[Rd] longint

[Hervé Pagès]

No segfault but a BIG warning from the compiler. That's because 
dereferencing the pointer inside your myfunc() function will
produce an int that is not predictable i.e. it is system-dependent.
Its value will depend on sizeof(long int) (which is not
guaranteed to be 8) and on the endianness of the system.

Also if the pointer you pass in the call to the function is
an array of long ints, then pointer arithmetic inside your myfunc()
won't necessarily take you to the array element that you'd expect.

Note that there are very specific situations where you can actually
do this kind of things e.g. in the context of writing a callback
function to pass to qsort(). See 'man 3 qsort' if you are on a Unix
system. In that case pointers to void and explicit casts should
be used. If done properly, this is portable code and the compiler won't
issue warnings.

H.


On 08/15/2018 07:05 AM, Brian Ripley wrote:
> 
> 
>> On 15 Aug 2018, at 12:48, Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
>>
>>> On 15/08/2018 7:08 AM, Benjamin Tyner wrote:
>>> Hi
>>> In my R package, imagine I have a C function defined:
>>>      void myfunc(int *x) {
>>>         // some code
>>>      }
>>> but when I call it, I pass it a pointer to a longint instead of a
>>> pointer to an int. Could this practice potentially result in a segfault?
>>
>> I don't think the passing would cause a segfault, but "some code" might be expecting a positive number, and due to the type error you could pass in a positive longint and have it interpreted as a negative int.
> 
> Are you thinking only of a little-endian system?  A 32-bit lookup of a pointer to a 64-bit area could read the wrong half and get a completely different value.
> 
>>
>> Duncan Murdoch
>>
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-- 
Hervé Pagès

Program in Computational Biology
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