[Python-Dev] Constifying C API

[Tres Seaver]

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On 12/18/2016 07:54 PM, Nick Coghlan wrote:
> On 18 December 2016 at 18:31, Serhiy Storchaka <storchaka at gmail.com>
> wrote:
> 
>> Later I'm planning following changes:
>> 
>> * Add the const qualifier to the result of functions that return 
>> references to internal representation of immutable objects, like 
>> PyBytes_AS_STRING() or PyUnicode_DATA(). While CPython internally
>> can modify the content of immutable objets, this is very dangerous,
>> because this can invalidates invariants and cached values.
>> Third-party code shouldn't do this.
>> 
>> * Add the const qualifier to the format field of Py_buffer. It is a 
>> reference to C string literal or to the content of bytes object.
>> Mutating its content is an error. Only _testbuffer overuses the
>> format field of internal Py_buffer object for owning a reference to
>> allocated memory. But this is not leaked outside.
>> 
>> What are you think about this?
>> 
> 
> As long as it's on the default branch with appropriate notes in the C 
> porting section of the 3.7 What's New, turning these kinds of runtime 
> errors into compilation errors sounds like the right thing to do to
> me.
> 
> One key aspect from my perspective is that code that is updated to 
> correctly declare the destination storage as a const pointer will
> still compile against the old API variants that return a mutable
> pointer, so any problems this finds in third party code are likely to
> be resolved for older 3.x releases as well.

Agreed.  Anything the compiler ralfs on after adding 'const' (where the
actual target must be immutable) already had the fuse smoldering.  FWIW I
help maintain some *old* C extensions (fifteen+ years and counting), and
am as likely to be affected as anyone.


Tres
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Tres Seaver          +1 540-429-0999          tseaver at palladion.com
Palladion Software   "Excellence by Design"    http://palladion.com
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