Initializing compound type containing opaque type

[Nate Eldredge]

Consider the following pseudo-code:

#include <opaque.h>

struct foo {
int a;
opaque_t op;
int b;
};

struct foo blah = { 17, /* ??? */ , 23 };

Here we suppose that `opaque_t' is defined in <opaque.has some type
not known to us, or which we cannot rely on. In particular, it may be
a scalar or compound type. The goal is to initialize `blah' in such a
way that `blah.a == 17' and `blah.b == 23'. We don't care about the
value of `blah.op'.

It seems to me that there is no way to do this in standard C. Is this
correct, or am I missing something?

A workaround would be to rearrange the members of `struct foo' as

struct foo2 {
int a;
int b;
opaque_t op;
};

struct foo2 blah2 = { 17, 23 };

I believe this is legal, but I could be wrong. My compiler warns
about a missing initializer, but accepts the code.

If `opaque_t' is known to be a compound type, we could write

struct foo blah3 = { 17, { }, 23 };

which again provokes a missing initializer warning but is accepted.
Again I am curious whether it is actually legal.

I thought of this issue after reading a post here a couple weeks ago,
"Exception handling crashes or exits" by Fabiano Sidler,
<48c4e24d$1_4@n ews.bluewin.ch> , in which the poster wants a struct
with a jmp_buf as one member. He initializes that member with { 0 },
which is certainly non-portable. But I wondered if there is a
portable way to do the same thing.

If not, it seems like it might be useful for the authors of <opaque.h>
to provide a macro which is suitable as an initializer for opaque_t.
E.g. <opaque.hmigh t contain

typedef opaque_t struct {
int x,
double d;
};

#define OPAQUE_INITIALI ZER { 0, 0.0 }

so that one could write

struct foo blah = { 17, OPAQUE_INITIALI ZER, 23 };

In particular, this could be a useful extension for library authors to
provide for opaque library types (e.g. jmp_buf, FILE, etc).

Any thoughts?

[vippstar@gmail.com]

Re: Initializing compound type containing opaque type

On Oct 2, 12:05 am, Nate Eldredge <n...@vulcan.la nwrote:

Consider the following pseudo-code:
>
#include <opaque.h>
>
struct foo {
int a;
opaque_t op;
int b;
>
};
>
struct foo blah = { 17, /* ??? */ , 23 };
>
Here we suppose that `opaque_t' is defined in <opaque.has some type
not known to us, or which we cannot rely on. In particular, it may be
a scalar or compound type. The goal is to initialize `blah' in such a
way that `blah.a == 17' and `blah.b == 23'. We don't care about the
value of `blah.op'.
>
It seems to me that there is no way to do this in standard C. Is this
correct, or am I missing something?
>
A workaround would be to rearrange the members of `struct foo' as
>
struct foo2 {
int a;
int b;
opaque_t op;
>
};
>
struct foo2 blah2 = { 17, 23 };
>
I believe this is legal, but I could be wrong. My compiler warns
about a missing initializer, but accepts the code.

Yes, it's legal.
(note: In POSIX it wouldn't be; *_t are reserved identifiers)

If `opaque_t' is known to be a compound type, we could write
>
struct foo blah3 = { 17, { }, 23 };
>
which again provokes a missing initializer warning but is accepted.
Again I am curious whether it is actually legal.

Also legal.

I thought of this issue after reading a post here a couple weeks ago,
"Exception handling crashes or exits" by Fabiano Sidler,
<48c4e24d$...@n ews.bluewin.ch> , in which the poster wants a struct
with a jmp_buf as one member. He initializes that member with { 0 },
which is certainly non-portable. But I wondered if there is a
portable way to do the same thing.

Yes, jmp_buf foo = {0};

Though that'd initialize all members to 0, 0.0 or NULL depending on
type, recursively applied for any aggregate

If not, it seems like it might be useful for the authors of <opaque.h>
to provide a macro which is suitable as an initializer for opaque_t.

Well, it is legal, so it's not useful.

E.g. <opaque.hmigh t contain

<snip>

[Eric Sosman]

Re: Initializing compound type containing opaque type

Nate Eldredge wrote:

Consider the following pseudo-code:
>
#include <opaque.h>
>
struct foo {
int a;
opaque_t op;
int b;
};
>
struct foo blah = { 17, /* ??? */ , 23 };
>
Here we suppose that `opaque_t' is defined in <opaque.has some type
not known to us, or which we cannot rely on. In particular, it may be
a scalar or compound type. The goal is to initialize `blah' in such a
way that `blah.a == 17' and `blah.b == 23'. We don't care about the
value of `blah.op'.

