Typedef structs

[stephen henry]

Hi all,

I have a question that I'm having difficulty answering. If I have a
struct:

typedef struct my_struct_tag{

struct my_other_struct *other;

} my_struct_tag

(I'm using the struct when declaring other because the definition of
my_other_struct is in another source file.)

Then, I want to access, from a different source file,

my_struct *tmp;
my_other_struct *other_struct;

tmp->other = other_struct;

But, for me, this gives a warning saying that the assignment assigns
different pointer types. If I change the code to:

tmp->other = (struct my_other_struct *)other_struct;

It works, but this is fugly. Is there anyway to assign like the first
method (ie without the cast).

Thanks,

Stephen Henry

[Michael Mair]

Re: Typedef structs

Hi,


I am not sure whether I understand your problem;
if I do
[color=blue]
> cat os.h[/color]
struct os {int c;};
[color=blue]
> cat s.h[/color]
typedef struct s {struct os *optr;} s_t;
[color=blue]
> cat test.c[/color]
#include "s.h"
#include "os.h"

int main (void)
{
s_t S, *pS;
struct os OS, *pOS;

pOS = &OS;
pS = &S;
pS->optr = pOS;

return 0;
}

then test.c compiles even without warning, irrespective
of the order of the #includes or the fact whether I use
s or s_t for the typedef. (gcc -Wall -ansi -pedantic test.c)

How does you setup differ from the above?


Cheers,
Michael

[Eric Sosman]

Re: Typedef structs

stephen henry wrote:[color=blue]
> Hi all,
>
> I have a question that I'm having difficulty answering. If I have a
> struct:
>
> typedef struct my_struct_tag{
>
> struct my_other_struct *other;
>
> } my_struct_tag
>
> (I'm using the struct when declaring other because the definition of
> my_other_struct is in another source file.)
>
> Then, I want to access, from a different source file,
>
> my_struct *tmp;
> my_other_struct *other_struct;
>
> tmp->other = other_struct;
>
> But, for me, this gives a warning saying that the assignment assigns
> different pointer types. If I change the code to:
>
> tmp->other = (struct my_other_struct *)other_struct;
>
> It works, but this is fugly. Is there anyway to assign like the first
> method (ie without the cast).[/color]

In some times and places, Royalty were considered to
be semi-divine, out of the general run of mankind (no doubt
the Royals encouraged this notion). Since it would be an
unthinkable sacrilege for a mere commoner to lay hands upon
the Royal person, some difficulty arose when there was
sickness or injury at the palace: How could the skilled
medicine man or barber-surgeon conduct a physical examination
of the ailing ruler without touching him?

The solution was to use a stand-in. The untouchable
Majesty would describe his symptoms to a common companion,
and the companion would submit to the specialist's examination
while doing his best to imitate his King's complaint. Given
the state of the medical arts and sciences of the time this
curious practice may not have made much practical difference
in the outcome, but today's doctors and nurses might be just
a wee bit hampered by such a roundabout and indirect custom,
don't you think?

The point of this story is that the pseudo-code you've
shown is just like the stand-in commoner. Is there something
untouchably holy about your actual code that prevents you from
revealing it to our unwashed gaze? Personally, I find it
surpassingly difficult to diagnose your problem when all I get
to see is your proxy. Please post a *complete* and *minimal*
example of *actual* code.

"A King of autocratic power We,
A despot whose tyrannic will is law,
Whose rule is paramount o'er land and sea,
A Presence of unutterable awe!
But though the awe that I inspire
Must shrivel with imperial fire
All foes whom it may chance to touch,
To judge by what I see and hear,
It does not seem to interfere
With popular enjoyment, much."

