[FFmpeg-user] Filter documentation -- PTSs

[Chris Angelico]

On Tue, Feb 16, 2021 at 12:45 PM Mark Filipak (ffmpeg)
<markfilipak at bog.us> wrote:
>
> On 02/15/2021 06:47 PM, Chris Angelico wrote:
> > On Tue, Feb 16, 2021 at 8:44 AM Mark Filipak (ffmpeg)
> > <markfilipak at bog.us> wrote:
> >>
> >> On 02/15/2021 04:32 PM, Carl Zwanzig wrote:
> >>> On 2/15/2021 1:26 PM, Mark Filipak (ffmpeg) wrote:
> >>>> Yes, I'm aware of C++. It's sort of a 'C' language architecture hack, isn't it?
> >>> More like butchery ...
> >>
> >> Ooooo... that's harsh.  :-)
> >>
> >>> Glad it's making more sense now.
> >>>
> >>> z!
> >>
> >> Thanks for the confirmation. Now, regarding pointers-to-pointers and '*' as a unary operator.....
> >>
> >
> > A pointer, at its core, is a number representing a place in memory. A
> > pointer to a pointer is a number representing the place in memory
> > where you can find another number that'll tell you where you want to
> > go. You want to know where the Sword Master can be found, but I won't
> > tell you where he is; all I'll tell you is that the shopkeeper knows
> > where to find the Sword Master. I point you to the shopkeeper, and
> > then you ask him to point you to the Sword Master.
> >
> > The asterisk, as a unary operator, is one step in that process.
> > Something like this:
> >
> > int **trial = find_sword_master();
>
> Oops, I'm getting lost in the story of the Sword Master. You're describing indirection. I have no
> problem with indirection. The problem I have is simply with syntax interpretation because to me,
> every variable (or constant) in every language is a location in memory, so when you write that a
> number represents a place in memory, to me it's undistinguished, it's not any different than any
> other variable (in any location in memory).
>
> Does '*' (a unary operator) that precedes a variable's name indicate that the named variable is a
> pointer? Or do I have that bit wrong?

Yes, it does. For "*trial" to be meaningful, "trial" has to be a pointer.

> Is '**' also a unary operator? Or is it 2 unary operators, '*'+'*'? I suspect that '**' is the
> infamous pointer-to-pointer (i.e. doubly indirect), or do I have that bit wrong?

It's two separate unary operators, which is why I wrote it in one
place as *(*trial) - has the same meaning. Yes, "**trial" implies that
trial is a pointer to a pointer.

> > "trial" is a number that says where in memory the shopkeeper is.
> > What's that number?
> >
> > printf("Shopkeeper is at %p\n", trial);
>
> I have no idea what '%p' means or what you're trying to demonstrate by the code. I don't know why
> the printf has a 2nd arg: 'trial'. '%' is modulo, but modulo is a binary operator, not a unary
> operator. That it's in a string just adds to the mystery.

Ah, printf is doing formatted output, so it interpolates a value.
Sorry, if you're not familiar with that, just ignore it.

> > "*trial" is the number that can be found there. What's that number?
> >
> > printf("Sword Master is at %p\n", *trial);
>
> Okay, now I'm lost. is 'trial' a variable or a pointer?

Both. In the same way that "int x = 5;" has x as a variable and an integer.

> Specifically, is '*trial' a pointer that's
> labeled "trial" or is it a pointer to a variable that's labeled "trial"?

It's a pointer that's labelled "trial". The name refers to the
variable that has the first pointer - or if you prefer, it refers to
the variable that has the address of the second step.

> Or are both labeled
> "trial"? Or does the string "*trial" just represent an entity that's just a relative memory address
> that neither the compiler nor the runtime system normally discloses? If your 'printf' functions
> print, what do they print? ...Yes, I know that 'print' doesn't mean print on a printer. The only
> thing I can guess is that "%p" is some sort of token, but I don't know its meaning.

Don't worry about the printf stuff, that's just a quick way to print
out the numbers involved.

> The rest of 'C' syntax, and precedent, and operators, etc. I know. It's just pointers that mystify
> me. Now I understand what a struct actually is but pointers?... not so much. I thought I understood,
> but '**' fuddled it. I'm afraid I'm one of the people who, for example, actually prefer BNF to
> textual descriptions.
>

A pointer, ultimately, is just a number - the address of the next
thing. So the pointer-to-pointer is saying "at this address, you can
find the address you want".

If this isn't helpful, feel free to ignore it; I'm sure there's BNF
around somewhere to explain it, but I'm not sure I could write it that
way, sorry.

ChrisA