Typecasting in C

[andynaik]

Hi,
Whenever we type in this code
int main()
{
printf("%f",10) ;
}
we get an error. We can remove that by using #pragma directive t
direct that to the 8087. Even after that the output is 0.00000 and no
10.0000. Can anybody tell me why it is like that and why typecasting i
not done in this case?
-
andynai
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Posted via http://www.codecomments.co
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[Dario (drinking coffee in the office…)]

andynaik wrote:

Hi,
Whenever we type in this code
int main()
{
printf("%f",10) ;
}
we get an error. We can remove that by using #pragma directive to
direct that to the 8087. Even after that the output is 0.00000 and not
10.0000. Can anybody tell me why it is like that and why typecasting is
not done in this case??

%f is for double, but 10 is an int.

Rewrite as:
#include <stdio.h>
int main(){printf(" %d",10);retur n 0;}
or as:
#include <stdio.h>
int main(){printf(" %f",10.0);retur n 0;}

- Dario

[Alex Fraser]

"andynaik" <an************ *@mail.codecomm ents.com> wrote in message
news:2e******** *************** *******@news.th enewsgroups.com ...

Whenever we type in this code
int main()
{
printf("%f",10) ;
}
we get an error. We can remove that by using #pragma directive to
direct that to the 8087.
Means nothing to me (compiler specifics are off-topic here).
Even after that the output is 0.00000 and not 10.0000. Can anybody tell
me why it is like that and why typecasting is not done in this case??

Read section 15 in the FAQ. Post back if you still have questions.
http://www.eskimo.com/~scs/C-faq/s15.html

Alex

[Emmanuel Delahaye]

In 'comp.lang.c', andynaik <an************ *@mail.codecomm ents.com> wrote:

Hi,
Whenever we type in this code
int main()
{
printf("%f",10) ;
}
we get an error.

Compiling MAIN.C:
Warning MAIN.C 4: Call to function 'printf' with no prototype
Warning MAIN.C 5: Function should return a value
Linking EXE\PROJ.EXE:

This code invokes an undefined behaviour (UB). It is mandatory to supply a
prototype when using a variadic function. Add

#include <stdio.h>

That said, 10 is a int. "%f" is expecting a double. If you want to printf a
double, use 10.0, or the (double) typecast.

Finally, some old Borland C compilers anre buggy and forget to link the
floating point library un such a case. This little hack can help:

#include <stdio.h>

#ifdef __BORLANDC__
/* The pesky "floating point formats not linked" killer hack : */
extern unsigned _floatconvert;
#pragma extref _floatconvert
#endif

int main (void)
{
printf ("%f\n", 10.0);

return 0;
}

--
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The C-language FAQ: http://www.eskimo.com/~scs/C-faq/top.html
C-reference: http://www.dinkumware.com/manuals/reader.aspx?lib=c99
FAQ de f.c.l.c : http://www.isty-info.uvsq.fr/~rumeau/fclc/

[jacob navia]

"Alex Fraser" <[e-mail] > a écrit dans le message de
news:2k******** ***@uni-berlin.de...

"andynaik" <an************ *@mail.codecomm ents.com> wrote in message
news:2e******** *************** *******@news.th enewsgroups.com ...

Whenever we type in this code
int main()
{
printf("%f",10) ;
}
we get an error. We can remove that by using #pragma directive to
direct that to the 8087.

Means nothing to me (compiler specifics are off-topic here).

Even after that the output is 0.00000 and not 10.0000. Can anybody tell
me why it is like that and why typecasting is not done in this case??

Read section 15 in the FAQ. Post back if you still have questions.
http://www.eskimo.com/~scs/C-faq/s15.html

Alex

The FAQ should mention that some compilers DO test the arguments
for sprintf/printf/fprintf etc for validity.

This CAN be done. lcc-win32 does it, and other compilers too.

For instance the above code produces
Warning tx.c: 4 printf argument mismatch for format f. Expected double got
int
0 errors, 1 warnings

[jacob navia]

Some compilers DO test the arguments of printf for
validity.
Under lcc-win32 the above code produces:

Warning tx.c: 4 printf argument mismatch for format f. Expected double got
int
0 errors, 1 warnings

[Dan Pop]

In <cb**********@n ews-reader3.wanadoo .fr> "jacob navia" <ja***@jacob.re mcomp.fr> writes:

The FAQ should mention that some compilers DO test the arguments
for sprintf/printf/fprintf etc for validity.

