avcall, callback, trampoline, vacall, etc

[gnuist007]

Dear Programming Gurus,

These concepts are rather well talked about .. over the internet and
usenet, however, I cant find any clear explanation.

avcall, callback, trampoline, vacall, etc

I looked in a book of C programming, ie k&r and also the rationale for
C, an old copy and could not find any explanation.

I know that javascript books talk a lot about callbacks and there is
the style of programming involving writing callbacks and registering
them. Unfortunately, in the age when I took my C class, there was no
discussion along these lines.

Can anyone give clear definition, example, benefits, variations,
origin or citation to a paper where it originated and explained first,
as well as some modern text that does the better than most job of
illustrating it and its applications.

 

[James Kuyper]

Dear Programming Gurus,

These concepts are rather well talked about .. over the internet and
usenet, however, I cant find any clear explanation.

avcall, callback, trampoline, vacall, etc

I looked in a book of C programming, ie k&r and also the rationale for
C, an old copy and could not find any explanation.

I know that javascript books talk a lot about callbacks and there is
the style of programming involving writing callbacks and registering
them. Unfortunately, in the age when I took my C class, there was no
discussion along these lines.

Can anyone give clear definition, example, benefits, variations,
origin or citation to a paper where it originated and explained first,
as well as some modern text that does the better than most job of
illustrating it and its applications.

When a C function takes as an argument a pointer to another function,
the function pointed at is often referred to as a callback function.
Examples from the C standard library include qsort() and bsearch(),
where the callback function is called in order to determine the relative
order of the records processed by those functions.

I know nothing about vacall or avcall. I've heard of trampolines in the
context of computer programming, but know little about them.

For all questions like this, I would recommend starting with wikipedia -
it generally does a good job of explaining computer-oriented technical
terms like these; if you don't find it there, try google.

 

[fmassei]

I know nothing about vacall or avcall. I've heard of trampolines in the
context of computer programming, but know little about them.

On a multi-processor system, one CPU boots, and the others use a "trampoline" code (usually prepared by the first one) to boot.

Ciao!

 

[James Kuyper]

On a multi-processor system, one CPU boots, and the others use a "trampoline" code (usually prepared by the first one) to boot.

That's one type of trampoline described at
<http://en.wikipedia.org/wiki/Trampoline_(computing)>

Most of the other types of trampolines described there seem more likely
to be relevant to his question that that type.

 

[fmassei]

That's one type of trampoline described at
<http://en.wikipedia.org/wiki/Trampoline_(computing)>
Most of the other types of trampolines described there seem more likely
to be relevant to his question that that type.

Curious! I never heard this term used outside the SMP development. There's always something to learn..

Ciao!

 

[Cortez]

Dear Programming Gurus,

These concepts are rather well talked about .. over the internet and
usenet, however, I cant find any clear explanation.

avcall, callback, trampoline, vacall, etc

I looked in a book of C programming, ie k&r and also the rationale for
C, an old copy and could not find any explanation.

I know that javascript books talk a lot about callbacks and there is
the style of programming involving writing callbacks and registering
them. Unfortunately, in the age when I took my C class, there was no
discussion along these lines.

Can anyone give clear definition, example, benefits, variations,
origin or citation to a paper where it originated and explained first,
as well as some modern text that does the better than most job of
illustrating it and its applications.

There's a trampoline function in Clojure, but I really don't know much
about it, or whether it's even the same thing as you're talking about.
This article seems to suggest it is:

http://pramode.net/clojure/2010/05/08/clojure-trampoline/

 

[Gene Wirchenko]

^^^1^^ ^^^^2^^^ ^^^^^3^^^^ ^^^4^^

GIYF!

1) Google for "avcall". For me, it was the first link.

2) Google for "callback". For me, it was the first link.

3) Google for "programming trampoline". For me, it was the first
link.

4) This one is a bit tougher. Noting that avcall got me a .h, I
Googled for "vacall.h". For me, the fourth link is for the vacall(3)
Linux man page.

