C Question

C Question

[Randi Botse]

Hi All,

Im looking at lscpu.c shipped by util-linux-ng-2.17, can you explain
me how the *vir_types[] declared, is this valid C declaration?

/* virtualization types */
enum {
	VIRT_NONE	= 0,
	VIRT_PARA,
	VIRT_FULL
};
const char *virt_types[] = {
	[VIRT_NONE]	= N_("none"),
	[VIRT_PARA]	= N_("para"),
	[VIRT_FULL]	= N_("full")
};

Re: C Question

[Xiaotian Feng]

On Tue, May 25, 2010 at 6:08 PM, Randi Botse <nightdecoder@gmail.com> wrote:
> Hi All,
>
> Im looking at lscpu.c shipped by util-linux-ng-2.17, can you explain
> me how the *vir_types[] declared, is this valid C declaration?
>

N_(blah) must be defined in some include files

> /* virtualization types */
> enum {
>        VIRT_NONE       = 0,
>        VIRT_PARA,
>        VIRT_FULL
> };
> const char *virt_types[] = {
>        [VIRT_NONE]     = N_("none"),
>        [VIRT_PARA]     = N_("para"),
>        [VIRT_FULL]     = N_("full")
> };
> --
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Re: C Question

[Randi Botse]

Yes, that's i was talking about, i got the idea, gcc even let me
declared "unordered" subobject index like this..

const char *my_strings[] = {
            [0] = "string1",
            [2] = "string2",
            [1] = "string3",
            etc...
}

Actually, this is my first time seeing codes like this, it was strange
for me :p, many thanks for let me know!

- Randi,


On Tue, May 25, 2010 at 5:24 PM, Guillaume Leconte
<guillaume.leconte@gmail.com> wrote:

> I think he is talking about the [VIRT_NONE] = ... style, which is C99.
>
>>
>> > /* virtualization types */
>> > enum {
>> >        VIRT_NONE       = 0,
>> >        VIRT_PARA,
>> >        VIRT_FULL
>> > };
>> > const char *virt_types[] = {
>> >        [VIRT_NONE]     = N_("none"),
>> >        [VIRT_PARA]     = N_("para"),
>> >        [VIRT_FULL]     = N_("full")
>> > };
>
> It is equivalent to:
>
> const char *virt_types[3];
>
> virt_types[VIRT_NONE] = N_("none");
> virt_types[VIRT_PARA] = N_("para");
> virt_types[VIRT_FULL] = N_("full");
>
>
>>
>> > --
>> > To unsubscribe from this list: send the line "unsubscribe
>> > linux-c-programming" in
>> > the body of a message to majordomo@vger.kernel.org
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>> >
>> --
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>
>
>
> --
> « I've seen things you people wouldn't believe.  Attack ships on fire
> off the shoulder of Orion.  I watched C-beams glitter in the dark near
> the Tanhauser gate.  All those moments will be lost in time like tears
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>
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Re: C Question

[Uriel Corfa]

That's "designated initializers". It's C99.

It allows you to initialize some members to a specific value without
specifying the default value manually for the other ones.

Refs :
http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf  chapter 6.7.8.6
http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/index.jsp?topic=/com.ibm.xlcpp8a.doc/language/ref/designators.htm

On Tue, May 25, 2010 at 12:08 PM, Randi Botse <nightdecoder@gmail.com> wrote:
> Hi All,
>
> Im looking at lscpu.c shipped by util-linux-ng-2.17, can you explain
> me how the *vir_types[] declared, is this valid C declaration?
>
> /* virtualization types */
> enum {
>        VIRT_NONE       = 0,
>        VIRT_PARA,
>        VIRT_FULL
> };
> const char *virt_types[] = {
>        [VIRT_NONE]     = N_("none"),
>        [VIRT_PARA]     = N_("para"),
>        [VIRT_FULL]     = N_("full")
> };
> --
> To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>



-- 
Uriel Corfa
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Re: C Question

[Michal Nazarewicz]

Randi Botse <nightdecoder@gmail.com> writes:

> Hi All,
>
> Im looking at lscpu.c shipped by util-linux-ng-2.17, can you explain
> me how the *vir_types[] declared, is this valid C declaration?
>
> /* virtualization types */
> enum {
> 	VIRT_NONE	= 0,
> 	VIRT_PARA,
> 	VIRT_FULL
> };
> const char *virt_types[] = {
> 	[VIRT_NONE]	= N_("none"),
> 	[VIRT_PARA]	= N_("para"),
> 	[VIRT_FULL]	= N_("full")
> };

As Xiaotian Feng said, N_(x) must be defined somewhere and additionally
to this this definition uses a new syntax where you can specify indexes
of elements you want to set value to, ie. the above is the same as:

const char *virt_types[] = {
	[0]	= N_("none"),
	[1]	= N_("para"),
	[2]	= N_("full")
};

which in turn is the same as:

const char *virt_types[] = {
	N_("none"),
	N_("para"),
	N_("full")
};

The advantage of the new syntax is that you can specify values in any
order and omit some.

