On 2019-09-11 20:59, Brian Inglis wrote:
On 2019-09-09 11:13, Petr Skočík wrote:
There's been a twitter discussion on how different POSIX platforms
handle malloc(0):
https://twitter.com/sortiecat/status/1170697927804817412 .
As for Cygwin, the answer appears to be "not well", but this should be
easy to fix.
POSIX SUS V4 2018 says:
"RETURN VALUE
Upon successful completion with size not equal to 0, malloc() shall
return a
pointer to the allocated space. If size is 0, either:
A null pointer shall be returned [CX] [Option Start] and errno may be
set to
an implementation-defined value, [Option End] or
A pointer to the allocated space shall be returned. The application
shall
ensure that the pointer is not used to access an object.
Otherwise, it shall return a null pointer [CX] [Option Start] and set
errno to
indicate the error. [Option End]"
The second option could be implemented by a pointer to an unmapped
page, or a
reference to an inaccessible mmap-ed area length zero.
That's easy: the null pointer, plus some small offset that observes
alignment, like 16.
(Alignment is important even if the memory isn't accessed, because
nonportable programs
depend on it for other reasons, like being able to use the least
significant few bits
of a pointer for tagging.)
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