[snip]

Did you even bother looking first?

I had only heard of two of them and was able to find all four
fairly easily.

When a C function takes as an argument a pointer to another function,
the function pointed at is often referred to as a callback function.
Examples from the C standard library include qsort() and bsearch(),
where the callback function is called in order to determine the relative
order of the records processed by those functions.

I know nothing about vacall or avcall. I've heard of trampolines in the
context of computer programming, but know little about them.

For all questions like this, I would recommend starting with wikipedia -
it generally does a good job of explaining computer-oriented technical
terms like these; if you don't find it there, try google.

Agreed about Wikipedia's reliability in this area, but I prefer
to use Google first. One can find out about more groups that way and
even lanaguage standards. If Google returns a Wikipedia link, I will
usually have a look.

Sincerely,

Gene Wirchenko

 

[Joe Pfeiffer]

Curious! I never heard this term used outside the SMP development. There's always something to learn..

Ironically, SMP development is a use I hadn't come across!

 

[Joe Pfeiffer]

Dear Programming Gurus,

These concepts are rather well talked about .. over the internet and
usenet, however, I cant find any clear explanation.

avcall, callback, trampoline, vacall, etc

Speaking only for myself, I found the idea of a 'callback' one that was
really easy to describe, but really hard to understand for a long time.
I see some other posters have already pointed you at google on this one,
but I'll give it a shot anyway.

The big thing about callbacks is that everything you know about writing
a program is turned completely on its head. It sounds like you may have
learned C when I did (or at any rate before these newfangles concepts
came about). If so, you're used to the idea that your main() is
basically in control, and calls functions you write, more or less one at
a time in an order you specify. So first you call a function (or some
functions) that read your input; then you call a function that gets some
work done; then you call a function that writes your output. It's
likely your code isn't that clean in real life; you probably actually
read a little bit of input, do a little bit of work, write a little bit
of output, lather rinse repeat. Doesn't matter, you're still in control
of when your functions get called.

For callback-based programming, forget all that. Flush it.

There are areas where a completely different paradigm is used. A good example
of one of these areas is writing a GUI interface. The big thing about a
GUI interface is that you don't have control of the order in which input
arrives, and you need to do something about whatever the crazy user
does. In this world, somebody else writes something called an "event
loop". The event loop just sits out there waiting for input (maybe a
button click, maybe information that you need to redraw your window,
maybe a timeout, maybe something else), and when the input happens the
event loop code calls a function you wrote to handle that particular
input. Your function is called a 'callback'.

So that's the idea. You write a function to handle the user clicking
the 'OK' button. You write a function to handle a window resize event.
You write a function to... whatever. You pass a pointer to your
function to the event loop code (this is called 'registering' your
callback), and just sit back and wait for your code to be called.

Another application of callbacks is in FUSE (Filesystem in User SpacE)
filesystems. I will somewhat immodestly suggest that a tutorial on FUSE
that you can find at http://www.cs.nmsu.edu/~pfeiffer/fuse-tutorial/ may
help you.

 

[Les Cargill]

Dear Programming Gurus,

These concepts are rather well talked about .. over the internet and
usenet, however, I cant find any clear explanation.

avcall, callback, trampoline, vacall, etc

I looked in a book of C programming, ie k&r and also the rationale for
C, an old copy and could not find any explanation.

I know that javascript books talk a lot about callbacks and there is
the style of programming involving writing callbacks and registering
them. Unfortunately, in the age when I took my C class, there was no
discussion along these lines.

Can anyone give clear definition, example, benefits, variations,
origin or citation to a paper where it originated and explained first,
as well as some modern text that does the better than most job of
illustrating it and its applications.

"Callbacks" are a method of storing pointers to functions
and exploiting them.

