I've heard of things called "trap values". From what I understand, if
you set an integer variable to a trap value, then the program crashes.
Is this right?
From what I understand, it seems that unsigned integers shouldn't
have any trap values seeing as how every bit-pattern is a unique number.
Also, from what I understand, the only time you could have a "trap
value" is where you're working with a sign-magnitude machine and the
machine doesn't like negative 0. Presumably, on such a machine, the
following would crash:
int i, j;
i = 0;
i |= 0x800;
j = i; /* Crashes because we've got a trap pattern */
Do I understand this alright? Are there any other trap values?
Also, what happens when you set padding bits within integers. For
instance, let's day we have a machine where:
CHAR_BIT == 8
sizeof(int) == 6 (4 bytes for value, 2 bytes of padding)
On this machine, is it OK to do the following:
unsigned i, j;
memset(&i,0x3f,sizeof(int));
j = i;
Can this cause a crash? If so, what kind of crash is it? Do we call this
a "trap value" also?
--
Tomás Ó hÉilidhe 9 1715
"=?iso-8859-1?q?Tom=E1s_=D3_h=C9ilidhe?=" <to*@lavabit.comwrote:
I've heard of things called "trap values". From what I understand, if
you set an integer variable to a trap value, then the program crashes.
Is this right?
Not quite. It causes undefined behaviour, which means that it _may_
crash. Or raise a signal, or ignore the occurrance, or anything else.
Do I understand this alright? Are there any other trap values?
AFAICT, there aren't any in integers; but floating point types have more
room for trap values.
Also, what happens when you set padding bits within integers. For
instance, let's day we have a machine where:
Can this cause a crash?
It can cause UB. Note that a recent TC has (IIRC) outlawed padding bits
within normal unsigned ints, and someone will now undoubtedly remind me
whether it also does so for signed ones.
If so, what kind of crash is it? Do we call this a "trap value" also?
We call it undefined behaviour; the Standard does not distinguish
between various kinds of undefined behaviour, and they're all equally
undefined.
Richard rl*@hoekstra-uitgeverij.nl (Richard Bos) writes:
"=?iso-8859-1?q?Tom=E1s_=D3_h=C9ilidhe?=" <to*@lavabit.comwrote:
> I've heard of things called "trap values". From what I understand, if you set an integer variable to a trap value, then the program crashes. Is this right?
Not quite. It causes undefined behaviour, which means that it _may_
crash. Or raise a signal, or ignore the occurrance, or anything else.
>Do I understand this alright? Are there any other trap values?
AFAICT, there aren't any in integers; but floating point types have more
room for trap values.
>Also, what happens when you set padding bits within integers. For instance, let's day we have a machine where:
>Can this cause a crash?
It can cause UB.
Note that a recent TC has (IIRC) outlawed padding bits
within normal unsigned ints, and someone will now undoubtedly remind me
whether it also does so for signed ones.
Well, I can't do that (because I don't know), but from my reading of
n1256.pdf (which I though had all the TCs merged) both signed and
unsigned ints can have trap representations made up from some
combinations of padding bits (a parity bit is given as an example).
They can't result from normal arithmetic (except from overflow in a
signed int) on normal values, but you could make one using memset as
shown.
You say the example is UB, but what causes the UB if unsigned int
can't have padding bits?
--
Ben.
Tomás Ó hÉilidhe wrote:
>
I've heard of things called "trap values". From what I understand,
if you set an integer variable to a trap value, then the program
crashes. Is this right?
No. You can't set an int to a trap value, when accessing it as an
int (barring illegal operations). If such values exist they have
to be set by the system, or possibly by accessing the object as a
series of bytes (which have no trap values).
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.
--
Posted via a free Usenet account from http://www.teranews.com
"Tomás Ó hÉilidhe" <to*@lavabit.comwrites:
I've heard of things called "trap values". From what I understand, if
you set an integer variable to a trap value, then the program crashes.
Is this right?
[...]
