Should I do that...?

[info]

Hello World,

The code below compiles without warnings and the resulting program runs
without any error.

....somehow I am not sure if I should do something like this and/or if
there are any dangers...

Any comments?

#include <stdio.h>

void Func1()
{
printf("func1\n");
return;
}

void Func2()
{
printf("func2\n");
return;
}

void Func3()
{
printf("func3\n");
return;
}

void StartFunctions( void (*f)(void) )
{
printf("Starting Function...\n");
f();
return;
}

int main()
{
char i;
void *v[] = { (void*)Func1, (void*)Func2, (void*)Func3 };
for ( i = 0; i < 3; i++ ) StartFunctions( v );
return 0;
}

 

[Richard Heathfield]

(e-mail address removed) said:

Hello World,

The code below compiles without warnings and the resulting program runs
without any error.

...somehow I am not sure if I should do something like this and/or if
there are any dangers...

Any comments?

Although it is a common extension, the conversion between function pointers
and void pointers is not guaranteed by the Standard to be lossless. I have
corrected that problem in your code, below. I have also introduced a
typedef, since it makes the whole thing easier to read, and added a couple
of comments which I recommend that you read.

#include <stdio.h>

typedef void VFUNCV(void);

void Func1(void)
{
printf("func1\n");
return;
}

void Func2(void)
{
printf("func2\n");
return;
}

void Func3(void)
{
printf("func3\n");
return;
}

void StartFunctions( VFUNCV *f )
{
printf("Starting Function...\n");
f(); /* I would prefer (*f)(); on the grounds that it reminds
* us a function pointer is being used here. Either works.
*/
return;
}

int main()
{
char i; /* Why char? What are you trying to save, and why? */
VFUNCV *v[] = { Func1, Func2, Func3 };
for ( i = 0; i < 3; i++ ) StartFunctions( v );
return 0;
}

 

[Irrwahn Grausewitz]

Hello World,

The code below compiles without warnings and the resulting program runs
without any error.

...somehow I am not sure if I should do something like this and/or if
there are any dangers...

Any comments?

See below.

#include <stdio.h>

void Func1()

Change this to:

void Func1( void )

to match the pointer declarations below.

{
printf("func1\n");
return;
}

void Func2()

Same here.

{
printf("func2\n");
return;
}

void Func3()

Same here.

{
printf("func3\n");
return;
}

void StartFunctions( void (*f)(void) )
{
printf("Starting Function...\n");
f();
return;
}

int main()

This isn't one of the two legitimate declarations of main, use:

int main( void )

{
char i;

Using a character value for array subscripting is at least suspicious;
no, wait, it's actually a constraint violation. Use:

int i;

void *v[] = { (void*)Func1, (void*)Func2, (void*)Func3 };

In order to avoid pointer type mismatches you presumably want:

void (*v[])(void) = { Func1, Func2, Func3 };

Note: cluttering your code with spurious casts is the wrong approach.
Better use correct declarations in the first place.

for ( i = 0; i < 3; i++ ) StartFunctions( v );

Minor quibble: beware of magic numbers (3).

return 0;

That's fine. )

}

HTH
Best regards.

 

[info]

Dear Richard,

thank you for your corrections

char i; /* Why char? What are you trying to save, and why? */

That was one of my questions. I wonder if I save _any_ memory by using
char instead of int.
The target of my application is a microcontroller where I really need
to save memory. The application will be compiled with 1 byte alignment.

yours,

Herwig

 

[Irrwahn Grausewitz]

That is better, however you can use any integer type for an array
subscript (or more specifically pointer arithemtic) and character types
are perfectly good integer types.

</me slaps forehead> Argh, I did it again. Note to self:

I shall not read the standard hastily.
I shall not read the standard hastily.
I shall not ...

