SSCANF

[Superfox il Volpone]

Hello

I have some problem with sscanf, I tryed this code but it doesn't works
:

char* stringa = "18/2005"
char mese[3]; char anno[5];
int i_letture;

i_letture = sscanf(stringa, "%2s/%4s", &mese, &anno);
mese[2] = anno[4] = '\0';

The values ar random and I don't understand the motive
I tryed either this variant :

char* porka_vakka = strchr(stringa, '/');
*porka_vakka = '\0'; // but either ' ' e '\n'
i_letture = sscanf(stringa, "%s", &mese);

But neither this works...

Could anyone help me ?

Bye
- Atari

p.s. happy new year :)

[nelu]

Re: SSCANF


Superfox il Volpone wrote:[color=blue]
> Hello
>
> I have some problem with sscanf, I tryed this code but it doesn't works
> :
>
> char* stringa = "18/2005"
> char mese[3]; char anno[5];
> int i_letture;
>
> i_letture = sscanf(stringa, "%2s/%4s", &mese, &anno);
> mese[2] = anno[4] = '\0';[/color]

You have mese[3] and anno[5]. When you read with sscanf you tell it to
store the character arrays in &mese and &anno. mese=&mese[0] and
anno=&anno[0].
It should be: i_letture=sscan f(stringa,"%2s/%4s",mese,anno) , otherwise
you are trying to write those values at locations that hold the
addresses of mese and anno instead of writing them to the addresses
where mese and anno point.
If you really want to use & then write:
i_letture=sscan f(stringa,"%2s/%4s",&mese[0],&anno[0]);
[color=blue]
>
> The values ar random and I don't understand the motive
> I tryed either this variant :
>
> char* porka_vakka = strchr(stringa, '/');
> *porka_vakka = '\0'; // but either ' ' e '\n'
> i_letture = sscanf(stringa, "%s", &mese);[/color]
Same problem as above. Reading into the wrong location.

[Mike Wahler]

Re: SSCANF

"Superfox il Volpone" <atari@email.it > wrote in message
news:1136310085 .816491.322760@ g14g2000cwa.goo glegroups.com.. .[color=blue]
> Hello
>
> I have some problem with sscanf, I tryed this code but it doesn't works
> :
>
> char* stringa = "18/2005"[/color]

"18/2005" is a string literal. Any attempts to modify
any of its characters produces undefined behavior.
[color=blue]
> char mese[3]; char anno[5];
> int i_letture;
>
> i_letture = sscanf(stringa, "%2s/%4s", &mese, &anno);[/color]

This gives undefined behavior. You're trying to modify
a string literal.

Also, since 'mese' and 'anno' are arrays, their names
in this context will evalutate to pointers to their
first element. So e.g. use 'mese', not '&mese'.
%s must match with type 'char*'. The expression &mese
does not have that type. Its type is (*)[3] (pointer to
array of three char). Not the correct type.
[color=blue]
> mese[2] = anno[4] = '\0';[/color]

'sscanf()' already applies the string terminator for you
(in the proper location)

Also (disregarding for now the string literal problem)
note that if the size of data stored by 'sscanf()' is
less than the size of the array, your arbitrary placement of
'\0' as the last array element will not terminate the string
properly (there will be 'garbage' between the data and the
terminator).
[color=blue]
>
> The values ar random and I don't understand the motive[/color]

You don't understand how 'sscanf()' works, or how arrays
and pointers work.
[color=blue]
> I tryed either this variant :
>
> char* porka_vakka = strchr(stringa, '/');
> *porka_vakka = '\0'; // but either ' ' e '\n'[/color]

There are two possible results of these two lines, both of
which are undefined behavior:

1) The character '/' is not found in the string (which
causes 'strchr()' to return NULL), in which case you
try to dereference a NULL pointer. Undefined behavior.

2) The character '/' is found (and 'strchr()' returns its
address. You then try to modify it, but it's part of
a string literal. Undefined behavior.
[color=blue]
> i_letture = sscanf(stringa, "%s", &mese);[/color]

More undefined behavior. Attemt to modify string literal.
Wrong data type used with '%s'.

Finally, even if you do have writable storage for 'sscanf()'
note that you have no protection against the data overflowing
your array. Look up the 'width' flag for sscanf() format
specifiers.
[color=blue]
>
> But neither this works...
>
> Could anyone help me ?[/color]

I think the best advice I can give is to recommend you get
some good textbooks.

http://www.accu.org/bookreviews/public/reviews/0sb/beginner_s_c.htm

-Mike

[Michael Mair]

Re: SSCANF

Mike Wahler wrote:[color=blue]
> "Superfox il Volpone" <atari@email.it > wrote in message
> news:1136310085 .816491.322760@ g14g2000cwa.goo glegroups.com.. .
>[color=green]
>>Hello
>>
>>I have some problem with sscanf, I tryed this code but it doesn't works
>>:
>>
>>char* stringa = "18/2005"[/color]
>
> "18/2005" is a string literal. Any attempts to modify
> any of its characters produces undefined behavior.
>[color=green]
>>char mese[3]; char anno[5];
>>int i_letture;
>>
>>i_letture = sscanf(stringa, "%2s/%4s", &mese, &anno);[/color]
>
> This gives undefined behavior. You're trying to modify
> a string literal.[/color]

Look again.
The first parameter of sscanf() is of type const char *
(qualified by restrict in C99 IIRC).

