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POboost::weak_ptr<T>.lock() Crashes with a SIGSEGV Segmentation Fault
 

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  1. POboost::weak_ptr<T>.lock() Crashes with a SIGSEGV Segmentation Fault
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    <p>(EDIT) Environment:</p> <pre><code>plee@sos-build:/usr/local/include/boost$ lsb_release -a No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 11.10 Release: 11.10 Codename: oneiric plee@sos-build:/usr/local/include/boost$ uname -a Linux sos-build 3.0.0-12-generic #20-Ubuntu SMP Fri Oct 7 14:56:25 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux plee@sos-build:/usr/local/include/boost$ plee@sos-build:/usr/local/include/boost$ cat version.hpp // BOOST_LIB_VERSION must be defined to be the same as BOOST_VERSION #define BOOST_LIB_VERSION "1_47" </code></pre> <p>I have been working on a server-side project. I use boost libraries, such as, <code>boost::asio</code>, <code>boost::shared_ptr</code>, and <code>boost::weak_ptr</code>.</p> <p>The Boost documentation (<a href="http://www.boost.org/doc/libs/1_47_0/libs/smart_ptr/weak_ptr.htm#lock" rel="nofollow">http://www.boost.org/doc/libs/1_47_0/libs/smart_ptr/weak_ptr.htm#lock</a>) says that the <code>weak_ptr&lt;T&gt;.lock</code> never throws:</p> <blockquote> <p>lock</p> <p>shared_ptr lock() const; Returns: expired()? shared_ptr(): shared_ptr(*this).</p> <p>Throws: nothing.</p> </blockquote> <p>However, in my application, it's even crashed:</p> <pre><code>Program received signal SIGSEGV, Segmentation fault. [Switching to Thread 0x7fffeffff700 (LWP 5102)] 0x000000000066fe08 in boost::detail::atomic_conditional_increment (pw=0x800000000007) at /usr/local/include/boost/smart_ptr/detail/sp_counted_base_gcc_x86.hpp:92 92 ); (gdb) (gdb) bt #0 0x000000000066fe08 in boost::detail::atomic_conditional_increment (pw=0x800000000007) at /usr/local/include/boost/smart_ptr/detail/sp_counted_base_gcc_x86.hpp:92 #1 0x000000000066fe5c in boost::detail::sp_counted_base::add_ref_lock (this=0x7fffffffffff) at /usr/local/include/boost/smart_ptr/detail/sp_counted_base_gcc_x86.hpp:138 #2 0x000000000068009b in boost::detail::shared_count::shared_count (this=0x7fffefffe658, r=...) at /usr/local/include/boost/smart_ptr/detail/shared_count.hpp:518 #3 0x0000000000691599 in boost::shared_ptr&lt;RtmpConnection&gt;::shared_ptr&lt;RtmpConnection&gt; ( this=0x7fffefffe650, r=...) at /usr/local/include/boost/smart_ptr/shared_ptr.hpp:216 #4 0x000000000068db48 in boost::weak_ptr&lt;RtmpConnection&gt;::lock (this=0x7fffe0e87e68) at /usr/local/include/boost/smart_ptr/weak_ptr.hpp:157 </code></pre> <p>I checked the line crashed in <code>/usr/local/include/boost/smart_ptr/detail/sp_counted_base_gcc_x86.hpp</code></p> <pre><code> 69 inline int atomic_conditional_increment( int * pw ) 70 { 71 // int rv = *pw; 72 // if( rv != 0 ) ++*pw; 73 // return rv; 74 75 int rv, tmp; 76 77 __asm__ 78 ( 79 "movl %0, %%eax\n\t" 80 "0:\n\t" 81 "test %%eax, %%eax\n\t" 82 "je 1f\n\t" 83 "movl %%eax, %2\n\t" 84 "incl %2\n\t" 85 "lock\n\t" 86 "cmpxchgl %2, %0\n\t" 87 "jne 0b\n\t" 88 "1:": 89 "=m"( *pw ), "=&amp;a"( rv ), "=&amp;r"( tmp ): // outputs (%0, %1, %2) 90 "m"( *pw ): // input (%3) 91 "cc" // clobbers 92 ); 93 94 return rv; 95 } </code></pre> <p>The line 92 is assembly code. I really don't know what that means.</p> <p>I always do the check if the returned <code>boost::weakptr&lt;RtmpConnection&gt;.lock()</code> (type of <code>boost::shared_ptr&lt;RtmpConnection&gt;</code> is empty before I used it.</p> <p>So I googled, I saw this <a href="http://wiki.inkscape.org/wiki/index.php/Boost_shared_pointers" rel="nofollow">http://wiki.inkscape.org/wiki/index.php/Boost_shared_pointers</a></p> <blockquote> <p>Weak pointers cannot be dereferenced for thread safety reasons. If some other thread destroyed the object after you checked the weak pointer for expiry but before you used it, you would get a crash</p> </blockquote> <ol> <li>So what should I do to deal with it, why does it crash (it seems <code>boost::weakptr&lt;RtmpConnection&gt;.lock()</code> should never crash)?</li> <li>Since my program is multithreaded. It's possible that after I get and check the returned value of <code>boost::weakptr&lt;RtmpConnection&gt;.lock()</code>, the <code>RtmpConnection</code> might be destroyed by other thread, does the Boost Library guarantee that it won't be destroyed, because the return type is <code>boost::shared_ptr&lt;RtmpConnection&gt;</code>?</li> </ol>
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    1. USPeter Lee
    1. USPeter Lee
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    1. COMost likely, the weak pointer itself was deallocated, the weak pointer was corrupted by having its memory overwritten, the object was deleted even though a strong pointer still pointed to it, or you have more than one chain of smart pointers that refer to the same object. (It's also possible that Boost was compiled incorrectly. For example, your platform may have options that need to be enabled to compile thread-safe code.)
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      1. POboost::weak_ptr<T>.lock() Crashes with a SIGSEGV Segmentation Fault
      1. USDavid Schwartz
    2. CO@David, but the `lock()` method itself should NOT crash, right? If it's already destroyed, it should return an empty `shared_ptr<>`, right? BTW, what's the option to be enabled to compile thread-safe code in Boost?
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      1. POboost::weak_ptr<T>.lock() Crashes with a SIGSEGV Segmentation Fault
      1. USPeter Lee
    3. COThe lock method will crash if the object was *improperly* destroyed. For example, if it was destroyed while a strong pointer still referenced it or if it was referenced through more than one chain of smart pointers. There is no option to compile thread-safe code in Boost -- that would be an option in the compiler for whatever platform you are using.
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      1. POboost::weak_ptr<T>.lock() Crashes with a SIGSEGV Segmentation Fault
      1. USDavid Schwartz
 

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