how to reset all structure fields to 0 or NULL

**Eric Sosman** · Nov 14 '05, 11:40 PM

Re: how to reset all structure fields to 0 or NULL

Roman Mashak wrote:[color=blue]
> Hello, All!
>
> I have a structure:
>
> typedef struct cmd_line_s {
> char *cl_action;
> char *cl_args[10];
> unsigned char cl_args_num;
> void *cl_handler;
> } cmd_line_t;
>
> At initialization time I'm dooing cmd_line_t cli = { NULL, NULL, 0, NULL };
>
> After dealing with 'cli' I'd like to reset fields values to { NULL, NULL, 0,
> NULL } state again, what is the most correct and easy way?[/color]

One way is to copy from an already-initialized instance:

const cmd_line_t empty_line = { NULL, NULL, 0, NULL };
cmd_line_t cli = { NULL, NULL, 0, NULL };
...
/* do things with cli */
...
cli = empty_line;
/* cli is now re-initialized */

C99 provides another way to do this, but it's not all that much
more convenient than the above in this sort of usage.

The initializer you've shown isn't quite complete, and may
cause some compilers to emit warning messages. You might try
rewriting it as

{ NULL, { NULL }, 0, NULL }

or even as

{ NULL,
{ NULL, NULL, NULL, NULL, NULL,
NULL, NULL, NULL, NULL, NULL } ,
0,
NULL }

or perhaps just as

{ 0 }

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

**Walter Roberson** · Nov 14 '05, 11:40 PM

Re: how to reset all structure fields to 0 or NULL

In article <d8lenp$lb7$1@r elay.tomsk.ru>, Roman Mashak <mrv@tusur.ru > wrote:[color=blue]
>I have a structure:[/color]
[color=blue]
> unsigned char cl_args_num;
> void *cl_handler;[/color]
[color=blue]
>After dealing with 'cli' I'd like to reset fields values to { NULL, NULL, 0,
>NULL } state again, what is the most correct and easy way?[/color]

Keep a null copy around and do a structure assignment. After, that
is, having freed whatever dynamic memory is appropriate.

--
Entropy is the logarithm of probability -- Boltzmann

**Roman Mashak** · Nov 14 '05, 11:40 PM

Re: how to reset all structure fields to 0 or NULL

Hello, Eric!
You wrote on Mon, 13 Jun 2005 22:45:55 -0400:

[skip]
ES> The initializer you've shown isn't quite complete, and may
ES> cause some compilers to emit warning messages. You might try
ES> rewriting it as

ES> { NULL, { NULL }, 0, NULL }

ES> or even as

ES> { NULL,
ES> { NULL, NULL, NULL, NULL, NULL,
ES> NULL, NULL, NULL, NULL, NULL } ,
ES> 0,
ES> NULL }

Thanks a lot!

With best regards, Roman Mashak. E-mail: mrv@tusur.ru

**Jean-Claude Arbaut** · Nov 14 '05, 11:40 PM

Re : how to reset all structure fields to 0 or NULL

Le 14/06/2005 04:17, dans d8lenp$lb7$1@re lay.tomsk.ru, « Roman Mashak »
<mrv@tusur.ru > a écrit :
[color=blue]
> Hello, All!
>
> I have a structure:
>
> typedef struct cmd_line_s {
> char *cl_action;
> char *cl_args[10];
> unsigned char cl_args_num;
> void *cl_handler;
> } cmd_line_t;
>
> At initialization time I'm dooing cmd_line_t cli = { NULL, NULL, 0, NULL };
>
> After dealing with 'cli' I'd like to reset fields values to { NULL, NULL, 0,
> NULL } state again, what is the most correct and easy way?
>
> Thank you!
>
> With best regards, Roman Mashak. E-mail: mrv@tusur.ru
>
>[/color]

Most correct I don't know, but "memset" should work...

