%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]

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)

 

[Andreas Kahari]

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).

 

[Irrwahn Grausewitz]

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

 

[Peter Nilsson]

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.

 

[Irrwahn Grausewitz]

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


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...