I'm answering from comp.lang.c so one thing I would start with is to
learn the concept of an ioctl(). It's one of the oldest ways to
do this sort of thing. Just google for "device driver programming in
linux" and follow some of the simpler examples. ioctl() is a pattern
that occurs quite frequently, in and out of kernel/driver
development.

After that, just do plain old callbacks in 'C' - declare...

//a typedef

typedef void (*cbtype)(char *);

// an array of them

cbtype cbtab[3];

// an initializer

void set_cb(cbtype a,int idx)
{
....
}

// and reference the callbacks

void thing(char *s)
{
....
}

....

set_cb(thing,2);
cbtab[2]("Calling Elvis");

 

[gnuist007]

Speaking only for myself, I found the idea of a 'callback' one that was
really easy to describe, but really hard to understand for a long time.
I see some other posters have already pointed you at google on this one,
but I'll give it a shot anyway.

The big thing about callbacks is that everything you know about writing
a program is turned completely on its head.  It sounds like you may have
learned C when I did (or at any rate before these newfangles concepts
came about).  If so, you're used to the idea that your main() is
basically in control, and calls functions you write, more or less one at
a time in an order you specify.  So first you call a function (or some
functions) that read your input; then you call a function that gets some
work done; then you call a function that writes your output.  It's
likely your code isn't that clean in real life; you probably actually
read a little bit of input, do a little bit of work, write a little bit
of output, lather rinse repeat.  Doesn't matter, you're still in control
of when your functions get called.

For callback-based programming, forget all that.  Flush it.

There are areas where a completely different paradigm is used.  A good example
of one of these areas is writing a GUI interface.  The big thing about a
GUI interface is that you don't have control of the order in which input
arrives, and you need to do something about whatever the crazy user
does.  In this world, somebody else writes something called an "event
loop".  The event loop just sits out there waiting for input (maybe a
button click, maybe information that you need to redraw your window,
maybe a timeout, maybe something else), and when the input happens the
event loop code calls a function you wrote to handle that particular
input.  Your function is called a 'callback'.

So that's the idea.  You write a function to handle the user clicking
the 'OK' button.  You write a function to handle a window resize event.
You write a function to...  whatever.  You pass a pointer to your
function to the event loop code (this is called 'registering' your
callback), and just sit back and wait for your code to be called.

I think, the above paras were what I was wanting as a reply although
all replies were useful. What I want to consolidate is really some
good and illustrative toy examples. Can you cook any or have a link to
some that you have whetted are good?

Another application of callbacks is in FUSE (Filesystem in User SpacE)
filesystems.  I will somewhat immodestly suggest that a tutorial on FUSE
that you can find athttp://www.cs.nmsu.edu/~pfeiffer/fuse-tutorial/may
help you.

I come to this after you give some examples of the first idea.

 

[gnuist007]

"Callbacks" are a method of storing pointers to functions
and exploiting them.

I'm answering from comp.lang.c so one thing I would start with is to
learn the concept of an ioctl(). It's one of the oldest ways to
do this sort of thing. Just google for "device driver programming in
linux" and follow some of the simpler examples. ioctl() is a pattern
that occurs quite frequently, in and out of kernel/driver
development.

For starters, I always start with google and wikipedia. I could not
find any examples.

I am asking here so that you guys can give me whetted and good
examples for a newbie.

there are books on defice driver programming. I need the part that you
can make into a small paper. Give me a reference to read that you have
read yourself and appreciate, maybe a citeseer link to paper etc.

After that, just do plain old callbacks in 'C' - declare...

//a typedef

typedef void (*cbtype)(char *);

// an array of them

cbtype cbtab[3];

// an initializer

void set_cb(cbtype a,int idx)
{
...

}

// and reference the callbacks

void thing(char *s)
{
...

}

...

   set_cb(thing,2);
   cbtab[2]("Calling Elvis");

 

[Les Cargill]

For starters, I always start with google and wikipedia. I could not
find any examples.

I am asking here so that you guys can give me whetted and good
examples for a newbie.

