syntax: struct in a struct

[fkater@googlemail.com]

I have got these two structs where the second is embedding the first:

typedef struct{
int i1;
int i2;
}struct1;

typedef struct{
int i3;
struct1 s;
}struct2;

Then, the second struct is passed as a void pointer argument in the
callback function f:

void f(void* struct_of_type_ struct2_expecte d){...};

My question is why I simpy can't access i2 like this:

((struct2*)stru ct_of_type_stru ct2_expected)->s.i2

but have to cast s into struct1 explicitly first:

(struct1)(((str uct2*)struct_of _type_struct2_e xpected)->s).i2


Felix

[Jordan Abel]

Re: syntax: struct in a struct

On 2006-03-23, fkater@googlema il.com <fkater@googlem ail.com> wrote:[color=blue]
> I have got these two structs where the second is embedding the first:
>
> typedef struct{
> int i1;
> int i2;
>}struct1;
>
> typedef struct{
> int i3;
> struct1 s;
>}struct2;
>
> Then, the second struct is passed as a void pointer argument in the
> callback function f:
>
> void f(void* struct_of_type_ struct2_expecte d){...};
>
> My question is why I simpy can't access i2 like this:
>
> ((struct2*)stru ct_of_type_stru ct2_expected)->s.i2[/color]

Why can't you? What error message are you getting?
[color=blue]
> but have to cast s into struct1 explicitly first:
>
> (struct1)(((str uct2*)struct_of _type_struct2_e xpected)->s).i2
>
>
> Felix
>[/color]

[Richard Bos]

Re: syntax: struct in a struct

"fkater@googlem ail.com" <fkater@googlem ail.com> wrote:
[color=blue]
> I have got these two structs where the second is embedding the first:
>
> typedef struct{
> int i1;
> int i2;
> }struct1;
>
> typedef struct{
> int i3;
> struct1 s;
> }struct2;
>
> Then, the second struct is passed as a void pointer argument in the
> callback function f:
>
> void f(void* struct_of_type_ struct2_expecte d){...};
>
> My question is why I simpy can't access i2 like this:
>
> ((struct2*)stru ct_of_type_stru ct2_expected)->s.i2[/color]

You can. Are you sure you're using a C compiler?

Richard

[fkater@googlemail.com]

Re: syntax: struct in a struct

Jordan Abel wrote:[color=blue]
> On 2006-03-23, fkater@googlema il.com <fkater@googlem ail.com> wrote:[/color]

[...]
[color=blue][color=green]
> > My question is why I simpy can't access i2 like this:
> >
> > ((struct2*)stru ct_of_type_stru ct2_expected)->s.i2[/color]
>
> Why can't you? What error message are you getting?[/color]

Unfortunatelly I don't get an error message. It just turned out that i2
never contained the expected results, and explicitly casting s before
surrounded this problem.

Felix

[fkater@googlemail.com]

Re: syntax: struct in a struct


Richard Bos wrote:[color=blue]
> "fkater@googlem ail.com" <fkater@googlem ail.com> wrote:[/color]

[...]
[color=blue][color=green]
> > My question is why I simpy can't access i2 like this:
> >
> > ((struct2*)stru ct_of_type_stru ct2_expected)->s.i2[/color]
>
> You can. Are you sure you're using a C compiler?[/color]

Hm. I am using gcc-3.4.2 (mingw-special).

Felix

[Richard Bos]

Re: syntax: struct in a struct

"fkater@googlem ail.com" <fkater@googlem ail.com> wrote:
[color=blue]
> Richard Bos wrote:[color=green]
> > "fkater@googlem ail.com" <fkater@googlem ail.com> wrote:[/color]
>[color=green][color=darkred]
> > > My question is why I simpy can't access i2 like this:
> > >
> > > ((struct2*)stru ct_of_type_stru ct2_expected)->s.i2[/color]
> >
> > You can. Are you sure you're using a C compiler?[/color]
>
> Hm. I am using gcc-3.4.2 (mingw-special).[/color]

And you're compiling as C, not as, say, C++? There are areas surrounding
conversion where C++ makes different demands from C; I do not know
whether this is one of them.
If you're compiling C, post a complete program, as small as you can cut
it, which exhibits the problem. It doesn't appear in my test:

#include <stdio.h>

typedef struct {
int i1;
int i2;
} struct1;

typedef struct {
int i3;
struct1 s;
} struct2;

void f(void *str2)
{
printf("%d\n", ((struct2 *)str2)->s.i2);
return;
}

int main(void) {
struct2 s2={3, {1,2}};

f(&s2);

getchar();
return 0;
}

I get an output of 2, as expected. This with Dev-C++, which is also gcc
plus MinGW.