-- Paramount, Rex

--
Eric.Sosman@sun .com

[xarax]

Re: Typedef structs

"stephen henry" <stehenry@yahoo .com> wrote in message
news:845db307.0 408270229.41864 044@posting.goo gle.com...[color=blue]
> Hi all,
>
> I have a question that I'm having difficulty answering. If I have a
> struct:
>
> typedef struct my_struct_tag{
>
> struct my_other_struct *other;
>
> } my_struct_tag
>
> (I'm using the struct when declaring other because the definition of
> my_other_struct is in another source file.)
>
> Then, I want to access, from a different source file,
>
> my_struct *tmp;
> my_other_struct *other_struct;[/color]

You probably want that line to say:

struct my_other_struct *other_struct;
^^^^^^

That is, you forgot the "struct" keyword.
[color=blue]
> tmp->other = other_struct;
>
> But, for me, this gives a warning saying that the assignment assigns
> different pointer types. If I change the code to:
>
> tmp->other = (struct my_other_struct *)other_struct;
>
> It works, but this is fugly. Is there anyway to assign like the first
> method (ie without the cast).
>
> Thanks,
>
> Stephen Henry[/color]


--
----------------------------
Jeffrey D. Smith
Farsight Systems Corporation
24 BURLINGTON DRIVE
LONGMONT, CO 80501-6906

http://www.farsight-systems.com

z/Debug debugs your Systems/C programs running on IBM z/OS for FREE!

[ChokSheak Lau]

Re: Typedef structs

write a "typedef struct my_other_struct *my_other_struc t_ptr;"
before you declare the code typedef struct my_struct_tag { ...} ...;

this is doubly-independent definition. C actually allows typedef of a
struct pointer (pointer only) without knowing the definition.

chok


stephen henry wrote:
[color=blue]
> Hi all,
>
> I have a question that I'm having difficulty answering. If I have a
> struct:
>
> typedef struct my_struct_tag{
>
> struct my_other_struct *other;
>
> } my_struct_tag
>
> (I'm using the struct when declaring other because the definition of
> my_other_struct is in another source file.)
>
> Then, I want to access, from a different source file,
>
> my_struct *tmp;
> my_other_struct *other_struct;
>
> tmp->other = other_struct;
>
> But, for me, this gives a warning saying that the assignment assigns
> different pointer types. If I change the code to:
>
> tmp->other = (struct my_other_struct *)other_struct;
>
> It works, but this is fugly. Is there anyway to assign like the first
> method (ie without the cast).
>
> Thanks,
>
> Stephen Henry[/color]

[ChokSheak Lau]

Re: Typedef structs

"doubly-dependent", not "doubly-independent". my bad.

chok

ChokSheak Lau wrote:
[color=blue]
> write a "typedef struct my_other_struct *my_other_struc t_ptr;"
> before you declare the code typedef struct my_struct_tag { ...} ...;
>
> this is doubly-independent definition. C actually allows typedef of a
> struct pointer (pointer only) without knowing the definition.
>
> chok
>
>
> stephen henry wrote:
>[color=green]
>> Hi all,
>>
>> I have a question that I'm having difficulty answering. If I have a
>> struct:
>>
>> typedef struct my_struct_tag{
>>
>> struct my_other_struct *other;
>>
>> } my_struct_tag
>>
>> (I'm using the struct when declaring other because the definition of
>> my_other_struct is in another source file.)
>>
>> Then, I want to access, from a different source file,
>> my_struct *tmp;
>> my_other_struct *other_struct;
>>
>> tmp->other = other_struct;
>>
>> But, for me, this gives a warning saying that the assignment assigns
>> different pointer types. If I change the code to:
>>
>> tmp->other = (struct my_other_struct *)other_struct;
>>
>> It works, but this is fugly. Is there anyway to assign like the first
>> method (ie without the cast).
>>
>> Thanks,
>>
>> Stephen Henry[/color][/color]

[stephen henry]