This CAN be done.
ONLY if the compiler can "see" the contents of the format string.
Which is usually the case, but exceptions are not that rare either.

At some point, gcc had the annoying habit of warning if it couldn't
perform such a check (if enabled), because the format was not a
string literal.
lcc-win32 does it, and other compilers too.
Other compilers do a much better job than lcc-win32, when it comes to
format string consistency checks. See below.
For instance the above code produces
Warning tx.c: 4 printf argument mismatch for format f. Expected double got
int
0 errors, 1 warnings

OTOH, lcc-win32 silently accepts printf("%d\n", "foo"), which is very
bad, considering the relative positions of the D and S keys on most
keyboard layouts (i.e. it is a fairly frequent mistake to type d
when you mean s).

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[jacob navia]

"Dan Pop" <Da*****@cern.c h> a écrit dans le message de
news:cb******** **@sunnews.cern .ch...

OTOH, lcc-win32 silently accepts printf("%d\n", "foo"), which is very
bad, considering the relative positions of the D and S keys on most
keyboard layouts (i.e. it is a fairly frequent mistake to type d
when you mean s).

I pondered a long time about that one, since it is perfectly legal to
do:

char *p;
....

printf("The address is %d\n",p);

specially in debugging code.

Granted, this is weird, but how to discriminate between
legal and wrong usage?

But maybe you are right. I added a warning when the "d" format
is used with a pointer.

The "x" format will NOT provoke any warnings.

But this is at the limit of what a compiler can do.

[Dan Pop]

In <cb**********@n ews-reader1.wanadoo .fr> "jacob navia" <ja***@jacob.re mcomp.fr> writes:

"Dan Pop" <Da*****@cern.c h> a écrit dans le message de
news:cb******* ***@sunnews.cer n.ch...

OTOH, lcc-win32 silently accepts printf("%d\n", "foo"), which is very
bad, considering the relative positions of the D and S keys on most
keyboard layouts (i.e. it is a fairly frequent mistake to type d
when you mean s).
I pondered a long time about that one, since it is perfectly legal to
do: ^^^^^^^^^^^^^^^ ^^^

char *p;
...

printf("The address is %d\n",p);

What have you been smoking recently?
specially in debugging code.

Granted, this is weird, but how to discriminate between
legal and wrong usage? ^^^^^
Can I have a chapter and verse? When did they drop the following
paragraph from the C standard?

9 If a conversion specification is invalid, the behavior is
undefined. If any argument is not the correct type for the
corresponding conversion specification, the behavior is undefined.

What is the type expected by %d? What is the type of p?
So, the legal usage would be (int)p instead of a plain p, right?
But maybe you are right. I added a warning when the "d" format
is used with a pointer.

The "x" format will NOT provoke any warnings.
Which is just as bad, especially given that x is also a typo candidate
for s.
But this is at the limit of what a compiler can do.

In your humble opinion, what is the purpose of the %p conversion
specification? Why support *anything else* for displaying pointer values?

And if a user *really* wants to use the extra flexibility of the
signed or unsigned integer conversion descriptors, what is preventing
him from casting the pointer to the desired type?

As I said in other threads, you have nothing to lose by using gcc's
behaviour as a guide. If you can benefit from the thought many competent
people have put into the same issue, why reinvent the wheel?

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Martin Ambuhl]

andynaik wrote:

Hi,
Whenever we type in this code
int main()
{
printf("%f",10) ;
}
we get an error. We can remove that by using #pragma directive to
direct that to the 8087. Even after that the output is 0.00000 and not
10.0000. Can anybody tell me why it is like that and why typecasting is
not done in this case??

Because you forgot
#include <stdio.h>
You also forgot to terminate the last line of output with an end-of-line
character.

The first mistake, omission of the prototype for a variadic function, is
always an error. The second is an error in code designed to be
portable, and may result in behavior you were not expecting.