 

[Lawrence Kirby]

On Wed, 14 Sep 2005 12:48:07 +0200, Irrwahn Grausewitz wrote:

....

int main( void )


Using a character value for array subscripting is at least suspicious;
no, wait, it's actually a constraint violation. Use:

int i;

That is better, however you can use any integer type for an array
subscript (or more specifically pointer arithemtic) and character types
are perfectly good integer types.

void *v[] = { (void*)Func1, (void*)Func2, (void*)Func3 };

In order to avoid pointer type mismatches you presumably want:

void (*v[])(void) = { Func1, Func2, Func3 };

Note: cluttering your code with spurious casts is the wrong approach.
Better use correct declarations in the first place.

for ( i = 0; i < 3; i++ ) StartFunctions( v );

This would also have been an error in the original code, you cannot
convert from void * to a funciton pointer type without a cast.

Lawrence

 

[Martin Ambuhl]

Hello World,

The code below compiles without warnings and the resulting program runs
without any error.

Then either you have an unhelpful compiler or you have your diagnostics
incorrectly set. I get, for example

a.c: In function 'main':
a.c:42: warning: ISO C forbids conversion of function pointer to object
pointer type
a.c:42: warning: ISO C forbids conversion of function pointer to object
pointer type
a.c:42: warning: ISO C forbids conversion of function pointer to object
pointer type
a.c:44: warning: array subscript has type 'char'
a.c:44: warning: ISO C forbids passing argument 1 of 'StartFunctions'
between function pointer and 'void *'

To fix this is easy. Change

> void *v[] = { (void*)Func1, (void*)Func2, (void*)Func3 };

to
void (*v[])() = { Func1, Func2, Func3 };

...somehow I am not sure if I should do something like this and/or if
there are any dangers...

Of course there are dangers in writing broken code.

Any comments?

#include <stdio.h>

void Func1()
{
printf("func1\n");
return;
}

void Func2()
{
printf("func2\n");
return;
}

void Func3()
{
printf("func3\n");
return;
}

void StartFunctions( void (*f)(void) )
{
printf("Starting Function...\n");
f();
return;
}

int main()
{
char i;
void *v[] = { (void*)Func1, (void*)Func2, (void*)Func3 };
for ( i = 0; i < 3; i++ ) StartFunctions( v );
return 0;
}

 

[John Bode]

Hello World,

The code below compiles without warnings and the resulting program runs
without any error.

...somehow I am not sure if I should do something like this and/or if
there are any dangers...

Any comments?

#include <stdio.h>

void Func1()

/*
** make sure function prototypes match
** the prototype in StartFunctions
*/
void Func1(void)

{
printf("func1\n");
return;
}

void Func2()

void Func2(void)

{
printf("func2\n");
return;
}

void Func3()

void Func3(void)

{
printf("func3\n");
return;
}

void StartFunctions( void (*f)(void) )
{
printf("Starting Function...\n");
f();
return;
}

int main()
{
char i;
void *v[] = { (void*)Func1, (void*)Func2, (void*)Func3 };

void (*v[])(void) = {Func1, Func2, Func3};

Converting between object and function pointers is bad juju.

for ( i = 0; i < 3; i++ ) StartFunctions( v );
return 0;
}

 

[Tim Woodall]

Dear Richard,

thank you for your corrections

char i; /* Why char? What are you trying to save, and why? */

That was one of my questions. I wonder if I save _any_ memory by using
char instead of int.
The target of my application is a microcontroller where I really need
to save memory. The application will be compiled with 1 byte alignment.

Depends on the compiler. Some compilers for 8 bit microcontrollers also
include an option to make int 8 bits.

You may also find that this is one place where register might make a
difference.

Tim.

 

[Keith Thompson]

On 14 Sep 2005 04:45:27 -0700,

Depends on the compiler. Some compilers for 8 bit microcontrollers also
include an option to make int 8 bits.