Yes, you made that clear . There is a superset of the callback()
concept embedded in developing device drivers. That's where Google
is helpful.

They offer trivial "device drivers" so you can learn the toolset.
The point is that device drivers are one of the most venerable
ways to do callbacks.

there are books on defice driver programming. I need the part that you
can make into a small paper. Give me a reference to read that you have
read yourself and appreciate, maybe a citeseer link to paper etc.

So fill in the code I posted directly. It's a simple enough
example template. Try it, and ask questions if you get stuck.

It's not necessarily *worth* a paper; it's a couple dozen lines
of 'C'...

After that, just do plain old callbacks in 'C' - declare...

//a typedef

typedef void (*cbtype)(char *);

// an array of them

cbtype cbtab[3];

// an initializer

void set_cb(cbtype a,int idx)
{
...

}

// and reference the callbacks

void thing(char *s)
{
...

}

...

set_cb(thing,2);
cbtab[2]("Calling Elvis");

 

[James Kuyper]

On 10/26/2012 01:16 PM, (e-mail address removed) wrote:
....

For starters, I always start with google and wikipedia. I could not
find any examples.

I am asking here so that you guys can give me whetted and good
examples for a newbie.

I pointed out that qsort() and bsearch() are good simple examples of how
callbacks are used, and I recommended searching
Wikipedia.<http://en.wikipedia.org/wiki/Callback_(computer_programming)>
has two completely filled out toy examples of code using callback
functions. Joe Pfeiffer pointed you at a tutorial for FUSE, which is a
more complicated system making use of callbacks. Les Cargill gave you a
basic template for implementing callbacks in C.

Callback are a relatively simple concept. Everything that makes a
callback be a callback is present in both Wikipedia's examples and in
Les Cargill's template. If there's something more that you need, that
wasn't provided by those examples, then you're looking for something
more than just a callback. Perhaps you could explain in more detail what
you're looking for that was missing from these examples?

 

[Öö Tiib]

In C++, callbacks have been mostly replaced by functors and, more
recently, lambdas.

Callbacks and virtual functions can not carry any state so you have usually to
provide some "context pointer" or "interface pointer" together with those.

Functors and lambdas have usually both function and state embedded into them.
So those are (or can be) more advanced beast than callback.

 

[fmassei]

Callbacks and virtual functions can not carry any state so you have usually to
provide some "context pointer" or "interface pointer" together with those..

Functors and lambdas have usually both function and state embedded into them.
So those are (or can be) more advanced beast than callback.

Maybe I'm wrong (or just pedant) but aren't "callbacks" and "functors/lambdas" concepts belonging to different levels? ;-)
For example, I could write a lambda function where a callback is needed (this is actually pretty common in C#/C mixtures).
Ciao!

 

[Jorgen Grahn]

[Trimmed the newsgroups a bit ... probably not enough]

Dear Programming Gurus,

These concepts are rather well talked about .. over the internet and
usenet, however, I cant find any clear explanation.

avcall, callback, trampoline, vacall, etc

Speaking only for myself, I found the idea of a 'callback' one that was
really easy to describe, but really hard to understand for a long time.
I see some other posters have already pointed you at google on this one,
but I'll give it a shot anyway.
[snip]

For callback-based programming, forget all that. Flush it.

I think you're exaggerating a bit here. But sure, callbacks are a bit
funny, and worth thinking about before trying to use them.

There are areas where a completely different paradigm is used. A good example
of one of these areas is writing a GUI interface. The big thing about a
GUI interface is that you don't have control of the order in which input
arrives, and you need to do something about whatever the crazy user
does. In this world, somebody else writes something called an "event
loop".

It seems to be very popular to design GUI frameworks that way, but
there is nothing that /forces/ them to do it. The fact that a GUI
application is (at leasr partly) driven by user input doesn't mean you
have to resort to callbacks.

Of course if a certain GUI framework forces you to use callbacks, you
have to do it.

