How do I create a function in my library for passing user callback function

[Angus]

Hello

I am writing a library which will write data to a user defined callback
function. The function the user of my library will supply is:

int (*callbackfunct ion)(const char*);

In my libary do I create a function where user passes this callback
function? How would I define the function?

I tried this

callbackfunctio n clientfunction;

void SpecifyCallback function(cbFunc tion cbFn)
{
clientfunction = cbFn;
}

Then called like this:
clientfunction( sz); // sz is a C-string.

But program crashes with access violation when attempt to call
clientfunction

What am I doing wrong?

[Ian Collins]

Angus wrote:

Hello

I am writing a library which will write data to a user defined callback
function. The function the user of my library will supply is:

int (*callbackfunct ion)(const char*);

In my libary do I create a function where user passes this callback
function? How would I define the function?

I tried this

callbackfunctio n clientfunction;

void SpecifyCallback function(cbFunc tion cbFn)
{
clientfunction = cbFn;
}

Then called like this:
clientfunction( sz); // sz is a C-string.

You should be passing the address of the function, not a string.

int f( const char* );

clientfunction( f );

--
Ian Collins.

[Richard Heathfield]

Ian Collins said:

Angus wrote:

>Hello

I am writing a library which will write data to a user defined callback
function. The function the user of my library will supply is:

int (*callbackfunct ion)(const char*);

In my libary do I create a function where user passes this callback
function? How would I define the function?

I tried this

callbackfuncti on clientfunction;

void SpecifyCallback function(cbFunc tion cbFn)
{
clientfunction = cbFn;
}

Then called like this:
clientfunction (sz); // sz is a C-string.

You should be passing the address of the function, not a string.

int f( const char* );

clientfunction( f );

I doubt it. Since clientfunction is an instance of callbackfunctio n (and
presumably the definition of callbackfunctio n, above, is supposed to be a
typedef), it takes a const char *, not an int(*)(const char *).

>

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Ian Collins]

Richard Heathfield wrote:

Ian Collins said:

>Angus wrote:

>>Hello

I am writing a library which will write data to a user defined callback
function. The function the user of my library will supply is:

int (*callbackfunct ion)(const char*);

In my libary do I create a function where user passes this callback
function? How would I define the function?

I tried this

callbackfunct ion clientfunction;

void SpecifyCallback function(cbFunc tion cbFn)
{
clientfunction = cbFn;
}

Then called like this:
clientfunctio n(sz); // sz is a C-string.

You should be passing the address of the function, not a string.

int f( const char* );

clientfunction ( f );

I doubt it. Since clientfunction is an instance of callbackfunctio n (and
presumably the definition of callbackfunctio n, above, is supposed to be a
typedef), it takes a const char *, not an int(*)(const char *).

OK, I promise never to post pre-caffeine ever again!

I read the OP as passing a string to SpecifyCallback function.

--
Ian Collins.

[Ben Bacarisse]

"Angus" <no****@gmail.c omwrites:

I am writing a library which will write data to a user defined callback
function. The function the user of my library will supply is:

int (*callbackfunct ion)(const char*);

In my libary do I create a function where user passes this callback
function? How would I define the function?

I tried this

callbackfunctio n clientfunction;

This won't compile. I suspect you have a typedef that you are not
showing us!

<snip>

But program crashes with access violation when attempt to call
clientfunction

What am I doing wrong?

Try to post a short, compilable, example of the problem. The outline
you posted is sound, the error is in the detail (and is somewhere
else).

--
Ben.

[Bill Reid]

Angus <no****@gmail.c omwrote in message
news:ft******** ***********@new s.demon.co.uk.. .

Hello

I am writing a library which will write data to a user defined callback
function. The function the user of my library will supply is:

int (*callbackfunct ion)(const char*);

In my libary do I create a function where user passes this callback
function? How would I define the function?