**pete** · Nov 14 '05, 11:40 PM

Re: Re : how to reset all structure fields to 0 or NULL

Jean-Claude Arbaut wrote:[color=blue]
>
> Le 14/06/2005 04:17, dans d8lenp$lb7$1@re lay.tomsk.ru, « Roman Mashak »
> <mrv@tusur.ru > a écrit :
>[color=green]
> > Hello, All!
> >
> > I have a structure:
> >
> > typedef struct cmd_line_s {
> > char *cl_action;
> > char *cl_args[10];
> > unsigned char cl_args_num;
> > void *cl_handler;
> > } cmd_line_t;
> >
> > At initialization time I'm dooing cmd_line_t
> > cli = { NULL, NULL, 0, NULL };
> >
> > After dealing with 'cli' I'd like to reset
> > fields values to { NULL, NULL, 0,
> > NULL } state again, what is the most correct and easy way?
> >
> > Thank you!
> >
> > With best regards, Roman Mashak. E-mail: mrv@tusur.ru
> >
> >[/color]
>
> Most correct I don't know, but "memset" should work...[/color]

memset(&pointer s, 0, sizeof pointers)
is the wrong way to set pointers to NULL.

--
pete

**Jean-Claude Arbaut** · Nov 14 '05, 11:40 PM

Re : Re : how to reset all structure fields to 0 or NULL

[color=blue]
> memset(&pointer s, 0, sizeof pointers)
> is the wrong way to set pointers to NULL.[/color]

Oh, excuse me, I thought NULL was null :-)

**Dik T. Winter** · Nov 14 '05, 11:41 PM

Re: Re : Re : how to reset all structure fields to 0 or NULL

In article <BED491DD.3131% jean-claude.arbaut@l aposte.net> Jean-Claude Arbaut <jean-claude.arbaut@l aposte.net> writes:[color=blue]
>[color=green]
> > memset(&pointer s, 0, sizeof pointers)
> > is the wrong way to set pointers to NULL.[/color]
>
> Oh, excuse me, I thought NULL was null :-)[/color]

The null pointer does not have necessarily all bits 0 (as a floating
point 0.0 has not necessarily all bits 0).
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/

**Jean-Claude Arbaut** · Nov 14 '05, 11:41 PM

Re : Re : Re : how to reset all structure fields to 0 or NULL

Le 14/06/2005 14:21, dans II2qB6.7tC@cwi. nl, « Dik T. Winter »
<Dik.Winter@cwi .nl> a écrit :
[color=blue]
> In article <BED491DD.3131% jean-claude.arbaut@l aposte.net> Jean-Claude Arbaut
> <jean-claude.arbaut@l aposte.net> writes:[color=green]
>>[color=darkred]
>>> memset(&pointer s, 0, sizeof pointers)
>>> is the wrong way to set pointers to NULL.[/color]
>>
>> Oh, excuse me, I thought NULL was null :-)[/color]
>
> The null pointer does not have necessarily all bits 0 (as a floating
> point 0.0 has not necessarily all bits 0).[/color]

Ok. I don't know all architectures, so assigning 0 may be non portable,
although on some OS (MacOSX for example), NULL is defigned as 0:

#define NULL __DARWIN_NULL
#define __DARWIN_NULL ((void *)0)

And the example of floating point 0.0 is irrelevant, +0.0 and -0.0
are two distinct values.

Just one question unrelated with NULLs, is it allowed to write "sizeof int",
or only "sizeof(int )" ? I wonder if GCC correctly follows the Standard...
I get:

test.c: In function 'main':
test.c:5: error: parse error before 'int'

**Dik T. Winter** · Nov 14 '05, 11:41 PM

Re: Re : Re : Re : how to reset all structure fields to 0 or NULL

In article <BED4A06D.31DD% jean-claude.arbaut@l aposte.net> Jean-Claude Arbaut <jean-claude.arbaut@l aposte.net> writes:[color=blue]
> Le 14/06/2005 14:21, dans II2qB6.7tC@cwi. nl, « Dik T. Winter »
> <Dik.Winter@cwi .nl> a écrit :[color=green]
> > In article <BED491DD.3131% jean-claude.arbaut@l aposte.net> Jean-Claude Arbaut
> > <jean-claude.arbaut@l aposte.net> writes:[/color][/color]
....[color=blue][color=green][color=darkred]
> >> Oh, excuse me, I thought NULL was null :-)[/color]
> >
> > The null pointer does not have necessarily all bits 0 (as a floating
> > point 0.0 has not necessarily all bits 0).[/color]
>
> Ok. I don't know all architectures, so assigning 0 may be non portable,
> although on some OS (MacOSX for example), NULL is defigned as 0:
>
> #define NULL __DARWIN_NULL
> #define __DARWIN_NULL ((void *)0)[/color]

