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Questions about "alignment" in memory

I posted a question some time back about accessing a char array as an
array of words. In order not to overrun the char array, I padded it
with enough 0x00 bytes to ensure that when accessed as words I
wouldn't overrun the array. I was told that this is dangerous and
that there could be alignment problems if, for example, I wanted to
access the char array elements from non-even multiples of sizeof(int).
For example, if I had the array:

char a[10];

and I wanted to access the 8 bytes (a[2], a[3],..., a[8], a[9]) as the
array:

int b[2];

where (b[0] contains the data in a[2] to a[5], and b[1] contains a[6]
to a[9])

I understand the alignment issue in this example. My question
is...can I turn this problem on its head...for example, create an
empty array of ints, then access this memory space as a char?

Here's what I'm talking about:
unsigned int* a_words;
char* a_bytes;

fstream in("myfile.dat" , ios::in | ios::binary | ios::ate);
int filesize_bytes = in.tellg();
int filesize_words = filesize_bytes / sizeof(int) + ((filesize_byte s %
sizeof(int)) > 0); // add 1 if there is a remander...

a_words = new unsigned int[filesize_words];
a_bytes = reinterpret_cas t<char*>(a_word s);

in.seekg(2, ios::beg); //note...out of (word) alignment...sta rts on
3rd byte
in.read(a_bytes , filesize_bytes-3);
in.close();

at which point the file is in memory and can be accessed as bytes (by
indexing a_bytes[0 to filesize_bytes]) or as words (by indexing
a_words[0 to filesize_words].

This seems to work fine. Additionally, it shouldn't suffer potential
alignment problems since the array is defined to align with words, and
word addresses should be accessable to a byte address, even if the
converse of this is not true.

I can see that there will be compatibility problems with this system
if ported to a system where CHAR_BIT != 8. However, I don't care
about these systems. If I'm only doing logical operators on the bits
in the file, I don't even see any endian issues with doing this.

Thanks for the slap-in-the-face I'm sure I'll get for performing such
blastphomous operations in c++. Seriously, does this treatment
circumvent potential alignment issues?
Jul 19 '05 #1
14 4088
WW
J. Campbell wrote:
I posted a question some time back about accessing a char array as an
array of words. In order not to overrun the char array, I padded it
with enough 0x00 bytes to ensure that when accessed as words I
wouldn't overrun the array. I was told that this is dangerous and
that there could be alignment problems if, for example, I wanted to
access the char array elements from non-even multiples of sizeof(int).
For example, if I had the array:

char a[10];

and I wanted to access the 8 bytes (a[2], a[3],..., a[8], a[9]) as the
array:

int b[2];

where (b[0] contains the data in a[2] to a[5], and b[1] contains a[6]
to a[9])

I understand the alignment issue in this example. My question
is...can I turn this problem on its head...for example, create an
empty array of ints, then access this memory space as a char?


Yes you can, but only with char being the "other" thing. Another solution
is to define a union, with a char and an int array inside.

--
WW aka Attila
Jul 19 '05 #2
WW wrote:
Yes you can, but only with char being the "other" thing. Another solution
is to define a union, with a char and an int array inside.

This is not guaranteed. It is implementation-defined behavior if the
value of a member of a union object is used when the most recent store
to the object was to a different member, other than structs sharing a
common initial sequence.

Many implementations do allow it.

There are more portable ways, basically shifting and or-ing the bytes
onto an int.


Brian Rodenborn
Jul 19 '05 #3
WW
Default User wrote:
WW wrote:
Yes you can, but only with char being the "other" thing. Another
solution is to define a union, with a char and an int array inside.

This is not guaranteed. It is implementation-defined behavior if the
value of a member of a union object is used when the most recent store
to the object was to a different member, other than structs sharing a
common initial sequence.


yep. But we are talking about a char and an int array so far.

--
WW aka Attila
Jul 19 '05 #4
WW wrote:

Default User wrote:
WW wrote:
Yes you can, but only with char being the "other" thing. Another
solution is to define a union, with a char and an int array inside.

This is not guaranteed. It is implementation-defined behavior if the
value of a member of a union object is used when the most recent store
to the object was to a different member, other than structs sharing a
common initial sequence.


yep. But we are talking about a char and an int array so far.