I tried this

callbackfunctio n clientfunction;

void SpecifyCallback function(cbFunc tion cbFn)
{
clientfunction = cbFn;
}

Then called like this:
clientfunction( sz); // sz is a C-string.

But program crashes with access violation when attempt to call
clientfunction

What am I doing wrong?

Well, just about everything, and most pertinently, asking a question
here, the land of the technically-incompetent trolls...but here's how
you do it:

In the header file for your library, declare the function as follows:

extern void my_library_func tion(int (*)(const char*));

(Note: as somebody may tell you, "extern" is a redundant
linkage specifier for function declarations, but I use it anyway
and therefore you should too!)

Now write your library function that takes the callback as
a parameter in the source file for your library:

void my_library_func tion(int my_callback_fun ction(const char*)) {
int my_callback_ret urn;
char *my_string;

... /* generic stuff done here, probably "build up" my_string */

my_callback_ret urn=my_callback _function(my_st ring);

... /* more generic stuff maybe, maybe check my_callback_ret urn */
}

Now, for any source file that you want to use that generic
my_library_func tion(), you can call it by first #include'ing the
library header file, then defining a specific callback function that
matches the declaration in the header file:

int my_specific_fun ction(const char* my_string) {

... /* do something with string, probably print it, right? */
}

Then you can call your library function with the callback anywhere
in your source file, as well as any other functions that you have defined
that match the callback signature:

void my_function(voi d) {

... /* stuff happens here, whatever, maybe nothing, who knows */

my_library_func tion(my_specifi c_function);

... /* and whatever else */
}

And that's "all" there is to it...not that bad once you get the hang of
it, just follow the pattern above, sometimes you have to really "think"
about what the perfect "signature" will be for all the various callbacks
you want for a generic library function, what all data you need to
pass for all possible conditions...

---
William Ernest Reid

[Richard Heathfield]

Bill Reid said:

Angus <no****@gmail.c omwrote in message
news:ft******** ***********@new s.demon.co.uk.. .

<snip>

>>
What am I doing wrong?

Well, just about everything, and most pertinently, asking a question
here, the land of the technically-incompetent trolls...but here's how
you do it:

When you make a claim like that, you should back it up with working code.
You didn't. A conforming implementation *must* diagnose, and *may* refuse
to translate, your code.

Your track record of ignoring or even railing against those who notify you
of your mistakes gives me little or no hope that you'll pay any attention
to this, but the OP will at least have fair warning of the kind of
"competence " to which you are exposing him.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Bill Reid]

Richard Heathfield <rj*@see.sig.in validwrote in message
news:a9******** *************@b t.com...

Bill Reid said:

Angus <no****@gmail.c omwrote in message
news:ft******** ***********@new s.demon.co.uk.. .

<snip>

>
What am I doing wrong?

Well, just about everything, and most pertinently, asking a question
here, the land of the technically-incompetent trolls...but here's how
you do it:

When you make a claim like that, you should back it up with working code.
You didn't. A conforming implementation *must* diagnose, and *may* refuse
to translate, your code.

Well, I didn't really write any code, you insane troll, so I guess
you're "right" again, as always...

Your track record of ignoring or even railing against those who notify you
of your mistakes gives me little or no hope that you'll pay any attention
to this, but the OP will at least have fair warning of the kind of
"competence " to which you are exposing him.

Well, no, I couldn't help but notice that you didn't specify any
actual problems with what I posted, so how would he know what's
wrong with it? Believe me, for his sake, you should let HIM know,
and let ALL of us know, including me, so we can all benefit from
your tremendous "wisdom"... I'd be the first to admit that I make
a LOT of mistakes, have made many here, and there may be
errors or omissions in my post, and I'd genuinely like to be
"set straight", but somehow I think you're just blowin' troll
smoke again, as usual...

So if you got anything of substance, post it; I'm not saying it
would be a first, but along the lines of a rarity...