In C99 you could use a "compound literal:"

struct foo blah = (struct foo){.a = 17, .b = 23};

(Double-check my syntax; I'm typing this hurriedly.)

In C90 you could initialize the unknown member to "zero of
the appropriate type:"

struct foo blah = { 17, { 0 }, 23 };

Some compilers may emit warnings for too few initializers, but
the initialization is valid; if opaque_t is compound, its sub-
elements are initialized to appropriate zeroes. The warnings may
be a nuisance, but they do no actual harm.

--
Eric.Sosman@sun .com

[Barry Schwarz]

Re: Initializing compound type containing opaque type

On Wed, 01 Oct 2008 14:05:53 -0700, Nate Eldredge <nate@vulcan.la n>
wrote:

>Consider the following pseudo-code:
>
>#include <opaque.h>
>
>struct foo {
int a;
opaque_t op;
int b;
>};
>
>struct foo blah = { 17, /* ??? */ , 23 };
>
>Here we suppose that `opaque_t' is defined in <opaque.has some type
>not known to us, or which we cannot rely on. In particular, it may be
>a scalar or compound type. The goal is to initialize `blah' in such a
>way that `blah.a == 17' and `blah.b == 23'. We don't care about the
>value of `blah.op'.
>
>It seems to me that there is no way to do this in standard C. Is this
>correct, or am I missing something?
>
>A workaround would be to rearrange the members of `struct foo' as
>
>struct foo2 {
int a;
int b;
opaque_t op;
>};
>
>struct foo2 blah2 = { 17, 23 };
>
>I believe this is legal, but I could be wrong. My compiler warns
>about a missing initializer, but accepts the code.

An insufficient quantity of initializers is specifically allowed
(6.7.8-19 and -21). Compilers are allowed to issue diagnostic
messages for correct code. Some do so when assigning an int to a char
or when using gets. These are not so much warnings (a term not
defined in the standard) but more like reminders to the programmer
that something is "unusual" and worth a second look. As long as the
code complies with the standard, the compiler is obligated to accept
it.

>
>If `opaque_t' is known to be a compound type, we could write
>
>struct foo blah3 = { 17, { }, 23 };
>
>which again provokes a missing initializer warning but is accepted.
>Again I am curious whether it is actually legal.
>
>I thought of this issue after reading a post here a couple weeks ago,
>"Exception handling crashes or exits" by Fabiano Sidler,
><48c4e24d$1_4@ news.bluewin.ch >, in which the poster wants a struct
>with a jmp_buf as one member. He initializes that member with { 0 },
>which is certainly non-portable. But I wondered if there is a

It is portable, for both aggregate and scalar objects. See 6.7.8-16,
-11, and -19.

>portable way to do the same thing.
>
>If not, it seems like it might be useful for the authors of <opaque.h>
>to provide a macro which is suitable as an initializer for opaque_t.
>E.g. <opaque.hmigh t contain
>
>typedef opaque_t struct {
int x,
double d;
>};
>
>#define OPAQUE_INITIALI ZER { 0, 0.0 }
>
>so that one could write
>
>struct foo blah = { 17, OPAQUE_INITIALI ZER, 23 };
>
>In particular, this could be a useful extension for library authors to
>provide for opaque library types (e.g. jmp_buf, FILE, etc).

{0} should suffice. If the object requires non-zero initialization,
the library should provide an initialization function.

--
Remove del for email

[Nate Eldredge]

Re: Initializing compound type containing opaque type

Eric Sosman <Eric.Sosman@su n.comwrites:

Nate Eldredge wrote:

>Consider the following pseudo-code:
>>
>#include <opaque.h>
>>
>struct foo {
> int a;
> opaque_t op;
> int b;
>};
>>
>struct foo blah = { 17, /* ??? */ , 23 };
>>
>Here we suppose that `opaque_t' is defined in <opaque.has some type
>not known to us, or which we cannot rely on. In particular, it may be
>a scalar or compound type. The goal is to initialize `blah' in such a
>way that `blah.a == 17' and `blah.b == 23'. We don't care about the
>value of `blah.op'.

>
In C99 you could use a "compound literal:"
>
struct foo blah = (struct foo){.a = 17, .b = 23};
>
(Double-check my syntax; I'm typing this hurriedly.)

My compiler says "error: initializer element is not constant". It
does work if it is moved inside a function, so that blah is auto, but
that's of course entirely different. (Anyway, in that case we could
just initialize blah.a and blah.b by hand.)