Re: Typedef structs

"xarax" <xarax@email.co m> wrote in message news:<GPHXc.124 $w%6.2@newsread 1.news.pas.eart hlink.net>...[color=blue]
> "stephen henry" <stehenry@yahoo .com> wrote in message
> news:845db307.0 408270229.41864 044@posting.goo gle.com...[color=green]
> > Hi all,
> >
> > I have a question that I'm having difficulty answering. If I have a
> > struct:
> >
> > typedef struct my_struct_tag{
> >
> > struct my_other_struct *other;
> >
> > } my_struct_tag
> >
> > (I'm using the struct when declaring other because the definition of
> > my_other_struct is in another source file.)
> >
> > Then, I want to access, from a different source file,
> >
> > my_struct *tmp;
> > my_other_struct *other_struct;[/color]
>
> You probably want that line to say:
>
> struct my_other_struct *other_struct;
> ^^^^^^
>
> That is, you forgot the "struct" keyword.[/color]

Sorry, but I need to clarify somewhat:

Say I have two structures in two seperate files

file1.c
-------

typedef struct my_structure1_t ag {

// FOO;

} my_structure1;

file2.c
-------

typedef struct my_structure2_t ag {

my_structure1 *my_struct;

} my_structure2;

Now, in order to get file2.c to compile I have to change it so that it
reads:

typedef struct my_structure2_t ag {

STRUCT my_structure1 *my_struct;

} my_structure2;

Without the STRUCT i would get a syntax error because the compiler has
not compiled file1.c (which contains the decleration of
my_structure1). By inserting STRUCT I'm essentially saying to the
compiler that my_struct is a pointer to a structure, but I've not yet
defined what that structure actually is yet. What I would like to do,
would be to assigned a newly created my_structure1 as follows:

my_structure1 *structure1;
my_structure2 *structure2;

structure1->my_struct = structure2;

However, because in structure2, my_struct is declares as STRUCT
my_structure1 I have to cast the above assignment to:

structure1->my_struct = (struct my_structure1 *)structure2;

If I don't do this cast, I would get an error saying that I'm
assigning two incompatible pointer types.

So the question is: Is there anyway to compile C such that a structure
can contain another -I believe its called an incomplete- structure
from another, not yet compiled file, without having to cast in the
above manner.

Thanks,

Stephen Henry

[Michael Mair]

Re: Typedef structs

Hiho,
[color=blue]
> file1.c
> -------
>
> typedef struct my_structure1_t ag {
>
> // FOO;
>
> } my_structure1;[/color]

--> file1.h[color=blue]
>
> file2.c
> -------[/color]
#include "file1.h"[color=blue]
>
> typedef struct my_structure2_t ag {
>
> my_structure1 *my_struct;[/color]
without the #include, this type is not known.
[color=blue]
>
> } my_structure2;
>
> Now, in order to get file2.c to compile I have to change it so that it
> reads:
>
> typedef struct my_structure2_t ag {
>
> STRUCT my_structure1 *my_struct;[/color]
with the #include, both ways are possible; this shoul
read struct. If you want to mark something, use a comment.[color=blue]
>
> } my_structure2;[/color]

it would be better, to put that typedef into file2.h (which
includes file1.h).[color=blue]
>
> Without the STRUCT i would get a syntax error because the compiler has
> not compiled file1.c (which contains the decleration of
> my_structure1). By inserting STRUCT I'm essentially saying to the
> compiler that my_struct is a pointer to a structure, but I've not yet
> defined what that structure actually is yet. What I would like to do,
> would be to assigned a newly created my_structure1 as follows:
>
> my_structure1 *structure1;
> my_structure2 *structure2;
>
> structure1->my_struct = structure2;[/color]

That is not possible. structure1 only contains // FOO;
[color=blue]
> However, because in structure2, my_struct is declares as STRUCT
> my_structure1 I have to cast the above assignment to:
>
> structure1->my_struct = (struct my_structure1 *)structure2;
>
> If I don't do this cast, I would get an error saying that I'm
> assigning two incompatible pointer types.[/color]

In which file are you doing this, file1.c, file2.c, or some
yet unknown file3.c?
[color=blue]
> So the question is: Is there anyway to compile C such that a structure
> can contain another -I believe its called an incomplete- structure
> from another, not yet compiled file, without having to cast in the
> above manner.[/color]

If I understand you correctly, try whether
struct mystructure1;

typedef struct my_structure2_t ag {
struct my_structure1 *my_struct;
} my_structure2;
helps you.


