sprintf function

[Earth]

Hi all,

I am new to c and trying the sprintf function.

I have written a testing program to try the sprintf fuction and expect
the output is 1.234. However, the output shows nothing.

Am I missing sth here?

Thanks in advance for you inputs.

Earth

#include <stdio.h>
char* toString(double d)
{
char buffer[9];
sprintf(buffer, "%lf", d);
return buffer;
}

main()
{
printf("%s\n", toString(1.234) );
}

[Morris Dovey]

Re: sprintf function

Earth wrote:
[color=blue]
> #include <stdio.h>
> char* toString(double d)
> {
> char buffer[9];
> sprintf(buffer, "%lf", d);
> return buffer;
> }
>
> main()
> {
> printf("%s\n", toString(1.234) );
> }[/color]

What do you suppose happens to buffer[9] when you return from
toString()?

(BTW toString is not a valid identifier)

--
Morris Dovey
West Des Moines, Iowa USA
C links at http://www.iedu.com/c
Read my lips: The apple doesn't fall far from the tree.

[Ian Peattie]

Re: sprintf function

In article <ab940065.04020 80651.60d043ee@ posting.google. com>, earth_sw@yahoo. com.hk (Earth) wrote:
[color=blue]
>I have written a testing program to try the sprintf fuction and expect
>the output is 1.234. However, the output shows nothing.[/color]
[color=blue]
>#include <stdio.h>
>char* toString(double d)
>{
> char buffer[9];
> sprintf(buffer, "%lf", d);
> return buffer;
>}[/color]

The problem is with 'buffer' being local to the toString function.

See http://www.eskimo.com/~scs/C-faq/q7.5.html

Ian.

[nrk]

Re: sprintf function

Earth wrote:
[color=blue]
> Hi all,
>
> I am new to c and trying the sprintf function.
>
> I have written a testing program to try the sprintf fuction and expect
> the output is 1.234. However, the output shows nothing.
>
> Am I missing sth here?
>
> Thanks in advance for you inputs.
>
> Earth
>
> #include <stdio.h>
> char* toString(double d)
> {
> char buffer[9];
> sprintf(buffer, "%lf", d);
> return buffer;
> }
>
> main()
> {
> printf("%s\n", toString(1.234) );
> }[/color]

Congratulations !! You hit the FAQ jackpot. In fact, we even have an FAQ
that actually uses sprintf while answering your very question. You can
find this question and answer at:

http://www.eskimo.com/~scs/C-faq/q7.5.html

Please to take the time to go through the fine FAQ. It's available at:

http://www.eskimo.com/~scs/C-faq/top.html

-nrk.

--
Remove devnull for email

[Peter Pichler]

Re: sprintf function

"Morris Dovey" <mrdovey@iedu.c om> wrote:

<snippage>
[color=blue]
> (BTW toString is not a valid identifier)[/color]

Why?

[Tom St Denis]

Re: sprintf function


"Peter Pichler" <pichlo7@pobox. sk> wrote in message
news:40265cdd$1 _2@mk-nntp-2.news.uk.tisca li.com...[color=blue]
> "Morris Dovey" <mrdovey@iedu.c om> wrote:
>
> <snippage>
>[color=green]
> > (BTW toString is not a valid identifier)[/color]
>
> Why?[/color]

Can't start with "to". It's reserved for things like tolower, toupper,
etc...

Technically it's not "buggy C" just not portable. Just like using "errno"
for a local variable isn't a great idea neither.

Tom

[Peter Pichler]

Re: sprintf function

"Tom St Denis" <tom@securescie nce.net> wrote in message
news:_7tVb.3392 4$3YE1.27018@ne ws04.bloor.is.n et.cable.rogers .com...[color=blue]
> "Peter Pichler" <pichlo7@pobox. sk> wrote in message
> news:40265cdd$1 _2@mk-nntp-2.news.uk.tisca li.com...[color=green]
> > "Morris Dovey" <mrdovey@iedu.c om> wrote:
> >
> > <snippage>
> >[color=darkred]
> > > (BTW toString is not a valid identifier)[/color]
> >
> > Why?[/color]
>
> Can't start with "to".[/color]

....and a lowercase letter.
[color=blue]
> It's reserved for things like tolower, toupper,
> etc...[/color]

But not toString. 'S' is not a lowercase letter, AFAICS.
[color=blue]
> Technically it's not "buggy C" just not portable. Just like using "errno"
> for a local variable isn't a great idea neither.[/color]

Not only is it portable, it is also recommended. Either that or is_string().