I think your question has already been answered, but there's at least
one point nobody has explicitly raised.
They're not called "trap values", they're called "trap
representations". The whole point is that a trap representation
*doesn't* represent a value of the type.
Nothing in the C standard requires a program to "crash" (unless you
consider the result of calling abort() to be a crash). All the stuff
that you might expect to cause a crash (signed or floating-point
numeric overflow, access beyond an array, dereferencing a null or
invalid pointer, etc.) merely invokes undefined behavior. If you're
*lucky*, these things will cause your program to crash so you can find
the problem. If you're unlucky, your program will continue merrily
executing with bad data. The standard guarantees nothing.
--
Keith Thompson (The_Other_Keith) <ks***@mib.org>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Keith Thompson <ks***@mib.orgwrites:
"Tomás Ó hÉilidhe" <to*@lavabit.comwrites:
> I've heard of things called "trap values". From what I understand, if you set an integer variable to a trap value, then the program crashes. Is this right?
[...]
I think your question has already been answered,
But not, unambiguously!
--
Ben.
CBFalconer <cb********@yahoo.comwrites:
Tomás Ó hÉilidhe wrote:
>> I've heard of things called "trap values". From what I understand, if you set an integer variable to a trap value, then the program crashes. Is this right?
No. You can't set an int to a trap value, when accessing it as an
int (barring illegal operations). If such values exist they have
to be set by the system, or possibly by accessing the object as a
series of bytes (which have no trap values).
"by the system" is a bit vague. They can be the result of bit-wise
operations on signed values (always a worry, but probably outside of
your category "illegal") but not, I think, from any such operation on
unsigned integer values.
--
Ben.
On 20 Feb, 12:30, "Tomás Ó hÉilidhe" <t...@lavabit.comwrote:
* *I've heard of things called "trap values". From what I understand, if
you set an integer variable to a trap value, then the program crashes.
Is this right?
* *From what I understand, it seems that unsigned integers shouldn't
have any trap values seeing as how every bit-pattern is a unique number.
Also, from what I understand, the only time you could have a "trap
value" is where you're working with a sign-magnitude machine and the
machine doesn't like negative 0. Presumably, on such a machine, the
following would crash:
1's complement also gives you a -0 (negative zero)
and I've used a machine that used this to trap uninitialised
values (not with a C compiler though)
<snip>
--
Nick Keighley
Nick Keighley wrote:
"Tomás Ó hÉilidhe" <t...@lavabit.comwrote:
>I've heard of things called "trap values". From what I understand, if you set an integer variable to a trap value, then the program crashes. Is this right?
From what I understand, it seems that unsigned integers shouldn't have any trap values seeing as how every bit-pattern is a unique number. Also, from what I understand, the only time you could have a "trap value" is where you're working with a sign-magnitude machine and the machine doesn't like negative 0. Presumably, on such a machine, the following would crash:
1's complement also gives you a -0 (negative zero)
and I've used a machine that used this to trap uninitialised
values (not with a C compiler though)
1's complement never generates a -0 IF the arithmetic is performed
with a subtractor AND addition is performed by "complement and
subtract". With a little care you can have a C compiler system
that traps all uninitialized values.
--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.
--
Posted via a free Usenet account from http://www.teranews.com
On Wed, 20 Feb 2008 13:01:58 GMT, rl*@hoekstra-uitgeverij.nl (Richard
Bos) wrote:
<snip other points>
It can cause UB. Note that a recent TC has (IIRC) outlawed padding bits
within normal unsigned ints, and someone will now undoubtedly remind me
whether it also does so for signed ones.
YDNRC; you may have conflated some related things.
TC2 changed 6.2.6.2 to require that all-zero-bits is a valid
representation of value zero in all integer types (signed and
unsigned, and plain char which formally is neither). Thus IF there are
padding bits in an integer type, it must be valid to have padding = 0
when magnitude+sign_if_any = 0.