In C90 you could initialize the unknown member to "zero of
the appropriate type:"
>
struct foo blah = { 17, { 0 }, 23 };
>
Some compilers may emit warnings for too few initializers, but
the initialization is valid; if opaque_t is compound, its sub-
elements are initialized to appropriate zeroes. The warnings may
be a nuisance, but they do no actual harm.

Interesting, so the OP was correct after all. I never knew about
"zero of the appropriate type". I assumed that would be wrong if
opaque_t turned out to be a scalar type, or a compound whose first
member was also compound, but apparently not. Those clever language
designers, always one step ahead. :)

Thanks to all who replied.

[vippstar@gmail.com]

Re: Initializing compound type containing opaque type

On Oct 2, 12:05 am, Nate Eldredge <n...@vulcan.la nwrote:

Consider the following pseudo-code:
>
#include <opaque.h>
>
struct foo {
int a;
opaque_t op;
int b;
>
};
>
struct foo blah = { 17, /* ??? */ , 23 };
>
Here we suppose that `opaque_t' is defined in <opaque.has some type
not known to us, or which we cannot rely on. In particular, it may be
a scalar or compound type. The goal is to initialize `blah' in such a
way that `blah.a == 17' and `blah.b == 23'. We don't care about the
value of `blah.op'.
>
It seems to me that there is no way to do this in standard C. Is this
correct, or am I missing something?

Whoops, I forgot to reply to this question at my post.
In C99 you can.

struct foo blah = { .a = 17, .b = 23 };

Mr Sossman mentioned compound literals, but I'm unsure why he didn't
mention this which is even simpler.

You can also do this for array elements

char foo[2] = { .[1] = 'a' };

[vippstar@gmail.com]

Re: Initializing compound type containing opaque type

On Oct 2, 1:28 am, Eric Sosman <Eric.Sos...@su n.comwrote:
....

In C99 you could use a "compound literal:"
>
struct foo blah = (struct foo){.a = 17, .b = 23};
>
(Double-check my syntax; I'm typing this hurriedly.)

I think you mean

struct foo blah = *(struct foo[]){ { .a = 17, .b = 23 } };

[Ben Bacarisse]

Re: Initializing compound type containing opaque type

Nate Eldredge <nate@vulcan.la nwrites:

Eric Sosman <Eric.Sosman@su n.comwrites:
>

>Nate Eldredge wrote:

>>Consider the following pseudo-code:
>>>
>>#include <opaque.h>
>>>
>>struct foo {
>> int a;
>> opaque_t op;
>> int b;
>>};
>>>
>>struct foo blah = { 17, /* ??? */ , 23 };
>>>
>>Here we suppose that `opaque_t' is defined in <opaque.has some type
>>not known to us, or which we cannot rely on. In particular, it may be
>>a scalar or compound type. The goal is to initialize `blah' in such a
>>way that `blah.a == 17' and `blah.b == 23'. We don't care about the
>>value of `blah.op'.

>>
> In C99 you could use a "compound literal:"
>>
> struct foo blah = (struct foo){.a = 17, .b = 23};
>>
>(Double-check my syntax; I'm typing this hurriedly.)

Not allowed, I think, at file scope.

My compiler says "error: initializer element is not constant".

Just use the "initialise r list" rather than providing a compound
literal. I.e. write:

struct foo blah = {.a = 17, .b = 23};

This works for file-scope declarations as well as block-scope ones.

It
does work if it is moved inside a function, so that blah is auto, but
that's of course entirely different.

Yes. 6.7.8 paragraph 13 permits an expression of "compatible
structure or union type" when the object has automatic storage. You
have to read though to paragraph 16 to get to the text:

"Otherwise, the initializer for an object that has aggregate or
union type shall be a brace-enclosed list of initializers... "

to see that a compound literal is not allowed as the sole "top-level"
initialiser for an object with static storage duration. At least that
is my explanation of your error message.

--
Ben.

[Ben Bacarisse]

Re: Initializing compound type containing opaque type

vippstar@gmail. com writes:
<snip>

You can also do this for array elements
>
char foo[2] = { .[1] = 'a' };

There's no dot. I.e.:

char foo[2] = { [1] = 'a' };

--
Ben.

[Eric Sosman]

Re: Initializing compound type containing opaque type

vippstar@gmail. com wrote:

[...]
Whoops, I forgot to reply to this question at my post.
In C99 you can.
>
struct foo blah = { .a = 17, .b = 23 };
>
Mr Sossman mentioned compound literals, but I'm unsure why he didn't
mention this which is even simpler.