Peter

[Eric Amick]

Re: sprintf function

On 8 Feb 2004 06:51:20 -0800, earth_sw@yahoo. com.hk (Earth) wrote:
[color=blue]
>I am new to c and trying the sprintf function.
>
>I have written a testing program to try the sprintf fuction and expect
>the output is 1.234. However, the output shows nothing.
>
>Am I missing sth here?
>
>#include <stdio.h>
>char* toString(double d)
>{
> char buffer[9];
> sprintf(buffer, "%lf", d);
> return buffer;
>}[/color]

I see several problems:

1. %lf is not a valid specifier for sprintf; you want %f, which works
for both float and double types. Many implementations will let you get
away with this, but you should get in the habit of using the right one.

2. The buffer allows for only 8 characters in the representation of the
number. If the number has more digits than that, things could get ugly.

3. You're returning a pointer to a buffer that goes away when the
function returns, which will have unpredictable consequences. The
program could just as well have aborted. This is the one that is the
immediate issue.

There are several solutions to this last problem. One is to declare
buffer as static char so that it remains after the function exits;
another is to use the malloc() function to allocate the memory instead
and return a pointer to that memory; yet another is to allocate the
buffer in the main program and pass a pointer to that buffer to the
function and store the result there. Any of these will work for toy
programs, but the last two are generally better choices for real code.

--
Eric Amick
Columbia, MD

[nrk]

Re: sprintf function

Tom St Denis wrote:
[color=blue]
>
> "Peter Pichler" <pichlo7@pobox. sk> wrote in message
> news:40265cdd$1 _2@mk-nntp-2.news.uk.tisca li.com...[color=green]
>> "Morris Dovey" <mrdovey@iedu.c om> wrote:
>>
>> <snippage>
>>[color=darkred]
>> > (BTW toString is not a valid identifier)[/color]
>>
>> Why?[/color]
>
> Can't start with "to". It's reserved for things like tolower, toupper,
> etc...
>[/color]

Tom, you might want to think twice before giving an "obvious" answer when
one respected clc regular is questioning another respected clc regular.
IMHO, such threads and sub-threads must be marked clearly in bold red type:
"Here be dragons" :-)

-nrk.
[color=blue]
> Technically it's not "buggy C" just not portable. Just like using "errno"
> for a local variable isn't a great idea neither.
>
> Tom[/color]

--
Remove devnull for email

[Tom St Denis]

Re: sprintf function


"nrk" <ram_nrk2000@de vnull.verizon.n et> wrote in message
news:6rtVb.2457 $4e3.114@nwrddc 02.gnilink.net. ..[color=blue]
> Tom St Denis wrote:
>[color=green]
> >
> > "Peter Pichler" <pichlo7@pobox. sk> wrote in message
> > news:40265cdd$1 _2@mk-nntp-2.news.uk.tisca li.com...[color=darkred]
> >> "Morris Dovey" <mrdovey@iedu.c om> wrote:
> >>
> >> <snippage>
> >>
> >> > (BTW toString is not a valid identifier)
> >>
> >> Why?[/color]
> >
> > Can't start with "to". It's reserved for things like tolower, toupper,
> > etc...
> >[/color]
>
> Tom, you might want to think twice before giving an "obvious" answer when
> one respected clc regular is questioning another respected clc regular.
> IMHO, such threads and sub-threads must be marked clearly in bold red[/color]
type:[color=blue]
> "Here be dragons" :-)[/color]

True dat. I'll bow out of this conversation. Though I'll add my two cents.
Generally it isn't a good idea to start functions without a descriptive
name. e.g. mp_*() for multiple precision math, etc...

Tom

[Peter Pichler]

Re: sprintf function

"nrk" <ram_nrk2000@de vnull.verizon.n et> wrote:[color=blue]
> Tom St Denis wrote:[color=green]
> > "Peter Pichler" <pichlo7@pobox. sk> wrote in message
> > news:40265cdd$1 _2@mk-nntp-2.news.uk.tisca li.com...[color=darkred]
> >> "Morris Dovey" <mrdovey@iedu.c om> wrote:
> >>
> >> <snippage>
> >>
> >> > (BTW toString is not a valid identifier)
> >>
> >> Why?[/color]
> >
> > Can't start with "to". It's reserved for things like tolower, toupper,
> > etc...[/color]
>
> Tom, you might want to think twice before giving an "obvious" answer when
> one respected clc regular is questioning another respected clc regular.[/color]

*blush*

When did I become a respected clc regular? I thought that one needs to be
marked as an "obvious troll" by ERT first... ;-)

Peter

[Jack Klein]

Re: sprintf function

On Sun, 08 Feb 2004 09:03:39 -0600, Morris Dovey <mrdovey@iedu.c om>
wrote in comp.lang.c:
[color=blue]
> Earth wrote:
>[color=green]
> > #include <stdio.h>
> > char* toString(double d)
> > {
> > char buffer[9];
> > sprintf(buffer, "%lf", d);
> > return buffer;
> > }
> >
> > main()
> > {
> > printf("%s\n", toString(1.234) );
> > }[/color]
>
> What do you suppose happens to buffer[9] when you return from
> toString()?
>
> (BTW toString is not a valid identifier)[/color]

Actually toString is a valid identifier. The standard says:

"7.26.2 Character handling <ctype.h>
1 Function names that begin with either is or to, and a lowercase
letter may be added to the declarations in the <ctype.h> header."