You have been asked -- directly and indirectly -- to post
a minimal example. Please do not use Pseudo-Code, as this is
likely to produce more confusion than clarifying; apart from
that, yours is broken.

Some notes:
- Use header files for your types, #defines, prototypes and so on.

- Take the time to bring the example down to a minimal example
you can post here (c.f. my first answer to your question).

- Do not tell us what you think happens but give us actual code.


--
Michael

[Michael Mair]

Re: Typedef structs

Hiho,
[color=blue]
> file1.c
> -------
>
> typedef struct my_structure1_t ag {
>
> // FOO;
>
> } my_structure1;[/color]

--> file1.h[color=blue]
>
> file2.c
> -------[/color]
#include "file1.h"[color=blue]
>
> typedef struct my_structure2_t ag {
>
> my_structure1 *my_struct;[/color]
without the #include, this type is not known.
[color=blue]
>
> } my_structure2;
>
> Now, in order to get file2.c to compile I have to change it so that it
> reads:
>
> typedef struct my_structure2_t ag {
>
> STRUCT my_structure1 *my_struct;[/color]
with the #include, both ways are possible; this shoul
read struct. If you want to mark something, use a comment.[color=blue]
>
> } my_structure2;[/color]

it would be better, to put that typedef into file2.h (which
includes file1.h).[color=blue]
>
> Without the STRUCT i would get a syntax error because the compiler has
> not compiled file1.c (which contains the decleration of
> my_structure1). By inserting STRUCT I'm essentially saying to the
> compiler that my_struct is a pointer to a structure, but I've not yet
> defined what that structure actually is yet. What I would like to do,
> would be to assigned a newly created my_structure1 as follows:
>
> my_structure1 *structure1;
> my_structure2 *structure2;
>
> structure1->my_struct = structure2;[/color]

That is not possible. structure1 only contains // FOO;
[color=blue]
> However, because in structure2, my_struct is declares as STRUCT
> my_structure1 I have to cast the above assignment to:
>
> structure1->my_struct = (struct my_structure1 *)structure2;
>
> If I don't do this cast, I would get an error saying that I'm
> assigning two incompatible pointer types.[/color]

In which file are you doing this, file1.c, file2.c, or some
yet unknown file3.c?
[color=blue]
> So the question is: Is there anyway to compile C such that a structure
> can contain another -I believe its called an incomplete- structure
> from another, not yet compiled file, without having to cast in the
> above manner.[/color]

If I understand you correctly, try whether
struct mystructure1;

typedef struct my_structure2_t ag {
struct my_structure1 *my_struct;
} my_structure2;
(untested) helps you.


You have been asked -- directly and indirectly -- to post
a minimal example. Please do not use Pseudo-Code, as this is
likely to produce more confusion than clarifying; apart from
that, yours is broken.

Some notes:
- Use header files for your types, #defines, prototypes and so on.

- Take the time to bring the example down to a minimal example
you can post here (c.f. my first answer to your question).

- Do not tell us what you think happens but give us actual code.


--
Michael

[Chris Torek]

Re: Typedef structs

In article <news:845db307. 0408271113.3757 dccc@posting.go ogle.com>
stephen henry <stehenry@yahoo .com> wrote:[color=blue]
>Sorry, but I need to clarify somewhat:[/color]

[more sample code that is definitely not the actual code]

Suppose you were a car mechanic and I drove up in a vehicle and
said "my car isn't working right; it won't start." You might wonder
how I got the car to you. What would you think if I told you: "Oh,
this isn't my car, this is my neighbor's car, but we have the same
brand of vehicle. You should be able to fix mine by looking at
his, right?"

