};
template<class T>
class stack{
std::atomic<node<T>*> head;
public:
void push(const T&data)
{
node<T>* new_node=new node<T>(data);
new_node->next=head.load(std::memory_order_relaxed);
while(!std::atomic_compare_exchange_weak_explicit(
&head,
&new_node->next,
new_node,
std::memory_order_release,
std::memory_order_relaxed));
}
~stack()
{
node<T>* p=head;
while(p)
{
node<T>* next=p->next;
delete p;
p=next;
}
}
};
int main()
{
stack<int> s;
s.push(1);
s.push(2);
s.push(3);
return 0;
}
When the program executes till ~stack，the last delete function prompts crash, like below:
$ g++ myatomic.cpp -std=c++11
$ ./a.out
*** Error in `./a.out': munmap_chunk(): invalid pointer: 0x0000000000400b00 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x77725)[0x7f07173a2725]
/lib/x86_64-linux-gnu/libc.so.6(cfree+0x1a8)[0x7f07173aec18]
./a.out[0x4008f5]
./a.out[0x400829]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0)[0x7f071734b830]
./a.out[0x4006d9]
======= Memory map: ========
00400000-00401000 r-xp 00000000 08:01 2506641 /home/x/cpp/x01/a.out
00601000-00602000 r--p 00001000 08:01 2506641 /home/x/cpp/x01/a.out
00602000-00603000 rw-p 00002000 08:01 2506641 /home/x/cpp/x01/a.out
022f6000-02328000 rw-p 00000000 00:00 0 [heap]
7f0717022000-7f071712a000 r-xp 00000000 08:01 2102313 /lib/x86_64-linux-gnu/libm-2.23.so
7f071712a000-7f0717329000 ---p 00108000 08:01 2102313 /lib/x86_64-linux-gnu/libm-2.23.so
7f0717329000-7f071732a000 r--p 00107000 08:01 2102313 /lib/x86_64-linux-gnu/libm-2.23.so
7f071732a000-7f071732b000 rw-p 00108000 08:01 2102313 /lib/x86_64-linux-gnu/libm-2.23.so
7f071732b000-7f07174eb000 r-xp 00000000 08:01 2102243 /lib/x86_64-linux-gnu/libc-2.23.so
7f07174eb000-7f07176ea000 ---p 001c0000 08:01 2102243 /lib/x86_64-linux-gnu/libc-2.23.so
7f07176ea000-7f07176ee000 r--p 001bf000 08:01 2102243 /lib/x86_64-linux-gnu/libc-2.23.so
7f07176ee000-7f07176f0000 rw-p 001c3000 08:01 2102243 /lib/x86_64-linux-gnu/libc-2.23.so
7f07176f0000-7f07176f4000 rw-p 00000000 00:00 0
7f07176f4000-7f071770a000 r-xp 00000000 08:01 2102281 /lib/x86_64-linux-gnu/libgcc_s.so.1
7f071770a000-7f0717909000 ---p 00016000 08:01 2102281 /lib/x86_64-linux-gnu/libgcc_s.so.1
7f0717909000-7f071790a000 rw-p 00015000 08:01 2102281 /lib/x86_64-linux-gnu/libgcc_s.so.1
7f071790a000-7f0717a7c000 r-xp 00000000 08:01 1312401 /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f0717a7c000-7f0717c7c000 ---p 00172000 08:01 1312401 /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f0717c7c000-7f0717c86000 r--p 00172000 08:01 1312401 /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f0717c86000-7f0717c88000 rw-p 0017c000 08:01 1312401 /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f0717c88000-7f0717c8c000 rw-p 00000000 00:00 0
7f0717c8c000-7f0717cb2000 r-xp 00000000 08:01 2102215 /lib/x86_64-linux-gnu/ld-2.23.so
7f0717e8e000-7f0717e93000 rw-p 00000000 00:00 0
7f0717eae000-7f0717eb1000 rw-p 00000000 00:00 0
7f0717eb1000-7f0717eb2000 r--p 00025000 08:01 2102215 /lib/x86_64-linux-gnu/ld-2.23.so
7f0717eb2000-7f0717eb3000 rw-p 00026000 08:01 2102215 /lib/x86_64-linux-gnu/ld-2.23.so
7f0717eb3000-7f0717eb4000 rw-p 00000000 00:00 0
7ffec8e19000-7ffec8e3a000 rw-p 00000000 00:00 0 [stack]
7ffec8edb000-7ffec8edd000 r--p 00000000 00:00 0 [vvar]
7ffec8edd000-7ffec8edf000 r-xp 00000000 00:00 0 [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0 [vsyscall]
Terminated
I tried to gdb it, and the 3rd time of delete crashes. Is there anything wrong in my program?
Thanks.

Answer 1

The head member of stack is never initialized, so your program has undefined behaviour, which just happens to manifest by the pointer being non-null and some garbage memory being deleted.
Fix this by initialising head in the constructor for stack:
stack() : head(nullptr){}