Dynamic array ot not

[H. S. Teoh]

On Sun, Jan 16, 2022 at 08:21:29AM -0800, Ali Çehreli via Digitalmars-d-learn wrote:
> On 1/16/22 07:32, Salih Dincer wrote:
> > On Sunday, 16 January 2022 at 11:43:40 UTC, Ali Çehreli wrote:
> >>
> >> void main() {
> >>   enum count = 7;
> >>
> >>   // Allocate some memory
> >>   void* rawData = malloc(int.sizeof * count);
> 
> In practice, malloc'ed memory is cleared e.g. by memset(). Or, there is
> calloc() which returns memory filled with zeros.

Correction: malloc() is not guaranteed to clear the allocated memory.
Possibly for "performance reasons".  Usually, though, I'd recommend
using calloc() instead to initialize the allocated memory and prevent
surprising results. (E.g., you allocate a buffer expecting it would be
zeroed, but it isn't so the code that assumes it does produces garbled
results. Or worse, if you're allocating memory for, e.g., a packet
buffer to be transmitted across the network, failing to initialize it
may leak the past contents of that memory to the internet. Cf.
HeartBleed.)


[...]
> > If count is not equal to 8 I get weird results! The reason of
> > course, is the free():
> > // [93947717336544, 1, 2, 3, 4, 5, 6]
> 
> I didn't know free wrote into the freed buffer but since it's
> undefined behavior, we shouldn't even be able to know whether it did
> or not. :/
[...]

This is likely caused by the implementation of malloc/free. Some
implementations store pointers and other tracking information inside the
free blocks themselves instead of using a separate data structure to
track free memory (e.g., the start of the block may contain a pointer to
the next available block).  After calling free(), that memory is now a
free block, so the implementation may start storing such information
within the block.

Some memory allocator implementations may also store a canary value at
the beginning of freed blocks to detect double free's (i.e., a value
unlikely to occur in user data that, if detected by free(), may indicate
a bug in the code that's trying to free the same block twice).


T

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