%f undefined behavior?

[Mantorok Redgormor]

Why does the following print -1.997505? Is this because of undefined behavior?

#include <stdio.h>
int main(void)
{
printf("%f\n", 0x1f);
return 0;
}

[Christopher Benson-Manica]

Mantorok Redgormor <ne*****@tokyo.com> spoke thus:

Why does the following print -1.997505? Is this because of undefined behavior? #include <stdio.h>
int main(void)
{
printf("%f\n", 0x1f);
return 0;
}

Compiling with all warnings on...
test.c:6: warning: double format, different type arg (arg 2)

--
Christopher Benson-Manica | Jumonji giri, for honour.
ataru(at)cyberspace.org |

[Andreas Kahari]

In article <41*************************@posting.google.com> ,
Mantorok Redgormor wrote:

Why does the following print -1.997505? Is this because of undefined behavior?

#include <stdio.h>
int main(void)
{
printf("%f\n", 0x1f);
return 0;
}

Here, it prints
-14167097396369144661962308996344933237638
562294362303773503482977858904653824.000000
(except for the line break in the middle)

But then again, you're printing an integer with a floating point
format string, which makes it interpret the bit pattern of the
integer as if it was the bit pattern of a floating point value
(possibly going outside the data that was the integer).

--
Andreas Kähäri

[Irrwahn Grausewitz]

Andreas Kahari <ak*******@freeshell.org> wrote:

In article <41*************************@posting.google.com> ,
Mantorok Redgormor wrote:

Why does the following print -1.997505? Is this because of undefined behavior?

#include <stdio.h>
int main(void)
{
printf("%f\n", 0x1f);
return 0;
}

Here, it prints
-14167097396369144661962308996344933237638
562294362303773503482977858904653824.000000
(except for the line break in the middle)

But then again, you're printing an integer with a floating point
format string, which makes it interpret the bit pattern of the
integer as if it was the bit pattern of a floating point value
(possibly going outside the data that was the integer).

Even worse: it invokes Nasal Demons, aka undefined behaviour, because
%f forces printf to retrieve an argument of sizeof(double), but there
is only sth. of size of an int available.

[OT]
IOW: this will most certainly mess up your stack, whatever that is...
[/OT]

Regards

Irrwahn
--
What does this red button do?

[Peter Nilsson]

Irrwahn Grausewitz <ir*****@freenet.de> wrote in message news:<78********************************@4ax.com>. ..

Andreas Kahari <ak*******@freeshell.org> wrote:

In article <41*************************@posting.google.com> ,
Mantorok Redgormor wrote:

Why does the following print -1.997505? Is this because of undefined
behavior?

#include <stdio.h>
int main(void)
{
printf("%f\n", 0x1f);
return 0;
}
Here, it prints
-14167097396369144661962308996344933237638
562294362303773503482977858904653824.000000
(except for the line break in the middle)

But then again, you're printing an integer with a floating point
format string, which makes it interpret the bit pattern of the
integer as if it was the bit pattern of a floating point value
(possibly going outside the data that was the integer).

It needn't ever even attempt to interpret the bit pattern of the
integer.

Even worse: it invokes Nasal Demons, aka undefined behaviour, because
%f forces printf to retrieve an argument of sizeof(double), but there
is only sth. of size of an int available.
Huh? %f expects a double, not merely an object the same size of a
double.
[OT]
IOW: this will most certainly mess up your stack, whatever that is...
[/OT]

Not all the world is intel.

There are implementations (modern, in use) which pass parameters via
registers, not a hardware stack.

--
Peter

[Irrwahn Grausewitz]

ai***@acay.com.au (Peter Nilsson) wrote:

Irrwahn Grausewitz <ir*****@freenet.de> wrote in message news:<78********************************@4ax.com>. ..

Andreas Kahari <ak*******@freeshell.org> wrote: <SNIP>

>
>But then again, you're printing an integer with a floating point
>format string, which makes it interpret the bit pattern of the
>integer as if it was the bit pattern of a floating point value
>(possibly going outside the data that was the integer).

It needn't ever even attempt to interpret the bit pattern of the
integer.

Even worse: it invokes Nasal Demons, aka undefined behaviour, because
%f forces printf to retrieve an argument of sizeof(double), but there
is only sth. of size of an int available.

Huh? %f expects a double, not merely an object the same size of a
double.

Hm, I used strange wording here, indeed.

[OT]
IOW: this will most certainly mess up your stack, whatever that is...
[/OT]
Not all the world is intel.

Oh, really? :)

There are implementations (modern, in use) which pass parameters via
registers, not a hardware stack.

Right, and that's just the reason for [OT][/OT] tagging...
--
What does this red button do?