Segfault while using Builder

[Sune S]

Hi 

I have a Class that instantiates a MallocMessageBuilder and initializes its root to a custom struct. I notice a segfault when I construct the custom struct's builder in the constructor's initializer list.

  // Construction that leads to a seg fault

  MyMessageBuilder() 

  : m_message(),

    m_myMsgBuilder(m_message.initRoot<MyMessage>())

  {

    // The program seg faults in the following line.

    m_myMsgBuilder.initSomeStruct();

  }

  // Alternate construction does not crash

  MyMessageBuilder() 

  : m_message(),

    m_myMsgBuilder(nullptr)

  {

    m_myMsgBuilder = m_message.initRoot<MyMessage>();

    m_myMsgBuilder.initSomeStruct();

  }

0x00000000004059a0 in capnp::_::DirectWireValue<MyMessage::Which>::set(MyMessage::Which) ()

0x00000000004056fe in void capnp::_::StructBuilder::setDataField<MyMessage::Which>(unsigned int, kj::NoInfer_<MyMessage::Which>::Type) ()

0x000000000040551d in MyMessage::Builder::initSomeStruct() ()

What could be causing the seg fault?

Thanks

Sune

[Kenton Varda]

Hi Sune,

In what order are m_message and m_myMsgBuilder declared in the class? My guess is that m_myMsgBuilder is declared before m_message. As a result, the compiler will re-order the initializations such that m_myMsgBuilder is initialized first. This means your code is calling m_message.initRoot when m_message is not yet constructed, resulting in undefined behavior. (Most compilers will give you a warning about this...)

-Kenton

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[Sune S]

Hi Kenton,

You were right, it was the order of initialization. I should have checked that first.  I have now added the all warnings option to the compiler.

Thank you.

Sune.

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