[LLVMdev] malloc(), free(), and alloca() with zero size

[Jon Sargeant]

Dale Johannesen wrote:
> On Oct 6, 2007, at 9:14 AM, Jon Sargeant wrote:
> 
>> If <NumElements> is zero, what is the behavior of malloc() and  
>> alloca()?
>> Can I call free() using the pointer that malloc() returns?
> 
> alloca is not standard.
> The behavior of malloc is covered in 7.20.3p1:
> 
> If the size of the space requested is zero, the behavior is  
> implementation-defined:
> either a null pointer is returned, or the behavior is as if the size  
> were some
> nonzero value, except that the returned pointer shall not be used to  
> access an object.
> 
> Portable code can't depend on either behavior, obviously.  It is  
> usually simplest to avoid using malloc(0).
> 
>> Also, I'm assuming that free()ing a null pointer is a legal NOP?
> 
> Yes, 7.20.3.2p2.

Ok, just to be clear...
The semantics of the LLVM malloc and free instructions and the 
llvm.memcpy, llvm.memmove, and llvm.memset intrinsics are the same as 
their C counterparts (except for the extra alignment parameter).  The 
LLVM alloca instruction is not standard, so does that mean alloca()ing 
zero elements is undefined?

Regards,
Jon