[cfe-dev] createCXString reads one past end byte

[Argyrios Kyrtzidis]

On Jan 22, 2013, at 11:38 AM, Dmitri Gribenko <gribozavr at gmail.com> wrote:

> On Tue, Jan 22, 2013 at 9:29 PM, Argyrios Kyrtzidis <akyrtzi at gmail.com> wrote:
>> On Jan 22, 2013, at 6:49 AM, Dmitri Gribenko <gribozavr at gmail.com> wrote:
>> 
>>> Hello Argyrios,
>>> 
>>> On Mon, Jan 21, 2013 at 9:13 PM, Argyrios Kyrtzidis <akyrtzi at gmail.com> wrote:
>>>> We could change how CXString works; instead of eagerly malloc'ing in case of a StringRef, have it stored in a "StringRef-kind" form and malloc when clang_getCString is called.
>>> 
>>> This will add an extra backing storage mode to CXString, let's call
>>> that CXS_UnmanagedWithLength.  In this mode we will store the pointer
>>> in 'data', and length will be stored in the upper bits of
>>> 'private_flags'.  But who will own the memory returned by
>>> clang_getCString?  We can not change the original CXString from
>>> CXS_UnmanagedWithLength to CXS_Malloc because CXString is a value
>>> type.  Or did I misunderstand something?
>> 
>> You are right, it is so easy to get a copy of it and trigger a memory leak unintentionally.
>> 
>> I'll do some measurements to see if it makes much difference to malloc on every StringRef (in the meantime any suggestions on how to avoid the malloc would be greatly appreciated).
> 
> We can add an indirection layer.  Let's add a mode CXS_Lazy.  'data'
> would point to a struct:
> 
> struct {
>  const char *Str;
>  unsigned Length:31;
>  unsigned IsNulTerminated:1;
> };
> 
> Then CXString would remain a value type, and all copies would point to
> the same underlying data.

I like it! We can also enhance the mechanism a bit to be able to use a free list of those so that we avoid malloc'ing them for the normal use pattern of having a CXString as a temporary object to get at the underlying string, what do you think ?

> 
> Dmitri
> 
> -- 
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