-- 
Best regards,                                         _     _
 .o. | Liege of Serenly Enlightened Majesty of      o' \,=./ `o
 ..o | Computer Science,  Michal "mina86" Nazarewicz   (o o)
 ooo +--<mina86-tlen.pl>--<jid:mina86-jabber.org>--ooO--(_)--Ooo--

Re: C Question

[Randi Botse]

Yes, the N_(x) macro used for localization that i discard in my question.

Sorry but, i just having another question, consider this code:

const char *my_strings[] = {
    [0] = "index0",
    [4] = "index4",
    [6] = "index6"
};

How many pointer to char that my_string array holds now? is that 3?
where are index [1], [2], [3], and [5] ?

2010/5/25 Michal Nazarewicz <mina86@tlen.pl>:
>
> As Xiaotian Feng said, N_(x) must be defined somewhere and additionally
> to this this definition uses a new syntax where you can specify indexes
> of elements you want to set value to, ie. the above is the same as:
>
> const char *virt_types[] = {
>        [0]     = N_("none"),
>        [1]     = N_("para"),
>        [2]     = N_("full")
> };
>
> which in turn is the same as:
>
> const char *virt_types[] = {
>        N_("none"),
>        N_("para"),
>        N_("full")
> };
>
> The advantage of the new syntax is that you can specify values in any
> order and omit some.
>
> --
> Best regards,                                         _     _
>  .o. | Liege of Serenly Enlightened Majesty of      o' \,=./ `o
>  ..o | Computer Science,  Michal "mina86" Nazarewicz   (o o)
>  ooo +--<mina86-tlen.pl>--<jid:mina86-jabber.org>--ooO--(_)--Ooo--
>
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Re: C Question

[Michal Nazarewicz]

Randi Botse <nightdecoder@gmail.com> writes:
> Yes, the N_(x) macro used for localization that i discard in my question.
>
> Sorry but, i just having another question, consider this code:
>
> const char *my_strings[] = {
>     [0] = "index0",
>     [4] = "index4",
>     [6] = "index6"
> };

NULLs will be written at indexes 1, 2, 3 and 5.  There will be total of
7 pointers (the last one will point to "index6").

-- 
Best regards,                                         _     _
 .o. | Liege of Serenly Enlightened Majesty of      o' \,=./ `o
 ..o | Computer Science,  Michal "mina86" Nazarewicz   (o o)
 ooo +--<mina86-tlen.pl>--<jid:mina86-jabber.org>--ooO--(_)--Ooo--

Re: C Question

[Randi Botse]

That's clear now, i was though the 'unindexed' strings (in this case
index 1, 2, 3, and 5) remains unintialized (not initialized to NULL).

Thanks very much.

Randi,

2010/5/30 Michal Nazarewicz <mina86@tlen.pl>:
> Randi Botse <nightdecoder@gmail.com> writes:
>> Yes, the N_(x) macro used for localization that i discard in my question.
>>
>> Sorry but, i just having another question, consider this code:
>>
>> const char *my_strings[] = {
>>     [0] = "index0",
>>     [4] = "index4",
>>     [6] = "index6"
>> };
>
> NULLs will be written at indexes 1, 2, 3 and 5.  There will be total of
> 7 pointers (the last one will point to "index6").
>
> --
> Best regards,                                         _     _
>  .o. | Liege of Serenly Enlightened Majesty of      o' \,=./ `o
>  ..o | Computer Science,  Michal "mina86" Nazarewicz   (o o)
>  ooo +--<mina86-tlen.pl>--<jid:mina86-jabber.org>--ooO--(_)--Ooo--
>
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Re: C Question

[Glynn Clements]

Randi Botse wrote:

> >> Sorry but, i just having another question, consider this code:
> >>
> >> const char *my_strings[] = {
> >>     [0] = "index0",
> >>     [4] = "index4",
> >>     [6] = "index6"
> >> };
> >
> > NULLs will be written at indexes 1, 2, 3 and 5.  There will be total of
> > 7 pointers (the last one will point to "index6").
> 
> That's clear now, i was though the 'unindexed' strings (in this case
> index 1, 2, 3, and 5) remains unintialized (not initialized to NULL).

If you specify an initialiser, the entire object is initialised. If
you specify an incomplete initialiser, the unspecified elements are
initialised to the appropriate zero value (0, 0.0, '\0', NULL, etc).

The C99 standard says (6.7.8p21):

       [#21]  If  there  are fewer initializers in a brace-enclosed
       list than there are elements or members of an aggregate,  or
       fewer  characters  in a string literal used to initialize an
       array of known size than there are elements  in  the  array,
       the   remainder   of  the  aggregate  shall  be  initialized
       implicitly the same as  objects  that  have  static  storage
       duration.

This applies to both traditional C89 sequential initialisers and C99
designated initialisers. It allows you to initialise large arrays or
structures without having to specify every element; e.g.:

	int array[1000] = {0};
	struct foo f = {0};

will initialise the entire array or structure to zero.

-- 
Glynn Clements <glynn@gclements.plus.com>
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