TC2 also changed 7.18.1.1p3 about the conditions which require the
implemention to define [u]int{8,16,32,64}_t so they match (in fact
repeat) the requirements for those typedefs in p1 and p2.
C99 6.2.6.2p1 required that unsigned char have no padding bits, with
no such requirement on any other type, and this hasn't changed. Though
IF the implementation chooses to make plain char like unsigned char,
that must use the same representation and thus have no padding bits.
- formerly david.thompson1 || achar(64) || worldnet.att.net This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
by: Simon Reye |
last post by:
Is there a way to force a floating point value to be invalid? I'm
reading floating points from a device and every now and then I get an
invalid number, but it is too few and far between for me to...
|
by: adams114 |
last post by:
I am having a strange problem with invalid type casts. I am trying to
update a MS SQL Database with a stored procedure. When I setup the
parameters collection for the command object I get a invalid...
|
by: Ben Finney |
last post by:
Howdy all,
I've recently packaged 'enum' in PyPI. In its description, I make the
claim that it creates "immutable" enumeration objects, and that the
enumeration values are "constant" values.
...
|
by: Ben Finney |
last post by:
Antoon Pardon wrote:
> I just downloaded your enum module for python
> and played a bit with it. IMO some of the behaviour makes it less
> usefull.
Feedback is appreciated. I'm hoping to...
|
by: John Salerno |
last post by:
I'm a little confused. Why doesn't s evaluate to True in the first part,
but it does in the second? Is the first statement something different?
False
print 'hi'
hi
Thanks.
|
by: cschind |
last post by:
Does anyone know how I can get default values for integers (0) to not show up in my Xml when using XmlSerializer. I added the DefaultValueAttribute, but then those values get deleted if they are in...
|
by: haelly |
last post by:
Write a program that prompts the user to enter three different integer values.If the values are not different, the program prints a message"equal values" and terminates(hint: use the return...
|
by: haelly |
last post by:
write a program that prompts the user to enter three different positive integer values.If the values are not different, the program prints a message"equal value" and terminates(hint:use the return...
|
by: arnuld |
last post by:
PURPOSE: see the comments.
WHAT I GOT: infinite loop
/* This program will simply create an array of pointers to integers
* and will fill it with some values while using malloc...
|
by: ryjfgjl |
last post by:
In our work, we often receive Excel tables with data in the same format. If we want to analyze these data, it can be difficult to analyze them because the data is spread across multiple Excel files...
|
by: BarryA |
last post by:
What are the essential steps and strategies outlined in the Data Structures and Algorithms (DSA) roadmap for aspiring data scientists? How can individuals effectively utilize this roadmap to progress...
|
by: nemocccc |
last post by:
hello, everyone, I want to develop a software for my android phone for daily needs, any suggestions?
|
by: Sonnysonu |
last post by:
This is the data of csv file
1 2 3
1 2 3
1 2 3
1 2 3
2 3
2 3
3
the lengths should be different i have to store the data by column-wise with in the specific length.
suppose the i have to...
|
by: Hystou |
last post by:
There are some requirements for setting up RAID:
1. The motherboard and BIOS support RAID configuration.
2. The motherboard has 2 or more available SATA protocol SSD/HDD slots (including MSATA, M.2...
|
by: marktang |
last post by:
ONU (Optical Network Unit) is one of the key components for providing high-speed Internet services. Its primary function is to act as an endpoint device located at the user's premises. However,...
|
by: Hystou |
last post by:
Most computers default to English, but sometimes we require a different language, especially when relocating. Forgot to request a specific language before your computer shipped? No problem! You can...
|
by: jinu1996 |
last post by:
In today's digital age, having a compelling online presence is paramount for businesses aiming to thrive in a competitive landscape. At the heart of this digital strategy lies an intricately woven...
|
by: agi2029 |
last post by:
Let's talk about the concept of autonomous AI software engineers and no-code agents. These AIs are designed to manage the entire lifecycle of a software development project—planning, coding, testing,...
| |