if (my_pointer==nullptr) causes segmentation fault

Seb Tur

Hi
Basic question but drives me mad for the whole day.

I have a class inicio_common

For this class I declared a pointer and a method in

inicio_common.h :

private:
  parameter *param=nullptr;

public:
 QString parameter_read(QString param_name, bool force_read = false);

I define the function in
inicio_common.cpp

QString inicio_common::parameter_read(QString param_name, bool force_read)
{
    qDebug()<<"param read enter";

    qDebug()<<"param name="+param_name;


    if (this->param==nullptr)
    {
        qDebug()<<"param obj is NULL";
        this->param = new parameter(receiver_app);

    }
    else
    {
        qDebug()<<"param obj ok";

    }
    qDebug()<<"param obj checked";

    QString val=this->param->read(param_name,force_read);
    qDebug()<<"param val="<<val;
    return (val);
}

This function runs fine several times and suddenly on one iteration it throws a segmantation fault on
if (this->param==nullptr)
and never reaches anything after that.

Any hints for a scenario ?

Christian Ehrlicher

@Seb-Tur said in if (my_pointer==nullptr) causes segmentation fault:

Any hints for a scenario ?

Use a debugger, see what's going wrong. I would guess 'this' is either a nullptr or garbage.

JoeCFD

check if this = nullptr. you do not need if (this->param==nullptr) you can simply use if ( param==nullptr )

Seb Tur

with if(param==nullptr) I get the same segfault

I added this-> just in case after being clueless for hours

Christian Ehrlicher

@Seb-Tur said in if (my_pointer==nullptr) causes segmentation fault:

with if(param==nullptr) I get the same segfault

Because it's exactly the same...

JoeCFD

@Seb-Tur run your app with valgrind to check it out if you are working on Linux.

Chris Kawa

This will happen when you call a method on a deleted object. this points to garbage and any access to its members results in seg fault. Make sure your object is still alive when you call this function.

Seb Tur

@Chris-Kawa

I don't understand
I'm calling this check in a method belonging to inicio_common class and param also belongs there.
so when I create an object

inicio_common *common_object;
so then it gets its member from .h
common_object->param =nullptr

so
method common_object->parameter_read()
reffers to common_object->param

Param is a sibling member , so how come it's parent "this" can be garbage? If this was garbage than its method read_paramater wouldnt't be called either?
I don't delete param in any method in inicio_common class. It is a private member.

JoeCFD

@Seb-Tur said in if (my_pointer==nullptr) causes segmentation fault:

inicio_common *common_object;

how did you create this one: inicio_common *common_object{}; ? is it nullptr?

Christian Ehrlicher

@Seb-Tur said in if (my_pointer==nullptr) causes segmentation fault:

If this was garbage than its method read_paramater wouldnt't be called either?

Why not? Who would prevent it?

Seb Tur

@JoeCFD

declared in mainwindow.h as
inicio_common = *common_object =nullptr

than contructed at the begining of mainwindow.cpp

common_object = new inicio_common();

JoeCFD

@Seb-Tur I guess it is better for you to show more code.

Chris Kawa

@Seb-Tur It's not the param member that is deleted. It's the instance of class inicio_common.

One case where this would happen:

inicio_common *common_object;       // common_object is an uninitialized variable
common_object->parameter_read(...); // calling method with garbage "this" -> crash

another way the same happens:

inicio_common *common_object = new inicio_common(); // common_object is initialized, ok
common_object->parameter_read(...);                 // works
delete common_object;                               // common_object now points to garbage
common_object->parameter_read(...);                 // calling method with garbage "this" -> crash

If this was garbage than its method read_paramater wouldnt't be called either?

There's nothing in C++ language preventing that. You can create pointers pointing to random memory and call methods on them. For example:

QString* foo = (QString*)12345; // because why not
foo->clear();                   // garbage "this" -> crash

Seb Tur

Thanks all for the support, your hints directed me to the right path to find an error I had no idea was an error.

Here's the scenario :

I used common_object in mainwindow doing various activities and all was fine;

at some point I was passing the common_object pointer in a constructor to another object of action_class that needed some properties of inicio_common object.,

action_class.h contained

inicio_common *ref_common = nullptr;

but action class cunstructor was capturing the common_object

action_class::action_class(inicio_common *common)
{
  ref_common = common;
}

so in the mainwindow I had common_object used by mainwindow and action_class_object because I run:

action_class *action_class_object = new (common_object);

After I was done with the actions I deleted action_class_object with

delete(action_class_object);

and this seems to have deleted the common_object by deleting ref_common in cascade .... I guess
Does it make sense or is it just my wicked thinking?

Anyway I still don't get why if(param==nullptr) would crash instead of returning true;

Chris Kawa

@Seb-Tur said:

and this seems to have deleted the common_object by deleting ref_common in cascade .... I guess

delete something does not delete "something". It deletes the thing that "something" points to. "something" points to garbage after that. If you copy a pointer you copy a pointer. You're not copying the object it points to, so when you delete the object via one pointer both the original and the copied pointer point to the same garbage memory.

Anyway I still don't get why if(param==nullptr) would crash instead of returning true;

It's not the if that is crashing. It can't return anything because it's not even executed. It doesn't get to the comparison operator. this points to garbage, so this->param is trying to read a value from garbage memory. Reading from garbage memory is access violation and terminates your app.

Christian Ehrlicher

@Chris-Kawa said in if (my_pointer==nullptr) causes segmentation fault:

this points to garbage, so this->param is trying to read a value from garbage memory. Reading from garbage memory is access violation and terminates your app.

Not that I said it in my first post... :)