I guess you saw sscanf() and thought sprintf()...

<snip>


Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

[Christopher Benson-Manica]

Re: SSCANF

Mike Wahler <mkwahler@mkwah ler.net> wrote:
[color=blue][color=green]
> > i_letture = sscanf(stringa, "%2s/%4s", &mese, &anno);[/color][/color]
[color=blue]
> This gives undefined behavior. You're trying to modify
> a string literal.[/color]

I don't know that OP is "trying" to modify the string literal. In
practice, is sscanf() really likely to modify its first argument?

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Chris McDonald]

Re: SSCANF

Christopher Benson-Manica <ataru@nospam.c yberspace.org> writes:
[color=blue]
>Mike Wahler <mkwahler@mkwah ler.net> wrote:[/color]
[color=blue][color=green][color=darkred]
>> > i_letture = sscanf(stringa, "%2s/%4s", &mese, &anno);[/color][/color][/color]
[color=blue][color=green]
>> This gives undefined behavior. You're trying to modify
>> a string literal.[/color][/color]
[color=blue]
>I don't know that OP is "trying" to modify the string literal. In
>practice, is sscanf() really likely to modify its first argument?[/color]


Doesn't the prototype of sscanf() promise to *not* modify its first argument?

--
Chris.

[Emmanuel Delahaye]

Re: SSCANF

Superfox il Volpone a écrit :[color=blue]
> I have some problem with sscanf, I tryed this code but it doesn't works
> :
>
> char* stringa = "18/2005"
> char mese[3]; char anno[5];[/color]

These are arrays of char
[color=blue]
> int i_letture;
>
> i_letture = sscanf(stringa, "%2s/%4s", &mese, &anno);[/color]

why & ? %s expects exactly what the name of an array of char is : the
address of its first element.

The separator must be read too :

#include <stdio.h>

int main(void)
{
char* stringa = "18/2005";
char mese[3];
char anno[5];
char c;
int i_letture = sscanf(stringa, "%2s%c%4s", mese, &c, anno);

if (i_letture == 3)
{
printf ("'%s' '%s'\n", mese, anno);
}
else
{
printf ("sscanf() error\n");
}
return 0;
}
[color=blue]
> mese[2] = anno[4] = '\0';[/color]

No need for that.

--
A+

Emmanuel Delahaye

[Mike Wahler]

Re: SSCANF


"Michael Mair" <Michael.Mair@i nvalid.invalid> wrote in message
news:4202kbF1gg ba5U1@individua l.net...[color=blue]
> Mike Wahler wrote:[color=green]
>> "Superfox il Volpone" <atari@email.it > wrote in message
>> news:1136310085 .816491.322760@ g14g2000cwa.goo glegroups.com.. .
>>[color=darkred]
>>>Hello
>>>
>>>I have some problem with sscanf, I tryed this code but it doesn't works
>>>:
>>>
>>>char* stringa = "18/2005"[/color]
>>
>> "18/2005" is a string literal. Any attempts to modify
>> any of its characters produces undefined behavior.
>>[color=darkred]
>>>char mese[3]; char anno[5];
>>>int i_letture;
>>>
>>>i_letture = sscanf(stringa, "%2s/%4s", &mese, &anno);[/color]
>>
>> This gives undefined behavior. You're trying to modify
>> a string literal.[/color]
>
> Look again.
> The first parameter of sscanf() is of type const char *
> (qualified by restrict in C99 IIRC).
>
> I guess you saw sscanf() and thought sprintf()...[/color]

Um, yes. Oops. Blush. Sorry. Er, Happy New Year and all that. :-)

-Mike

[Emmanuel Delahaye]

Re: SSCANF

Mike Wahler a écrit :[color=blue][color=green]
>>char* stringa = "18/2005"[/color]
>
> "18/2005" is a string literal. Any attempts to modify
> any of its characters produces undefined behavior.[/color]

How would it be modified ? I'm curious. Isn't the first parameter of
sscanf() a char const * ?

--
A+

Emmanuel Delahaye

[Robert Harris]

Re: SSCANF

Superfox il Volpone wrote:[color=blue]
> Hello
>
> I have some problem with sscanf, I tryed this code but it doesn't works
> :
>
> char* stringa = "18/2005"[/color]
Missing ';' termination[color=blue]
> char mese[3]; char anno[5];
> int i_letture;
>
> i_letture = sscanf(stringa, "%2s/%4s", &mese, &anno);
> mese[2] = anno[4] = '\0';[/color]
Unnecessary here: sscanf NUL-terminates the results[color=blue]
>
> The values ar random and I don't understand the motive[/color]

Well, it works for me! What makes you think the result is wrong?

