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Re: Undefined behavior or not?

From: Segher Boessenkool <segher at kernel dot crashing dot org>
To: Liu Hao <lh_mouse at 126 dot com>
Cc: Bernd Edlinger <bernd dot edlinger at hotmail dot de>, "gcc-help at gcc dot gnu dot org" <gcc-help at gcc dot gnu dot org>
Date: Fri, 10 Mar 2017 10:12:38 -0600
Subject: Re: Undefined behavior or not?
Authentication-results: sourceware.org; auth=none
References: <AM4PR0701MB216210B1013279C3CF4273A7E4200@AM4PR0701MB2162.eurprd07.prod.outlook.com> <e83e8e5d-f082-d9bd-3d44-b746f98c0036@126.com>

On Fri, Mar 10, 2017 at 11:57:28PM +0800, Liu Hao wrote:
> >So does this mean, that the above C code uses undefined behavior?
> Yes.
> 
> 6.2.4 Storage durations of objects
> 1 ... Allocated storage is described in 7.22.3.
> 2 ... The value of a pointer becomes indeterminate when the object it 
> points to (or just past) reaches the end of its lifetime.
> 
> 7.22.3 Memory management functions
> 1 ... The lifetime of an allocated object extends from the allocation 
> until the deallocation. ...

But a null pointer does not point to an (allocated) object.  So the
code has undefined behaviour only if ever the pointer is not a null
pointer?

What a mess :-)


Segher

Follow-Ups:
- Re: Undefined behavior or not?
  - From: Martin Sebor

References:
- Undefined behavior or not?
  - From: Bernd Edlinger
- Re: Undefined behavior or not?
  - From: Liu Hao

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