[openssl-users] ECDSA_SIG_new and ECDSA_SIG_free details

[Erwann Abalea]

ISO/C 2011, clause 6.3.2.3:
----
An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to any object or function.

Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.
----

int *var1 = 0;
int *var2 = (void*)0;

result in var1 and var2 to both be null pointers (the null pointer constant being « 0 » or « (void*)0 »).

This doesn’t matter if your specific machine encodes null pointers as ‘0xffffffff'.

On your specific machine, however:

int *var1;
int *var2 = 0;
memset(var1, 0, sizeof(var1));

won’t make var1 be a null pointer, but var2 will internally contain this 0xffffffff, and will be a null pointer.


Cordialement,
Erwann Abalea

> Le 11 janv. 2017 à 17:18, Jeffrey Walton <noloader at gmail.com> a écrit :
> 
>> Could someone from the OpenSSL team please explain the rationale for this
>> decision? What is the problem with using assignments with 0 or NULL to
>> initialize pointers?
> 
> I'm not from the team, so take it for what its worth...
> 
> On some systems, NULL is _not_ 0. NULL can be anywhere in memory the
> architecture wants it to be. It can be in a high page in memory, too.
> One of my instructors in college was telling me about a system he
> worked on where NULL was an address in the last page in memory, so it
> took a value like `0xffffffff`.
> 
> Jeff
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