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Re: Error message for a dangling pointer

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  Re: Error message for a dangling pointer Keith Thompson <kst-u@mib.org> - 2017-02-06 18:02 -0800
    Re: Error message for a dangling pointer BartC <bc@freeuk.com> - 2017-02-07 15:07 +0000
      Re: Error message for a dangling pointer David Brown <david.brown@hesbynett.no> - 2017-02-07 16:24 +0100

#103344 — Re: Error message for a dangling pointer

ram@zedat.fu-berlin.de (Stefan Ram) writes:
>   For teaching purposes, I just wrote this program to show
>   dangling pointers:
>
> #include <stdio.h>
> int main( void )
> { int * p;
>   { int i = 27; p = &i; }
>   printf( "%d\n", *p ); }

(A side note: Your students might later have difficulties with more, um,
conventional C code layouts.)

>   I hope you all agree that »p« in the last line /is/
>   a dangling pointer.

Yes.

>   The error message and output is not what I expected,
>   but still interesting:
>
> main.c: In function 'main':
> main.c:6:3: warning: 'i' is used uninitialized in this function [-Wuninitialized]
>    printf( "%d\n", *p ); }
>    ^
> main.c:5:9: note: 'i' was declared here
>    { int i = 27; p = &i; }
>          ^
> 0
>
>   »i« /was/ initialized in the program,
>   so the error message is not accurate.
>
>   (I do not write »misleading«, since it is so
>   obviously wrong that it won't mislead anyone.)

I get that message with gcc 6.3.0, and with a gcc snapshot from
2017-01-12 that claims to be gcc 7.0.0, but not with gcc 6.2.0
(or with clang).

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"

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#103360

On 07/02/2017 02:02, Keith Thompson wrote:
> ram@zedat.fu-berlin.de (Stefan Ram) writes:
>>   For teaching purposes, I just wrote this program to show
>>   dangling pointers:
>>
>> #include <stdio.h>
>> int main( void )
>> { int * p;
>>   { int i = 27; p = &i; }
>>   printf( "%d\n", *p ); }
>
> (A side note: Your students might later have difficulties with more, um,
> conventional C code layouts.)
>
>>   I hope you all agree that »p« in the last line /is/
>>   a dangling pointer.
>
> Yes.
>
>>   The error message and output is not what I expected,
>>   but still interesting:
>>
>> main.c: In function 'main':
>> main.c:6:3: warning: 'i' is used uninitialized in this function [-Wuninitialized]
>>    printf( "%d\n", *p ); }
>>    ^
>> main.c:5:9: note: 'i' was declared here
>>    { int i = 27; p = &i; }
>>          ^
>> 0
>>
>>   »i« /was/ initialized in the program,
>>   so the error message is not accurate.
>>
>>   (I do not write »misleading«, since it is so
>>   obviously wrong that it won't mislead anyone.)
>
> I get that message with gcc 6.3.0, and with a gcc snapshot from
> 2017-01-12 that claims to be gcc 7.0.0, but not with gcc 6.2.0
> (or with clang).

I couldn't get any diagnostic with any compiler. That included gcc 5.1.x 
and 6.2.x with all warnings turned on.

-- 
bartc

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#103365

On 07/02/17 16:07, BartC wrote:
> On 07/02/2017 02:02, Keith Thompson wrote:
>> ram@zedat.fu-berlin.de (Stefan Ram) writes:
>>>   For teaching purposes, I just wrote this program to show
>>>   dangling pointers:
>>>
>>> #include <stdio.h>
>>> int main( void )
>>> { int * p;
>>>   { int i = 27; p = &i; }
>>>   printf( "%d\n", *p ); }
>>
>> (A side note: Your students might later have difficulties with more, um,
>> conventional C code layouts.)
>>
>>>   I hope you all agree that »p« in the last line /is/
>>>   a dangling pointer.
>>
>> Yes.
>>
>>>   The error message and output is not what I expected,
>>>   but still interesting:
>>>
>>> main.c: In function 'main':
>>> main.c:6:3: warning: 'i' is used uninitialized in this function
>>> [-Wuninitialized]
>>>    printf( "%d\n", *p ); }
>>>    ^
>>> main.c:5:9: note: 'i' was declared here
>>>    { int i = 27; p = &i; }
>>>          ^
>>> 0
>>>
>>>   »i« /was/ initialized in the program,
>>>   so the error message is not accurate.
>>>
>>>   (I do not write »misleading«, since it is so
>>>   obviously wrong that it won't mislead anyone.)
>>
>> I get that message with gcc 6.3.0, and with a gcc snapshot from
>> 2017-01-12 that claims to be gcc 7.0.0, but not with gcc 6.2.0
>> (or with clang).
> 
> I couldn't get any diagnostic with any compiler. That included gcc 5.1.x
> and 6.2.x with all warnings turned on.
> 

Testing with <https://gcc.godbolt.org/> I get that for gcc 5 onwards
(including 6.2 and 5.1), but not clang.

However, like many warnings you need to have optimisation enabled so
that gcc will do more analysis of the code.

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