Perfectly conforming to the standard. And if that is done on a machine
where the 0 pointer has not all bits 0, the compiler has to ensure that
the proper bit pattern is used for a null pointer, although the source
may contain the constant 0. 0 in a pointer context does *not* mean a
pointer with all bits 0, it means a null pointer.
[color=blue]
> And the example of floating point 0.0 is irrelevant, +0.0 and -0.0
> are two distinct values.[/color]

Not in C and not on all machines. But it is relevant. I know one
machine where all bits 0 is a non-normalized 0.0. So if a has that
value, and b is 3.5, a + b will be 3.0.
[color=blue]
> Just one question unrelated with NULLs, is it allowed to write "sizeof int",
> or only "sizeof(int )" ? I wonder if GCC correctly follows the Standard...
> I get:
>
> test.c: In function 'main':
> test.c:5: error: parse error before 'int'[/color]

Only sizeof(int), see the standard. The parenthesis are required when sizeof
is applied to a type.
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/

**Jean-Claude Arbaut** · Nov 14 '05, 11:41 PM

Re : Re : Re : Re : how to reset all structure fields to 0 or NULL

Le 14/06/2005 15:54, dans II2unJ.8pF@cwi. nl, « Dik T. Winter »
<Dik.Winter@cwi .nl> a écrit :
[color=blue]
> In article <BED4A06D.31DD% jean-claude.arbaut@l aposte.net> Jean-Claude Arbaut
> <jean-claude.arbaut@l aposte.net> writes:[color=green]
>> Le 14/06/2005 14:21, dans II2qB6.7tC@cwi. nl, « Dik T. Winter »
>> <Dik.Winter@cwi .nl> a écrit :[color=darkred]
>>> In article <BED491DD.3131% jean-claude.arbaut@l aposte.net> Jean-Claude Arbaut
>>> <jean-claude.arbaut@l aposte.net> writes:[/color][/color]
> ...
>[color=green][color=darkred]
>>>> Oh, excuse me, I thought NULL was null :-)
>>>
>>> The null pointer does not have necessarily all bits 0 (as a floating
>>> point 0.0 has not necessarily all bits 0).[/color]
>>
>> Ok. I don't know all architectures, so assigning 0 may be non portable,
>> although on some OS (MacOSX for example), NULL is defigned as 0:
>>
>> #define NULL __DARWIN_NULL
>> #define __DARWIN_NULL ((void *)0)[/color]
>
> Perfectly conforming to the standard. And if that is done on a machine
> where the 0 pointer has not all bits 0, the compiler has to ensure that
> the proper bit pattern is used for a null pointer, although the source
> may contain the constant 0. 0 in a pointer context does *not* mean a
> pointer with all bits 0, it means a null pointer.[/color]

You're right. But on MacOSX/PowerPC, the NULL pointer *is* 0. Not compiler
trick, you can check the assembly output. And the "segment zero" is
here to crash if the NULL pointer is dereferenced.
[color=blue][color=green]
>> And the example of floating point 0.0 is irrelevant, +0.0 and -0.0
>> are two distinct values.[/color]
>
> Not in C and not on all machines. But it is relevant. I know one
> machine where all bits 0 is a non-normalized 0.0. So if a has that
> value, and b is 3.5, a + b will be 3.0.[/color]

Sorry, I had IEEE 754 in mind. 0.0 and -0.0 are distinct (juste compute
1/x to check this), but 0.0 == -0.0. I'm pretty sure it's perfect IEEE
behaviour. But you're right: one must not use memset to initialize
a zero fp vector... funny.
[color=blue][color=green]
>> Just one question unrelated with NULLs, is it allowed to write "sizeof int",
>> or only "sizeof(int )" ? I wonder if GCC correctly follows the Standard...
>> I get:
>>
>> test.c: In function 'main':
>> test.c:5: error: parse error before 'int'[/color]
>
> Only sizeof(int), see the standard. The parenthesis are required when sizeof
> is applied to a type.[/color]

Thanks.

