www.digitalmars.com         C & C++   DMDScript  

digitalmars.D.learn - this is null

reply ANtlord <antlord92 gmail.com> writes:
Hello everyone! I've encountered the problem which I already 
encountered before. Unfortunately, I had no time in the previous 
time to report and to talk about it. So I decided to play making 
my own "malloc" function in pure D (betterC) at this time. And I 
encountered the issue one more time. `this` can be null. How? 
Please take a look at my project [0]. It gets the current heap 
break and tries to increase via a free list. So the segfault I 
meet happens there [1]. Tell me, please. What do I wrong?

[0] https://github.com/ANtlord/deadmemory
[1] 
https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/freelist.d#L56
Mar 09 2019
parent reply Paul Backus <snarwin gmail.com> writes:
On Saturday, 9 March 2019 at 19:18:38 UTC, ANtlord wrote:

 Hello everyone! I've encountered the problem which I already 
 encountered before. Unfortunately, I had no time in the 
 previous time to report and to talk about it. So I decided to 
 play making my own "malloc" function in pure D (betterC) at 
 this time. And I encountered the issue one more time. `this` 
 can be null. How? Please take a look at my project [0]. It gets 
 the current heap break and tries to increase via a free list. 
 So the segfault I meet happens there [1]. Tell me, please. What 
 do I wrong?

 [0] https://github.com/ANtlord/deadmemory
 [1] 
 https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/freelist.d#L56


You can end up with a null `this` reference if you dereference a 
null pointer to a struct and then call a method on the result. 
For example:

struct S
{
     bool isThisNull() { return &this is null; }
}

void main()
{
     import.std.stdio;
     S* p = null;
     writeln((*p).isThisNull); // true
}

Interactive version: https://run.dlang.io/is/fgT2rS
Mar 09 2019
next sibling parent reply ANtlord <antlord92 gmail.com> writes:
On Saturday, 9 March 2019 at 20:04:53 UTC, Paul Backus wrote:

 You can end up with a null `this` reference if you dereference 
 a null pointer to a struct and then call a method on the 
 result. For example:


I can but my reference is not null before calling. Take a look at 
the line of code [0]. There is a check before the line.

https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/mem.d#L20 [0]
Mar 09 2019
next sibling parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 03/09/2019 12:10 PM, ANtlord wrote:

 On Saturday, 9 March 2019 at 20:04:53 UTC, Paul Backus wrote:

 You can end up with a null `this` reference if you dereference a null 
 pointer to a struct and then call a method on the result. For example:


 
 I can but my reference is not null before calling. Take a look at the 
 line of code [0]. There is a check before the line.
 
 https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/mem.d#L20 
 [0]


I haven't run the code but which pointer is null? Try adding this check 
as well:

   auto node = this.list.getFisrtFreeOrAdd(memViewLen);
   assert(node !is null);

Ali
Mar 09 2019
parent ANtlord <antlord92 gmail.com> writes:
On Saturday, 9 March 2019 at 21:00:51 UTC, Ali Çehreli wrote:

 I haven't run the code but which pointer is null? Try adding


I mean `this` by "this" word. You can see that `this` is null if 
you run gdb and before that line make `p/x this` [0]

 this check as well:

   auto node = this.list.getFisrtFreeOrAdd(memViewLen);
   assert(node !is null);


I get segfault in `getFisrtFreeOrAdd` method. Before the line I 
have an assertion [1]. It looks like the program misses (I have 
no idea how) `list` object while calling its method 
`getFisrtFreeOrAdd`.

[0] 
https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/freelist.d#L56
[1] 
https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/mem.d#L19
Mar 09 2019
prev sibling parent reply spir <denis.spir gmail.com> writes:
On 09/03/2019 21:10, ANtlord via Digitalmars-d-learn wrote:

 On Saturday, 9 March 2019 at 20:04:53 UTC, Paul Backus wrote:

 You can end up with a null `this` reference if you dereference a null pointer 
 to a struct and then call a method on the result. For example:


 
 I can but my reference is not null before calling. Take a look at the line of 
 code [0]. There is a check before the line.
 
 https://github.com/ANtlord/deadmemory/blob/master/src/deadmemory/mem.d#L20 [0]


There is a typo in this instruction:

T* ptr = this.list.getFisrtFreeOrAdd(memViewLen).getPtr!T();
                         ^^
                         rs
(may this explain your null? the compiler should complain)

diniz
Mar 10 2019
parent ANtlord <antlord92 gmail.com> writes:
On Sunday, 10 March 2019 at 14:25:56 UTC, spir wrote:


 There is a typo in this instruction:

 T* ptr = this.list.getFisrtFreeOrAdd(memViewLen).getPtr!T();
                         ^^
                         rs
 (may this explain your null? the compiler should complain)

 diniz


Good catch! But I have the same typo within the definition of the 
method. I believe DMD screams about undefined method if it 
happens.
Mar 10 2019
prev sibling parent ANtlord <antlord92 gmail.com> writes:
On Saturday, 9 March 2019 at 20:04:53 UTC, Paul Backus wrote:

 struct S
 {
     bool isThisNull() { return &this is null; }
 }

 void main()
 {
     import.std.stdio;
     S* p = null;
     writeln((*p).isThisNull); // true
 }

 Interactive version: https://run.dlang.io/is/fgT2rS


Anyway, thank you! I didn't know about the feature.
Mar 09 2019