:-)

In any case, I have a method I recommend. Stop using typedef for
structs.

Just plain stop using it. You do not NEED it, and what it does is
rarely what most beginning or even intermediate-level C programmers
expect. Use structure tags, and most of your problems will go
away.
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[Tim Rentsch]

Re: Typedef structs

Chris Torek <nospam@torek.n et> writes:
[color=blue]
> In any case, I have a method I recommend. Stop using typedef for
> structs.
>
> Just plain stop using it. You do not NEED it, and what it does is
> rarely what most beginning or even intermediate-level C programmers
> expect. Use structure tags, and most of your problems will go
> away.[/color]

Usually I find myself in agreement with Chris Torek's advice;
in this case however I think the advice given is somewhat
shortsighted.

Right from the very beginning, C programmers should be taught
to follow the pattern


typedef struct some_type_name_ struct_tag some_type_name;


struct some_type_name_ struct_tag {

/* ... definition goes here ... */

};


and elsewhere always use 'some_type_name ' to refer to the structure
type.

I don't mean to suggest that they follow the pattern blindly; there
should be explanation of the two pieces. But the amount of
explanation needed isn't that much, and the development practice is a
good one to be in the habit of following. Surely at some point there
will also be a need to explain about 'struct' tags and their separate
namespace, and so forth, but the pattern above is helpful much more
often than it's harmful - it's better to learn it earlier rather than
later.

[pete]

Re: Typedef structs

Tim Rentsch wrote:[color=blue]
>
> Chris Torek <nospam@torek.n et> writes:
>[color=green]
> > In any case, I have a method I recommend. Stop using typedef for
> > structs.
> >
> > Just plain stop using it. You do not NEED it, and what it does is
> > rarely what most beginning or even intermediate-level C programmers
> > expect. Use structure tags, and most of your problems will go
> > away.[/color]
>
> Usually I find myself in agreement with Chris Torek's advice;
> in this case however I think the advice given is somewhat
> shortsighted.
>
> Right from the very beginning, C programmers should be taught
> to follow the pattern
>
> typedef struct some_type_name_ struct_tag some_type_name;
>
> struct some_type_name_ struct_tag {
>
> /* ... definition goes here ... */
>
> };
>
> and elsewhere always use 'some_type_name ' to refer to the structure
> type.
>
> I don't mean to suggest that they follow the pattern blindly; there
> should be explanation of the two pieces. But the amount of
> explanation needed isn't that much, and the development practice is a
> good one to be in the habit of following. Surely at some point there
> will also be a need to explain about 'struct' tags and their separate
> namespace, and so forth, but the pattern above is helpful much more
> often than it's harmful - it's better to learn it earlier rather than
> later.[/color]

Your explanation is missing the part about why it's good
to typedef a struct. I don't see any point in doing it.
Why is
some_type_name object;
superior to
struct some_type_name_ struct_tag object;
?

--
pete

[Chris Torek]

Re: Typedef structs

>> Chris Torek <nospam@torek.n et> writes:[color=blue][color=green][color=darkred]
>>> In any case, I have a method I recommend. Stop using typedef for
>>> structs. ...[/color][/color][/color]
[color=blue]
>Tim Rentsch wrote:[color=green]
>> Usually I find myself in agreement with Chris Torek's advice;
>> in this case however I think the advice given is somewhat
>> shortsighted.
>>
>> Right from the very beginning, C programmers should be taught
>> to follow the pattern
>>
>> typedef struct some_type_name_ struct_tag some_type_name;
>>
>> struct some_type_name_ struct_tag {
>>
>> /* ... definition goes here ... */
>>
>> };
>>
>> and elsewhere always use 'some_type_name ' to refer to the structure
>> type.[/color][/color]

