syntax: struct in a struct

[fkater]

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_expected){...};

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

((struct2*)struct_of_type_struct2_expected)->s.i2

but have to cast s into struct1 explicitly first:

(struct1)(((struct2*)struct_of_type_struct2_expect ed)->s).i2
Felix

[Jordan Abel]

On 2006-03-23, fk****@googlemail.com <fk****@googlemail.com> wrote:

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_expected){...};

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

((struct2*)struct_of_type_struct2_expected)->s.i2
Why can't you? What error message are you getting?
but have to cast s into struct1 explicitly first:

(struct1)(((struct2*)struct_of_type_struct2_expect ed)->s).i2
Felix

[Richard Bos]

"fk****@googlemail.com" <fk****@googlemail.com> wrote:

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_expected){...};

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

((struct2*)struct_of_type_struct2_expected)->s.i2

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

Richard

[fkater]

Jordan Abel wrote:

On 2006-03-23, fk****@googlemail.com <fk****@googlemail.com> wrote:

[...]

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

((struct2*)struct_of_type_struct2_expected)->s.i2

Why can't you? What error message are you getting?

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]

Richard Bos wrote:

"fk****@googlemail.com" <fk****@googlemail.com> wrote:

[...]

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

((struct2*)struct_of_type_struct2_expected)->s.i2

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

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

Felix

[Richard Bos]

"fk****@googlemail.com" <fk****@googlemail.com> wrote:

Richard Bos wrote:

"fk****@googlemail.com" <fk****@googlemail.com> wrote:

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

((struct2*)struct_of_type_struct2_expected)->s.i2

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

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

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]

fk****@googlemail.com wrote:

Richard Bos wrote:

"fk****@googlemail.com" <fk****@googlemail.com> wrote:

[...]

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

((struct2*)struct_of_type_struct2_expected)->s.i2

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

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

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_expected)
{
((struct2*)struct_of_type_struct2_expected)->s.i2 = 42;
}

int main(void)
{
struct2 s2;

f(&s2);

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

return 0;
}

[fkater]

Richard Bos wrote:

And you're compiling as C, not as, say, C++?
Yes, I compile as C.

If you're compiling C, post a complete program, as small as you can cut
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.
it, which exhibits the problem. It doesn't appear in my test:

Thanks so far for testing it.

Felix

[Rod Pemberton]

<fk****@googlemail.com> wrote in message
news:11**********************@u72g2000cwu.googlegr oups.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_expected){...};

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

((struct2*)struct_of_type_struct2_expected)->s.i2

This is correct and works with two compilers for me.
but have to cast s into struct1 explicitly first:

(struct1)(((struct2*)struct_of_type_struct2_expect ed)->s).i2

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*)struct_of_type_struct2_expected)->s).i2

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

((struct1*)&(((struct2*)struct_of_type_struct2_exp ected)->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]

fk****@googlemail.com wrote:

Richard Bos wrote:

And you're compiling as C, not as, say, C++?

Yes, I compile as C.

If you're compiling C, post a complete program, as small as you can cut

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.

it, which exhibits the problem. It doesn't appear in my test:

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