**Clark S. Cox III** · Nov 14 '05, 11:41 PM

Re: Re : Re : Re : how to reset all structure fields to 0 or NULL

On 2005-06-14 08:55:41 -0400, Jean-Claude Arbaut
<jean-claude.arbaut@l aposte.net> said:[color=blue]
>
> Le 14/06/2005 14:21, dans II2qB6.7tC@cwi. nl, « Dik T. Winter »
> <Dik.Winter@cwi .nl> a écrit :
>[color=green]
>> In article <BED491DD.3131% jean-claude.arbaut@l aposte.net> Jean-Claude Arbaut
>> <jean-claude.arbaut@l aposte.net> writes:[color=darkred]
>>>
>>>> memset(&pointer s, 0, sizeof pointers)
>>>> is the wrong way to set pointers to NULL.
>>>
>>> Oh, excuse me, I thought NULL was null :-)[/color]
>>
>> The null pointer does not have necessarily all bits 0 (as a floating
>> point 0.0 has not necessarily all bits 0).[/color]
>
> Ok. I don't know all architectures, so assigning 0 may be non portable,
> although on some OS (MacOSX for example),[/color]

Assigning zero to a pointer variable is *always* OK, and will always
result in a NULL pointer. But that doesn't mean that the actual, in
memory representation of that NULL pointer is "all-bits zero"
[color=blue]
> And the example of floating point 0.0 is irrelevant, +0.0 and -0.0
> are two distinct values.[/color]

Not always, and the example is quite relevant.
[color=blue]
>
> Just one question unrelated with NULLs, is it allowed to write "sizeof int",
> or only "sizeof(int )" ?[/color]

No, when applied to a type, only sizeof(type) is allowed.

--
Clark S. Cox, III
clarkcox3@gmail .com

**S.Tobias** · Nov 14 '05, 11:41 PM

Re: [OT] how to reset all structure fields to 0 or NULL

Jean-Claude Arbaut <jean-claude.arbaut@l aposte.net> wrote:
[snip all]

Could you please fix something in your reader, so that it doesn't
insert a space between "Re" and ":" in the subject? I think other
readers don't recognize "Re :" as "regarding" (or "reply"?) prefix
and add their own "Re:"; then probably your reader changes it to "Re :"
again, and the "game of life" restarts. In my reader in threads
menu I'll soon be seeing only "Re : Re : Re : Re : Re : Re : ...".

--
Stan Tobias
mailx `echo siXtY@FamOuS.Be dBuG.pAlS.INVALID | sed s/[[:upper:]]//g`

**Jean-Claude Arbaut** · Nov 14 '05, 11:41 PM

Re: [OT] how to reset all structure fields to 0 or NULL

Le 14/06/2005 17:11, dans 3h8adcFfh61qU1@ individual.net, « S.Tobias »
<siXtY@FamOuS.B edBuG.pAlS.INVA LID> a écrit :
[color=blue]
> Jean-Claude Arbaut <jean-claude.arbaut@l aposte.net> wrote:
> [snip all]
>
> Could you please fix something in your reader, so that it doesn't
> insert a space between "Re" and ":" in the subject? I think other
> readers don't recognize "Re :" as "regarding" (or "reply"?) prefix
> and add their own "Re:"; then probably your reader changes it to "Re :"
> again, and the "game of life" restarts. In my reader in threads
> menu I'll soon be seeing only "Re : Re : Re : Re : Re : Re : ...".[/color]

Sorry, I didn't see... The origin is Microsoft Entourage and its
interpretation of french language rule: it puts a space before a
colon, by default. I'll probably switch my news reader soon :-)

**CBFalconer** · Nov 14 '05, 11:41 PM

Re: Re : how to reset all structure fields to 0 or NULL

Jean-Claude Arbaut wrote:[color=blue]
> « Roman Mashak » <mrv@tusur.ru > a écrit :
>[color=green]
>> typedef struct cmd_line_s {
>> char *cl_action;
>> char *cl_args[10];
>> unsigned char cl_args_num;
>> void *cl_handler;
>> } cmd_line_t;
>>
>> At initialization time I'm dooing
>> cmd_line_t cli = { NULL, NULL, 0, NULL };
>>
>> After dealing with 'cli' I'd like to reset fields values to
>> { NULL, NULL, 0, NULL } state again, what is the most correct
>> and easy way?[/color]
>
> Most correct I don't know, but "memset" should work...[/color]

Definitely incorrect. Those may not be NULLs. Suggested means:

....
/* Very suitable use for a global */
const cmd_line_t clempty = {NULL, NULL, 0, NULL);
....
int main(...)
{
....
cmd_line_t cli = clempty;
....
do {
/* fool with cli */
cli = clempty; /* rezero it */
} while (something);
....
}

--
"If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson

how to reset all structure fields to 0 or NULL

how to reset all structure fields to 0 or NULL

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