Because I was (as I said) in a hurry, typing faster than
I was thinking. An insalubrious habit ...

--
Eric Sosman
esosman@ieee-dot-org.invalid

[William Pursell]

Re: Initializing compound type containing opaque type

On 1 Oct, 22:05, Nate Eldredge <n...@vulcan.la nwrote:

Consider the following pseudo-code:
>
#include <opaque.h>
>
struct foo {
       int a;
       opaque_t op;
       int b;
>
};
>

If opaque_t is truly opaque, and not specified
in opaque.h, then the compiler won't accept
this definition of struct foo. If the compiler
is accepting this, then the header is specifying
opaque_t completely, so (I would argue) has
misnamed it because it isn't opaque.

struct foo blah = { 17, /* ??? */ , 23 };
>
Here we suppose that `opaque_t' is defined in <opaque.has some type
not known to us, or which we cannot rely on.

I don't understand what you mean by "not known to us". If
the compiler can get the definition from opaque_t, then
you can, too! Am I using the word "opaque" in a
stricter sense than you? When I say opaque type,
I mean that the header contains no more than:
typedef struct opaque * opaque_t;
or just:
struct opaque;

<snip>

If not, it seems like it might be useful for the authors of <opaque.h>
to provide a macro which is suitable as an initializer for opaque_t.
E.g. <opaque.hmigh t contain
>
typedef opaque_t struct {
        int x,
        double d;
>
};

Is this an opaque type? It's got an int and
a double, so ... no, it's not. At least not
by my understanding of the word opaque.

[William Pursell]

Re: Initializing compound type containing opaque type

On 2 Oct, 00:45, Ben Bacarisse <ben.use...@bsb .me.ukwrote:

vipps...@gmail. com writes:
>
<snip>
>

You can also do this for array elements

>

char foo[2] = { .[1] = 'a' };

>
There's no dot.  I.e.:
>
  char foo[2] = { [1] = 'a' };

Is that C99? I thought that was a gnu extension.

[Nate Eldredge]

Re: Initializing compound type containing opaque type

William Pursell <bill.pursell@g mail.comwrites:

On 1 Oct, 22:05, Nate Eldredge <n...@vulcan.la nwrote:

>Consider the following pseudo-code:
>>
>#include <opaque.h>
>>
>struct foo {
>       int a;
>       opaque_t op;
>       int b;
>>
>};
>>

>
If opaque_t is truly opaque, and not specified
in opaque.h, then the compiler won't accept
this definition of struct foo. If the compiler
is accepting this, then the header is specifying
opaque_t completely, so (I would argue) has
misnamed it because it isn't opaque.

By "opaque" I mean an object whose format and contents are not
intended to be used by the programmer, and are subject to change.
This is how I've heard it used in the past.

jmp_buf is an example. Certainly you can open up <setjmp.hand find
out how it is defined on your system; maybe it's a struct with a
member for each of your CPU's registers. But you can't use any of
that information in a portable program, since on another system, or a
later version of the same system, it may be defined differently, or
used differently by the library, so in a more general sense you don't
really "know" how it's defined. All you "know" about jmp_buf is that
you can create one using setjmp() and pass it to longjmp() later on.
From the programmer's point of view it's just a magic cookie.

Other examples in the standard library would be time_t and FILE.
div_t is a library type that I would not call opaque; it is documented
as being a struct with two members, `quot' and `rem', whose types and
meanings are well described and guaranteed to be the same in all
conforming implementations .

[Ben Bacarisse]

Re: Initializing compound type containing opaque type

William Pursell <bill.pursell@g mail.comwrites:

On 2 Oct, 00:45, Ben Bacarisse <ben.use...@bsb .me.ukwrote:

>vipps...@gmail .com writes:
>>
><snip>
>>

You can also do this for array elements

>>

char foo[2] = { .[1] = 'a' };

>>
>There's no dot.  I.e.:
>>
>  char foo[2] = { [1] = 'a' };

>
Is that C99? I thought that was a gnu extension.

Yes, it is C99:

designation:
designator-list =

designator-list:
designator
designator-list designator

designator:
[ constant-expression ]
. identifier

--
Ben.

[James Kuyper]

Re: Initializing compound type containing opaque type

William Pursell wrote:

On 2 Oct, 00:45, Ben Bacarisse <ben.use...@bsb .me.ukwrote:

....

> char foo[2] = { [1] = 'a' };

>
Is that C99? ...

Yes.