Segfault in std::atomic load?

Question

On linux, using gcc 4.8.4, compiled with -std=c++11 -mcx16:

#include <atomic>

struct node_t;

struct pointer_t {
        node_t* ptr;
        unsigned int count;
        pointer_t() noexcept : ptr{nullptr}, count{0} {}
};

struct empty {};

struct node_t {
        empty value;
        std::atomic<pointer_t> next;
        node_t() : next{pointer_t{}} {}
};

int main() {
        node_t{}.next.load();
        return 0;
}

gives a segfault when load is called. How am I meant to initialize an atomic value?

Just as a sidenote, the `_t` suffix is reserved on POSIX systems. — Beta Carotin, Apr 23 '15 at 13:18
I don't see a declaration of `pointer_t::load()` anywhere... — Kevin, Apr 23 '15 at 13:20
@BetaCarotin yeah, I'm just directly translating https://www.cs.rochester.edu/research/synchronization/pseudocode/queues.html :) — Shea Levy, Apr 23 '15 at 13:23
@Kevin I'm calling load on a `std::atomic`, not on a `pointer_t` — Shea Levy, Apr 23 '15 at 13:23

score 9 · Accepted Answer · edited Apr 23 '15 at 14:45

9

Turns out this is a bug in gcc that has since been fixed in GCC 5.1. Specifying the alignment to be two words fixed it.

edited Apr 23 '15 at 14:45

Drew Dormann

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answered Apr 23 '15 at 13:24

Shea Levy

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@Drew Dormann : Is there any patch/solution that could avoid this in gcc 4.8.4 itself? – rahul.deshmukhpatil Jan 11 '16 at 04:03
@rahul.deshmukhpatil Yes, you need to specify the alignment of the pointer to be two words. – Shea Levy Apr 11 '16 at 22:02
I was accidentally changing the base class object address, so it was appearing that atomic data member variable was allocated on the odd address. thanks for the reply. – rahul.deshmukhpatil Apr 12 '16 at 08:42
I am just wondering if this bug has been fixed in default gcc compiler of CentOS 7.3? – Mickey Shine Aug 09 '17 at 07:19
2

How to specify the alignment to be two words, an example? Thanks – Mickey Shine Aug 09 '17 at 09:38
See the alignas keyword – Shea Levy Aug 09 '17 at 11:39

Segfault in std::atomic load?

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