Robert

[Mike Wahler]

Re: SSCANF


"Emmanuel Delahaye" <emdel@YOURBRAn oos.fr> wrote in message
news:43bae22d$0 $15331$79c14f64 @nan-newsreader-07.noos.net...[color=blue]
> Mike Wahler a écrit :[color=green][color=darkred]
>>>char* stringa = "18/2005"[/color]
>>
>> "18/2005" is a string literal. Any attempts to modify
>> any of its characters produces undefined behavior.[/color]
>
> How would it be modified ? I'm curious. Isn't the first parameter of
> sscanf() a char const * ?[/color]

Brain malfunction. Read 'sscanf', saw 'sprintf'.
My apologies.

-Mike

[Superfox il Volpone]

Re: SSCANF

Sorry people, what I posted it's part of a more large program and the
printf with I verified it was wrong :)

I solved but I don't understand one thing :
the sscanf works with '&mese' and 'mese' in the correct manner : maybe
it's the compiler (GCC) that does some correction ?

The second thing if I want to jump one argument is it correct the
behaviour ?
year[5]
sscanf(str_date , "%*s/%4s", year);

and at last, at year will be placed the string terminator ('\0')

Bye & thx all for the help
~ Superfox il Volpone :)

[Keith Thompson]

Re: SSCANF

"Superfox il Volpone" <atari@email.it > writes:[color=blue]
> Sorry people, what I posted it's part of a more large program and the
> printf with I verified it was wrong :)[/color]

For context, here's the code you posted:

char* stringa = "18/2005"
char mese[3]; char anno[5];
int i_letture;

i_letture = sscanf(stringa, "%2s/%4s", &mese, &anno);
mese[2] = anno[4] = '\0';

Please read <http://cfaj.freeshell. org/google/>.

This also demonstrates why you should always post a small, complete,
compilable program. We can't guess whether the part you didn't post
is what's causing the problem (in this case, it was). What you posted
wasn't even a correct code fragment; the declaration of stringa is
missing a semicolon, an error that the compiler would have caught.
Don't try to re-type your code; copy-and-paste *exactly* what you fed
to the compiler.
[color=blue]
> I solved but I don't understand one thing :
> the sscanf works with '&mese' and 'mese' in the correct manner : maybe
> it's the compiler (GCC) that does some correction ?[/color]

Given, for example,

char anno[5];

the expression "anno", an array name, is implicitly converted (in most
contexts) to a pointer to the array's first element; in this case, the
resulting expression is of type char*. The expression "&anno" yields
the address of the array; the resulting expression is of type
char (*p)[5], i.e., a pointer to an array of 5 chars.

These two expressions are of different types, but they're both
pointers, they both (in some sense) have the same value, and they
*probably* both have the same representation.

Since sscanf() takes a variable number and type(s) of arguments, the
compiler doesn't know what types are expected for the third and fourth
arguments in your call; it just blindly passes in whatever you
specify. It's your job to make sure you call it correctly. Since the
arguments you gave it were (probably) the same size and representation
as the correct arguments, it happened to work. (Passing something of
an incorrect type to sscanf() actually invokes undefined behavior;
working "correctly" is one of the many possible consequences.)

BTW, I believe there are real systems where char* and char (*p)5 would
have different representations , and your sscanf() call would fail.
[color=blue]
> The second thing if I want to jump one argument is it correct the
> behaviour ?
> year[5]
> sscanf(str_date , "%*s/%4s", year);
>
> and at last, at year will be placed the string terminator ('\0')[/color]

Yes, that should work (assuming the declaration is "char year[5]"
rather than "year[5]").

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Christopher Benson-Manica]

Re: SSCANF

Chris McDonald <chris@csse.uwa .edu.au> wrote:
[color=blue]
> Doesn't the prototype of sscanf() promise to *not* modify its first argument?[/color]

I would have sworn that my K&R2 at work gave the type of sscanf()'s
first argument as char*, but I see from n869 that either I or K&R2 am
mistaken. Apologies. It still begs the question of how it can modify
a string literal, however.

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cybers pace.org | don't, I need to know. Flames welcome.

[Richard Heathfield]

Re: SSCANF

Christopher Benson-Manica said:
[color=blue]
> Chris McDonald <chris@csse.uwa .edu.au> wrote:
>[color=green]
>> Doesn't the prototype of sscanf() promise to *not* modify its first
>> argument?[/color]
>
> I would have sworn that my K&R2 at work gave the type of sscanf()'s
> first argument as char*,[/color]

It does. It's listed in the errata.
[color=blue]
> but I see from n869 that either I or K&R2 am
> mistaken. Apologies. It still begs the question of how it can modify
> a string literal, however.[/color]

No, it doesn't beg the question. "To beg the question" means "to assume as
an implicit premise something you are seeking to prove".

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999

http://www.cpax.org.uk

email: rjh at above domain (but drop the www, obviously)