[snip]

/Jorgen

 

[Joe Pfeiffer]

[Trimmed the newsgroups a bit ... probably not enough]

Dear Programming Gurus,

These concepts are rather well talked about .. over the internet and
usenet, however, I cant find any clear explanation.

avcall, callback, trampoline, vacall, etc

Speaking only for myself, I found the idea of a 'callback' one that was
really easy to describe, but really hard to understand for a long time.
I see some other posters have already pointed you at google on this one,
but I'll give it a shot anyway.
[snip]

For callback-based programming, forget all that. Flush it.

I think you're exaggerating a bit here. But sure, callbacks are a bit
funny, and worth thinking about before trying to use them.

I don't think I'm exaggerating at all. This is one of those areas where
your previous intuitions about how a program fits together will do more
harm than good, and a necessary first step is to forget your previous
intuitions.

It seems to be very popular to design GUI frameworks that way, but
there is nothing that /forces/ them to do it. The fact that a GUI
application is (at leasr partly) driven by user input doesn't mean you
have to resort to callbacks.

Of course if a certain GUI framework forces you to use callbacks, you
have to do it.

And since the context of my post was somebody asking for help
understanding callbacks, that is the class of GUI program I described.

 

[Pascal J. Bourguignon]

For callback-based programming, forget all that. Flush it.

There are areas where a completely different paradigm is used. A good example
of one of these areas is writing a GUI interface. The big thing about a
GUI interface is that you don't have control of the order in which input
arrives, and you need to do something about whatever the crazy user
does. In this world, somebody else writes something called an "event
loop". The event loop just sits out there waiting for input (maybe a
button click, maybe information that you need to redraw your window,
maybe a timeout, maybe something else), and when the input happens the
event loop code calls a function you wrote to handle that particular
input. Your function is called a 'callback'.

Yes, but in general, there is still this direct and simple flow of
control: a function calls another function, and so on.

A callback is just a function that is given to some module, to be called
when the module needs to call it. This is a way to parameterize or
customize that module.

(defun my-callback (data)
(format t "Received ~A~%" data))

(defun receive-loop (data-callback)
(loop
:for line = (progn (format *query-io* "> ")
(finish-output *query-io*)
(read-line *query-io* nil nil))
:while (and line (plusp (length line)))
:do (funcall data-callback line)))

cl-user> (receive-loop 'my-callback)

Hello Received Hello
World Received World

nil
cl-user>

And then you can use the receive-loop module with a different callback,
making it do something else:


cl-user> (let ((c 0))
(receive-loop (lambda (data) (incf c (parse-integer data))))
c)

42
33
101

176
cl-user>


Now, there's another situation where you really don't know WHEN the
callback is called, it's when the module calling it uses threads, and
calls your callback function at "random" times, possibly in another
thread, or perhaps while handling a unix signal. In this situation,
there are a lot of technicalities to fulfill for calbacks.


But the in usual case, there's no complexity involved.

 

[Jorgen Grahn]

[Trimmed the newsgroups a bit ... probably not enough]

(e-mail address removed) writes:

Dear Programming Gurus,

These concepts are rather well talked about .. over the internet and
usenet, however, I cant find any clear explanation.

avcall, callback, trampoline, vacall, etc

Speaking only for myself, I found the idea of a 'callback' one that was
really easy to describe, but really hard to understand for a long time.
I see some other posters have already pointed you at google on this one,
but I'll give it a shot anyway.
[snip]

For callback-based programming, forget all that. Flush it.

I think you're exaggerating a bit here. But sure, callbacks are a bit
funny, and worth thinking about before trying to use them.

I don't think I'm exaggerating at all. This is one of those areas where
your previous intuitions about how a program fits together will do more
harm than good, and a necessary first step is to forget your previous
intuitions.

Then we disagree slightly. And that's fine -- the OP has seen two
opinions, and can make up his own mind.

Perhaps I'm overly sensitive to "forget everything you think you
know!" messages.

/Jorgen