Richard

[Vladimir S. Oka]

Re: syntax: struct in a struct


fkater@googlema il.com wrote:[color=blue]
> Richard Bos wrote:[color=green]
> > "fkater@googlem ail.com" <fkater@googlem ail.com> wrote:[/color]
>
> [...]
>[color=green][color=darkred]
> > > My question is why I simpy can't access i2 like this:
> > >
> > > ((struct2*)stru ct_of_type_stru ct2_expected)->s.i2[/color]
> >
> > You can. Are you sure you're using a C compiler?[/color]
>
> Hm. I am using gcc-3.4.2 (mingw-special).[/color]

I just tried it on exact same compiler, and it works.

Here's the exact code I tried:

#include<stdio. h>

typedef struct {
int i1;
int i2;
} struct1;

typedef struct{
int i3;
struct1 s;
} struct2;

void f(void* struct_of_type_ struct2_expecte d)
{
((struct2*)stru ct_of_type_stru ct2_expected)->s.i2 = 42;
}

int main(void)
{
struct2 s2;

f(&s2);

printf("%d\n", s2.s.i2);

return 0;
}

[fkater@googlemail.com]

Re: syntax: struct in a struct

Richard Bos wrote:
[color=blue]
> And you're compiling as C, not as, say, C++?[/color]

Yes, I compile as C.

[color=blue]
> If you're compiling C, post a complete program, as small as you can cut[/color]

Hm, I'll see what I can extract but this seems part of the problem
itself since the code I posted seems to be the relevant one as far as I
unterstand things here. So, it will take a little to extract the right
code from my program (which is quite large) to still generate the
error.
[color=blue]
> it, which exhibits the problem. It doesn't appear in my test:[/color]

Thanks so far for testing it.

Felix

[Rod Pemberton]

Re: syntax: struct in a struct


<fkater@googlem ail.com> wrote in message
news:1143100179 .229454.257450@ u72g2000cwu.goo glegroups.com.. .[color=blue]
> I have got these two structs where the second is embedding the first:
>
> typedef struct{
> int i1;
> int i2;
> }struct1;
>
> typedef struct{
> int i3;
> struct1 s;
> }struct2;
>
> Then, the second struct is passed as a void pointer argument in the
> callback function f:
>
> void f(void* struct_of_type_ struct2_expecte d){...};
>
> My question is why I simpy can't access i2 like this:
>
> ((struct2*)stru ct_of_type_stru ct2_expected)->s.i2
>[/color]

This is correct and works with two compilers for me.
[color=blue]
> but have to cast s into struct1 explicitly first:
>
> (struct1)(((str uct2*)struct_of _type_struct2_e xpected)->s).i2[/color]

Is that actually what you used? This is illegal in ANSI/ISO C. You can't
cast to a struct, union, array or function.

The precedence of the direct '.' and indirect '->' component selection
operators should be the same and both left associative. If the precedence
rules are incorrect for your compiler, you'd need this (without the cast to
struct1):

(((struct2*)str uct_of_type_str uct2_expected)->s).i2

If you want to use the cast to struct1, it would be written like so:

((struct1*)&((( struct2*)struct _of_type_struct 2_expected)->s))->i2

You must cast to a pointer to struct1 since casts to a struct are illegal.
Also, notice that you must take the address of struct s before casting to a
pointer to struct1 and now use the indirection operator for i2.


Rod Pemberton

[santosh]

Re: syntax: struct in a struct

fkater@googlema il.com wrote:[color=blue]
> Richard Bos wrote:
>[color=green]
> > And you're compiling as C, not as, say, C++?[/color]
>
> Yes, I compile as C.
>
>[color=green]
> > If you're compiling C, post a complete program, as small as you can cut[/color]
>
> Hm, I'll see what I can extract but this seems part of the problem
> itself since the code I posted seems to be the relevant one as far as I
> unterstand things here. So, it will take a little to extract the right
> code from my program (which is quite large) to still generate the
> error.
>[color=green]
> > it, which exhibits the problem. It doesn't appear in my test:[/color][/color]

Since your code compiles and since your construct is legal, it's
probable that some other area of your program is trashing i2.