The main difference here is where you are storing the value "20000000".

In the first example, p is located in addresses belonging to "main" memory
space (the stack presumably, beacuse "main" is the first function called
upon start). You are passing the memory address of p (where p is located)
to the function "change_value". The function assumes the memory address you
are passing it's valid (meaning that it can be written safely). If you call
"change_value" with null or with an invalid address (an address outside of
all segments) you will cause a segfault.

In the second example, "change_value" creates a new memory space (in the
heap) to store "20000000". This means you aren't "changing" anything. Every
time you call "change_value" it creates a new """instance""" of "20000000".
Plus, if you don't free p before calling it again ("change_value(&p)"), you
will be trashing memory (memory that cannot be released later, because you
don't know their addresses).


On Mon, Mar 28, 2016 at 3:50 AM, Edward Bartolo <edbarx@???> wrote:

> Hi,
>
> As the title of the email indicates, I am doing some exercises to make
> sense out of C pointer syntax. I have been using pointers for as long
> as I have been programming without issues, apart from the usual
> initial programmatic errors when new code is run for the first time.
> However, C pointer syntax is proving to be as unintuitive as it can
> be. For this reason, I am doing some exercises regarding C pointer
> use.
>
> I am attaching two short C programs that I created and which I tested
> to work although the mechanism by which they work is still somewhat
> hazy to me. Both programs use a function to change the value of a
> parameter. I want to understand, as opposed to knowing by rote, the
> mechanism why they work. Please note that I didn't consult any books
> to create the pointers. This is because I have already the concepts,
> but I cannot make sense, as in deeply understanding the details, of
> pointer syntax as used in C.
>
> Edward
>
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