Identifiers beginning with "is" or "to" followed by an underscore,
digit, or uppercase letter do not violate the reserved name space.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++

http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

[Martin Ambuhl]

Re: sprintf function

Earth wrote:
....[color=blue]
> I have written a testing program to try the sprintf fuction and expect
> the output is 1.234. However, the output shows nothing.[/color]
....[color=blue]
> #include <stdio.h>
> char* toString(double d)
> {
> char buffer[9];
> sprintf(buffer, "%lf", d);
> return buffer;
> }
>
> main()
> {
> printf("%s\n", toString(1.234) );
> }[/color]

But this code ...

#include <stdio.h>
char *toString(doubl e d)
{
static char buffer[9];
sprintf(buffer, "%lf", d);
return buffer;
}

int main(void)
{
printf("%s\n", toString(1.234) );
return 0;
}

Has this output:
1.234000

Notice the difference?

--
Martin Ambuhl

[Martin Ambuhl]

Re: sprintf function

Peter Pichler wrote:
[color=blue]
> "Morris Dovey" <mrdovey@iedu.c om> wrote:
>
> <snippage>
>[color=green]
>>(BTW toString is not a valid identifier)[/color]
>
>
> Why?[/color]

It is a valid identifier. But 'tostring' (with external linkage),
beginning with 'to' followed by a lowercase letter would be a name reserved
to the implementation no matter what headers are included.


--
Martin Ambuhl

[Peter Nilsson]

Re: sprintf function

"Jack Klein" <jackklein@spam cop.net> wrote in message
news:sksd20518j 5rj1smgejc6m4dt 25ov5epvu@4ax.c om...[color=blue]
> On 8 Feb 2004 16:21:07 -0800, airia@acay.com. au (Peter Nilsson) wrote
> in comp.lang.c:[color=green]
> > Jack Klein <jackklein@spam cop.net> wrote in message
> > news:<mnsc20h8g 3quu92re8e7cmea jgepknb5ks@4ax. com>...[color=darkred]
> > > On Sun, 08 Feb 2004 09:03:39 -0600, Morris Dovey <mrdovey@iedu.c om>
> > > wrote in comp.lang.c:
> > >
> > > > Earth wrote:
> > > >
> > > > > #include <stdio.h>
> > > > > char* toString(double d)
> > > > > {
> > > > > char buffer[9];
> > > > > sprintf(buffer, "%lf", d);
> > > > > return buffer;
> > > > > }
> > > > >
> > > > > main()
> > > > > {
> > > > > printf("%s\n", toString(1.234) );
> > > > > }
> > > >
> > > > (BTW toString is not a valid identifier)
> > >
> > > Actually toString is a valid identifier. The standard says:
> > >
> > > "7.26.2 Character handling <ctype.h>
> > > 1 Function names that begin with either is or to, and a lowercase
> > > letter may be added to the declarations in the <ctype.h> header."
> > >
> > > Identifiers beginning with "is" or "to" followed by an underscore,
> > > digit, or uppercase letter do not violate the reserved name space.[/color]
> >
> > Although, case insensitivity of external identifiers is implementation
> > defined in C90. [This was deprecated and indeed removed in C99.][/color]
>
> Immaterial, the patterns that are and are not reserved by this clause
> were identical in C90...[/color]

C90 _implicitly_ reserves a broader range of external identifiers for
implementations which do not have case sensitive external linkage.

I don't have C90 itself (only a draft), but I have seen numerous posts
quoting ANSI 4.1.2.1

All identifiers with external linkage in any of the following sections
(including the future library directions) are always reserved for use
as identifiers with external linkage.

So, if tostri* is reserved as an external identifier in C90 (as it is), then
using toStri* as an external identifier will preclude strict conformance.
It's undefined behaviour on machines with case insensitive external
linkage.[*]

If 'Earth' had declared toString static, there wouldn't be a problem.
[*] I don't know of any conforming implementations having this _feature_
exist, but nonetheless, C90 allows for them.

--
Peter