This is, of course, non-conforming; int is required to be able to hold
values in the range -32767 .. +32767. (But "non-conforming" is not
synonymous with "evil"; there's nothing wrong with creating an
implementation for something that's not quite C, as long as you don't
claim that it conforms to the C standard.)

 

[Simon Biber]

This isn't one of the two legitimate declarations of main, use:

int main( void )

The standard requires main to be defined with a type equivalent to one
of the two legitimate declarations. The OP's definition is equivalent,
and so perfectly valid.

To make it clear: leaving out the "void" in the parameter list of a
function _definition_ makes no difference. Leaving it out in a
_declaration_ does make a difference.

 

[pete]

Simon Biber wrote:

To make it clear: leaving out the "void" in the parameter list of a
function _definition_ makes no difference. Leaving it out in a
_declaration_ does make a difference.

Function definitions are also declarations.

 

[Simon Biber]

Function definitions are also declarations.

True, but this only matters if you're going to call the function from a
subsequent function.

As I was talking about the definition of the main function, this is
something which, hopefully, no-one does in practice.

 

[Walter Roberson]

pete wrote:

True, but this only matters if you're going to call the function from a
subsequent function.

As I was talking about the definition of the main function, this is
something which, hopefully, no-one does in practice.

clc often has examples of people calling main recursively
(at, at the very least, cursedly.)

I wouldn't expect calling recursing to main to be -common-, but
it apparently does happen in real life.

 

[info]

Of course there are dangers in writing broken code.
I'll gonna tell this my predecessor

 

[Emmanuel Delahaye]

Hello World,

The code below compiles without warnings and the resulting program runs
without any error.

Get a better compiler or learn how to tune it..

main.c:8: warning: function declaration isn't a prototype
main.c:14: warning: function declaration isn't a prototype
main.c:20: warning: function declaration isn't a prototype

main.c:26: warning: no previous prototype for 'StartFunctions'

main.c: In function `main_':
main.c:36: warning: array subscript has type `char'

...somehow I am not sure if I should do something like this and/or if
there are any dangers...

#include <stdio.h>

typedef void Tfun (void);

static void Func1 (void)
{
printf ("func1\n");
return;
}

static void Func2 (void)
{
printf ("func2\n");
return;
}

static void Func3 (void)
{
printf ("func3\n");
return;
}

static void StartFunctions (void (*f) (void))
{
printf ("Starting Function...\n");
f ();
return;
}

int main (void)
{
size_t i;
Tfun *v[] =
{
Func1,
Func2,
Func3,
};

for (i = 0; i < 3; i++)
{
StartFunctions (v);
}
return 0;
}

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
The C-library: http://www.dinkumware.com/refxc.html

"Mal nommer les choses c'est ajouter du malheur au
monde." -- Albert Camus.

 

[Huajian Luo]

(e-mail address removed) wrote on 14/09/05 :
int main (void)
{
size_t i;
Tfun *v[] =
{
Func1,
Func2,
Func3,
};

for (i = 0; i < 3; i++)
{
StartFunctions (v);
}
return 0;
}

I'm just curious about How can I traverse the array V with pointer,
I've tried something like
VFUNCV *p;
for (p = *v; *p; p++) StartFunctions(*p);

but that result core dump, Any hints to make it by poniter,
Thanks,
whatluo

 

[Simon Biber]

I'm just curious about How can I traverse the array V with pointer,
I've tried something like
VFUNCV *p;
for (p = *v; *p; p++) StartFunctions(*p);

but that result core dump, Any hints to make it by poniter,

#include <stdio.h>

typedef void VFUNCV(void);

void func1(void)
{
printf("func1\n");
}

void func2(void)
{
printf("func2\n");
}

void func3(void)
{
printf("func3\n");
}

VFUNCV *array[] =
{
func1,
func2,
func3
};

int main(void)
{
VFUNCV **p;
VFUNCV **end = array + sizeof array / sizeof *array;

for(p = array; p != end; ++p)
{
(*p)();
}

return 0;
}