---
William Ernest Reid

[Richard Heathfield]

Bill Reid said:

>
Richard Heathfield <rj*@see.sig.in validwrote in message
news:a9******** *************@b t.com...

>Bill Reid said:

Angus <no****@gmail.c omwrote in message
news:ft******** ***********@new s.demon.co.uk.. .

<snip>

>>
What am I doing wrong?

Well, just about everything, and most pertinently, asking a question
here, the land of the technically-incompetent trolls...but here's how
you do it:

When you make a claim like that, you should back it up with working
code. You didn't. A conforming implementation *must* diagnose, and *may*
refuse to translate, your code.

Well, I didn't really write any code, you insane troll, so I guess
you're "right" again, as always...

Here is the code you claim you didn't really write, which I've copied
verbatim from your article.

extern void my_library_func tion(int (*)(const char*));
void my_library_func tion(int my_callback_fun ction(const char*)) {
int my_callback_ret urn;
char *my_string;

... /* generic stuff done here, probably "build up" my_string */

my_callback_ret urn=my_callback _function(my_st ring);

... /* more generic stuff maybe, maybe check my_callback_ret urn */
}
int my_specific_fun ction(const char* my_string) {

... /* do something with string, probably print it, right? */
}
void my_function(voi d) {

... /* stuff happens here, whatever, maybe nothing, who knows */

my_library_func tion(my_specifi c_function);

... /* and whatever else */
}

Note that my observation about failure to compile does not relate to
obvious "more stuff goes here" conventions such as an occasional ellipsis.

>Your track record of ignoring or even railing against those who notify
you of your mistakes gives me little or no hope that you'll pay any
attention to this, but the OP will at least have fair warning of the
kind of "competence " to which you are exposing him.

Well, no, I couldn't help but notice that you didn't specify any
actual problems with what I posted,

No point. You never listen anyway.

so how would he know what's wrong with it?

Since it doesn't compile, he'll find out pretty quickly that it *is* wrong.
As to *why* it's wrong, that's easy. It was written by someone who doesn't
understand C very well.

So if you got anything of substance, post it;

After you.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Nick Keighley]

On 14 Apr, 08:10, Richard Heathfield <[e-mail].i nvalidwrote:

Bill Reid said:

Richard Heathfield <[e-mail].i nvalidwrote in message
news:a9******** *************@b t.com...

Bill Reid said:
Angus <nos...@gmail.c omwrote in message
news:ft******** ***********@new s.demon.co.uk.. .

What am I doing wrong?

Well, just about everything, and most pertinently, asking a question
here, the land of the technically-incompetent trolls...but here's how
you do it:

oh the irony...

When you make a claim like that, you should back it up with working
code. You didn't. A conforming implementation *must* diagnose, and *may*
refuse to translate, your code.

Well, I didn't really write any code, you insane troll, so I guess
you're "right" again, as always...

Here is the code you claim you didn't really write, which I've copied
verbatim from your article.

<snip code-like stuff>

Note that my observation about failure to compile does not relate to
obvious "more stuff goes here" conventions such as an occasional ellipsis.

Your track record of ignoring or even railing against those who notify
you of your mistakes gives me little or no hope that you'll pay any
attention to this, but the OP will at least have fair warning of the
kind of "competence " to which you are exposing him.

Well, no, I couldn't help but notice that you didn't specify any
actual problems with what I posted,

No point. You never listen anyway.

so how would he know what's wrong with it?

Since it doesn't compile, he'll find out pretty quickly that it *is* wrong.
As to *why* it's wrong, that's easy. It was written by someone who doesn't
understand C very well.

So if you got anything of substance, post it;

After you.

perhaps, Richard, if you tried to be just a little less gnomic...
Perhaps, even, point out the error :-)

Who remembers the Campaign Againt Grumpiness in c.l.c.?
--
Nick Keighley

If cosmology reveals anything about God, it is that He has
an inordinate fondness for empty space and non-baryonic dark
matter.
Sverker Johansson (talk.origins)