This is what I recommend as an alternative *if* one is stubbornly
insistent on using typedefs.
[color=blue][color=green]
>> I don't mean to suggest that they follow the pattern blindly; there
>> should be explanation of the two pieces.[/color][/color]

And indeed, in the past, I have done that (feel free to DejaGoogle
search for such articles). Sometimes I have less time to post,
though. :-)

In article <news:4132B169. 7282@mindspring .com>
pete <pfiland@mindsp ring.com> wrote:[color=blue]
>Your explanation is missing the part about why it's good
>to typedef a struct. I don't see any point in doing it.
>Why is
> some_type_name object;
>superior to
> struct some_type_name_ struct_tag object;
>?[/color]

I do not believe it is, myself, but I acknowledge that this is a
matter of opinion rather than fact.
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[Tim Rentsch]

Re: Typedef structs

pete <pfiland@mindsp ring.com> writes:
[color=blue]
> Tim Rentsch wrote:[/color]
[color=blue][color=green]
> > Right from the very beginning, C programmers should be taught
> > to follow the pattern
> >
> > typedef struct some_type_name_ struct_tag some_type_name;
> >
> > struct some_type_name_ struct_tag {
> >
> > /* ... definition goes here ... */
> >
> > };
> >
> > and elsewhere always use 'some_type_name ' to refer to the structure
> > type.[/color]
>
> Your explanation is missing the part about why it's good
> to typedef a struct. I don't see any point in doing it.
> Why is
> some_type_name object;
> superior to
> struct some_type_name_ struct_tag object;
> ?[/color]

Ahh, this is a good question. I would offer several reasons:

1. (minor) Slight economy of expression - shorter to omit 'struct'.

2. Using a typedef'ed name gives greater flexibility in the choice
of type - if later we want some_type_name to be a 'union' or 'int'
or 'enum', less of the program will need to change as a result.

3. Greater safety - if the type is defined in a header file (as it
very likely may be), and the header file is not #include'ed, using

some_type_name *x;

will produce an error, whereas

struct some_type_name_ struct_tag *x;

need no produce any diagnostic at all. Sometimes the second case is
perfectly easy to figure out, but the first case is *always* easy to
figure out.

4. Natural extension to opaque types - a commonly useful pattern is to
pass pointers where the pointer type is public but the type being
pointed at is private; if someone is used to separate definitions for
the type name and the 'struct' members, that leap will seem more
natural than if they aren't - just move the typedef to a public header
file and keep the struct definition local. In a sense this point is
similar to point (2), but there is also something else - for didactic
reasons we would like to be able introduce opaque types early in the
educational cycle, and having separately defined type names makes it
easier both to make use of opaque (pointer) types and to explain some
mechanics about how to express opaque types in C.

In connection with point 4, for a type that is opaque and will always
be accessed via pointers, there is nothing wrong with defining a
pointer type rather than an instance type as the defined type name.
(Of course it's important to get the conventions right so which is
which is always clear.)


Forgive me for not listing these earlier - I'd sort of assumed without
thinking about it that most long time C developers would be aware of
all of these already.

[Tim Rentsch]

Re: Typedef structs

Chris Torek <nospam@torek.n et> writes:
[color=blue]
> In article <news:4132B169. 7282@mindspring .com>
> pete <pfiland@mindsp ring.com> wrote:[color=green]
> >Your explanation is missing the part about why it's good
> >to typedef a struct. I don't see any point in doing it.
> >Why is
> > some_type_name object;
> >superior to
> > struct some_type_name_ struct_tag object;
> >?[/color]
>
> I do not believe it is, myself, but I acknowledge that this is a
> matter of opinion rather than fact.[/color]

To be fair here, I should say that in addition to some advantages that
the first form has (which I posted separately), there also are some
advantages to the second form. I think everyone would agree that
there are some advantages to each approach; where opinions might
differ is on the relative merits of the two sets of advantages. My
other posting didn't try to address either the advantages of the
second form or the question of the relative merits of the two
approaches.