Right, which don't come under the exemption. If I got the OP's problem
right, he had a buffer of char that he wanted to convert into a series
of ints. Using unions to do so would be implementation-defined behavior
(if I'm reading the standard correctly).

Here's a way from my personal library:

unsigned int CreateDataWord (unsigned char data[4])
{
unsigned int dataword = 0;

for (int i = 0; i < 4; i++)
{
dataword |= data[i] << (3-i) * 8;
}
return dataword;
}
Note that this uses unsigned char for the buffer, which is guaranteed to
be safe, requires CHAR_BIT == 8, and is predicated on 32-bit int, so it
has its own nonportabilitie s.

Brian Rodenborn
Jul 19 '05 #5
"WW" <wo***@freemail .hu> wrote in:
yep. But we are talking about a char and an int array so far.


Thanks...<so far :-)>

Indeed, the real question is: is it SAFE to access a region of
memory, defined as other than char, as a char array...if you are aware
of the issues? Your answer indicates a cautious "yes" if you are
gentle, and make sure never to overstep the char array bounds...as
long as CHAR_BIT is the length expected. Is this interpretation
correct??

Thanks for the response...stil l trying to learn...6 mos into the
process...still love QB45...;-)
Jul 19 '05 #6
Default User <fi********@boe ing.com.invalid > wrote in message news:<3F******* ********@boeing .com.invalid>.. .
WW wrote:
Yes you can, but only with char being the "other" thing. Another solution
is to define a union, with a char and an int array inside.

This is not guaranteed. It is implementation-defined behavior if the
value of a member of a union object is used when the most recent store
to the object was to a different member, other than structs sharing a
common initial sequence.

Many implementations do allow it.

There are more portable ways, basically shifting and or-ing the bytes
onto an int.

Brian Rodenborn


Brian,

So...you raise issue with the use of union...but what about my
original solution where I take a char array and put it into an int
array...which I then access as both an int and a char array. Are
there alignment problems with this, or are the problems more local???

I somehow get the feeling you are posting from Galviston...if this is
the case, then it explains the dissarray. Cheers, ciao, and thanks in
advance for the c++ help.
Jul 19 '05 #7
"J. Campbell" <ma**********@y ahoo.com> wrote in message
news:b9******** *************** ***@posting.goo gle.com...
[...]
So...you raise issue with the use of union...but what about my
original solution where I take a char array and put it into an int
array...which I then access as both an int and a char array. Are
there alignment problems with this, or are the problems more
local???
[...]


You would need to do a reinterpret cast, and that is not one of
the portable types for it. So technically, no. Doing what you
suggest will result in an ill-formed program (or maybe the
behaviour is just implementation-defined). On the other hand,
it will probably work on 99% of the compilers and systems out
there. Since it would be costly to do it the "right" way, I
personally would just run with it. But that's just me, and this is
a C++ newsgroup, so if I were toeing the party line like a good
programmer, I would revile you for suggesting a program which
might possibly contravene the sacred text which is the C++
standard. Anyway, good luck.

Dave

---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.521 / Virus Database: 319 - Release Date: 9/23/2003
Jul 19 '05 #8
"J. Campbell" wrote:
So...you raise issue with the use of union...but what about my
original solution where I take a char array and put it into an int
array...which I then access as both an int and a char array. Are
there alignment problems with this, or are the problems more local???

You can access any object as an array of unsigned char safely. That's
because unsigned char is guaranteed to have no trap representations . An
array of ints can be accessed as unsigned char. However, you then must
be cognizant of endianess of the ints in the array. It's generally kind
of tricky, I've found it easier and more portable (no method is
completely portable) to use bitwise operators.


Brian Rodenborn
Jul 19 '05 #9
"David B. Held" <dh***@codelogi cconsulting.com > wrote in message
news:bm******** **@news.astound .net...
[...]
On the other hand, it will probably work on 99% of the
compilers and systems out there.
[...]


After reading Default User's post, I realized I should have added
the caveat that it will probably work on 99% of the compilers
and systems out there *but in a generally non-portable way*.
That means that since you're reading raw bytes into an array
from a file, and assuming a certain byte order for int, the code
obviously won't work on a platform that has a different byte order.
But usually, people who do stuff like this aren't interested in
portability in the first place.

Dave

---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.521 / Virus Database: 319 - Release Date: 9/23/2003
Jul 19 '05 #10

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