Question about C Functions

[jobo]

If I have the function:

int f(int (*h)(int)) {
return (*h)(13);
}

What exactly does (int (*h)(int)) do? So it's taking a pointer but
what's with the two ints? Thanks.

[T.M. Sommers]

jobo wrote:

If I have the function:

int f(int (*h)(int)) {
return (*h)(13);
}

What exactly does (int (*h)(int)) do? So it's taking a pointer but
what's with the two ints? Thanks.

f is a function returning int. It takes as an argument a pointer
to a function returning int, which function takes an int as an
argument. f's argument is called h. f calls the function
pointed to by h with an argument of 13.

Look for the program called cdecl, which helps explain C
declarations.

--
Thomas M. Sommers -- tm*@nj.net -- AB2SB

[attn.steven.kuo]

jobo wrote:

If I have the function:

int f(int (*h)(int)) {
return (*h)(13);
}

What exactly does (int (*h)(int)) do? So it's taking a pointer but
what's with the two ints? Thanks.

Incrementally read as:

h -- a pointer
(*h) -- whose dereferenced value
(*h)(int) -- is a function that accepts a single int as a
parameter
int (*h)(int) -- and returns an int
Thus h is a pointer to a function with the aforementioned parameter
and return types. For example, h can be used to point to the 'toupper'
function defined in the C standard:
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

int main (void)
{
int (*h)(int);
h = toupper;
printf ("%c programming is hard!\n", (*h)('c'));
return 0;
}

--
Hope this helps,
Steven

[Keith Thompson]

"jobo" <jo*****@gmail. comwrites:

If I have the function:

int f(int (*h)(int)) {
return (*h)(13);
}

What exactly does (int (*h)(int)) do? So it's taking a pointer but
what's with the two ints? Thanks.

f is a function that returns a result of type int. It has a single
parameter, "h", of type int (*)(int). In other words, h is a pointer
to a function; that function has a single parameter of type int, and
returns a result of type int.

There's a program called "cdecl" that's often helpful for this kind of
thing (though it doesn't like the parameter name):

% cdecl
Type `help' or `?' for help
cdeclexplain int f(int (*h)(int))
syntax error
cdeclexplain int f(int (*)(int))
declare f as function (pointer to function (int) returning int) returning int
cdecl>

I got it from
<http://www.ibiblio.org/pub/Linux/devel/lang/c/cdecl-2.5.tar.gz>

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Simon Biber]

at************* @gmail.com wrote:

jobo wrote:

>What exactly does (int (*h)(int)) do? So it's taking a pointer but
what's with the two ints? Thanks.

Incrementally read as:

h -- a pointer

From 'h' all we can tell is that 'h' is an identifier.

(*h) -- whose dereferenced value

Only now can we see that 'h' is a pointer whose dereferenced value ...

(*h)(int) -- is a function that accepts a single int as a parameter
int (*h)(int) -- and returns an int

I would read it like this:

h -- object
*h -- pointer
(*h) -- pointer
int (*h)(int) -- pointer to function (int) returning int

--
Simon.

[Frederick Gotham]

jobo:

int f(int (*h)(int)) {
return (*h)(13);
}

The parentheses and dereference are redundant.

int f(int (*const Func)(int))
{
return Func(13);
}

--

Frederick Gotham

[Simon Biber]

Frederick Gotham wrote:

jobo:

>int f(int (*h)(int)) {
return (*h)(13);
}

The parentheses and dereference are redundant.

int f(int (*const Func)(int))
{
return Func(13);
}

For some reason cdecl chokes on your definition above. When I give it
just the parameter, it's fine:

cdeclexplain int (* const Func)(int)
declare Func as const pointer to function (int) returning int

But when I give it the whole thing (minus the name of the parameter), it
chokes:

cdeclexplain int f(int (*const)(int))
syntax error

On further exploration it seems to choke on all const pointers as
parameters:

cdeclexplain int f(int *const)
syntax error

However it works properly in the other direction:

cdecldeclare f as function (const pointer to function (int) returning
int) returning int
int f(int (* const )(int ))

--
Simon.

[Frederick Gotham]

Simon Biber:

For some reason cdecl chokes on your definition above.

Sorry, I don't know what cdecl is.

<snip cdecl stuff>

At first glance, all the ones that give syntax errors are the ones which
cannot serve as standalone declarations, e.g.:

int main(void)
{
int (*const)(int);
}

--

Frederick Gotham

[Richard Heathfield]

Frederick Gotham said:

Simon Biber:

>For some reason cdecl chokes on your definition above.

Sorry, I don't know what cdecl is.

Read K&R2 more closely.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)

[Mark McIntyre]

On Wed, 01 Nov 2006 15:43:46 GMT, in comp.lang.c , Frederick Gotham
<fg*******@SPAM .comwrote:

>Simon Biber:

>For some reason cdecl chokes on your definition above.

Sorry, I don't know what cdecl is.

This is obvious from one of your comments in a different thread.
cdecl is a programme that describes a C construct in human.

# cdecl explain int arr[2][2]
declare arr as array 2 of array 2 of int

I strongly recommend you obtain and play with it, its jolly useful not
to mention helping clarify some other questions.

--
Mark McIntyre

"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan