Is it required to comply byte alignment within a function?

[gamja]

Hi all.
This is my first post on this group. Nice to meet you, cool guys~!

I'm on system programming on various embedded systems and understand
very well the byte alignment issues. When I write C code, especially
design a structure, I pay attention to the order and size of member
variables. Because, my boss always says that all variables should be
aligned by 4byte boundary, if not a data abort will be occurred on a
specific machine such like ARM.

However, I've never seen such situation. Because, my compiler
automatically aligns the boundary and gives padding properly between
each variable in the structure, if needed. Of course, pack option of
the compiler may change its behavior and this assumption may be break
easily. So, I agree with my boss.

The problem I want to ask is that he wants me to align all local
variables even in a function. For example, see the following code.

Case #1 - THIS IS NOT ALLOWED!!
int foo1()
{
char a;
int b;
short c;
...
}

Case #2 - THIS IS OK. GOOD.
int foo2()
{
char a;
char dummy[3];
int b;
short c;
...
}

I think this is bothersome and even ridiculous.

How do you think about?
Is the case #1 can make a data abort or bus error?

[Flash Gordon]

I've re-instated the context so that people can see what is actually
being talked about.

gamja wrote:

Richard Bos wrote:

"gamja" <as****@gmail.c om> wrote:

The problem I want to ask is that he wants me to align all local
variables even in a function. For example, see the following code.

Case #1 - THIS IS NOT ALLOWED!!
int foo1()
{
char a;
int b;
short c;
...
}
If in assembler you did something like
a .space 1 # allocate 1 byte
b .space 4 # allocate 4 bytes
c .space 2 # allocate 2 bytes

on a system where alignment for integer access mattered then tried to
read b as a 4 byte integer then yes, this could lead to problems.

The wonder of high level languages is they *compiler* sorts out this
stuff for you except where you deliberately trick it.
Case #2 - THIS IS OK. GOOD.
int foo2()
{
char a;
char dummy[3];
int b;
short c;
...
}

I think this is bothersome and even ridiculous.

How do you think about?
The man is a fool, and that for more reasons than one.

The man is a fool probably thinking of assembler and applying the same
rules to C where they make no sense.

Or the OP is misunderstandin g his boss.

Is the case #1 can make a data abort or bus error?

If it does, you're not using a C compiler, but a bodge job.

May I know why it is? Hmm.. I need more information somewhat technical
to persuade my boss.

The language definition says that if you declare a variable of a given
type you can use it as that type. Anything else would not make sense.

You could tell you boss to read *any* C text book. If that won't do,
tell him/her to by a copy of the C standard.
--
Flash Gordon
Living in interesting times.
Although my email address says spam, it is real and I read it.

[Christian Bau]

In article <11************ **********@f14g 2000cwb.googleg roups.com>,
"gamja" <as****@gmail.c om> wrote:

Hi all.
This is my first post on this group. Nice to meet you, cool guys~!

I'm on system programming on various embedded systems and understand
very well the byte alignment issues. When I write C code, especially
design a structure, I pay attention to the order and size of member
variables. Because, my boss always says that all variables should be
aligned by 4byte boundary, if not a data abort will be occurred on a
specific machine such like ARM.

However, I've never seen such situation. Because, my compiler
automatically aligns the boundary and gives padding properly between
each variable in the structure, if needed. Of course, pack option of
the compiler may change its behavior and this assumption may be break
easily. So, I agree with my boss.

The problem I want to ask is that he wants me to align all local
variables even in a function. For example, see the following code.

Case #1 - THIS IS NOT ALLOWED!!
int foo1()
{
char a;
int b;
short c;
...
}

Case #2 - THIS IS OK. GOOD.
int foo2()
{
char a;
char dummy[3];
int b;
short c;
...
}

I think this is bothersome and even ridiculous.

I'd hate to work for your boss.
I'd hate even more to be the person who is your boss.

[pete]

Gordon Burditt wrote:

You CANNOT align local variables.

If they are union members, then you can.
I used to be a shop steward.

--
pete

[Jack Klein]

On Sat, 14 May 2005 02:13:04 GMT, pete <pf*****@mindsp ring.com> wrote
in comp.lang.c:

Gordon Burditt wrote:

You CANNOT align local variables.

If they are union members, then you can.
I used to be a shop steward.

Yes indeed, you can align them to the beginning of the union. But, of
course, you can't align the beginning of the union.

I used to eat shop stewards for breakfast.

--
Jack Klein
Home: http://JK-Technology.Com
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[pete]

Jack Klein wrote:

On Sat, 14 May 2005 02:13:04 GMT, pete <pf*****@mindsp ring.com> wrote
in comp.lang.c:

Gordon Burditt wrote:

You CANNOT align local variables.

If they are union members, then you can.
I used to be a shop steward.

Yes indeed, you can align them to the beginning of the union. But, of
course, you can't align the beginning of the union.

You can align the union with every type that is a member of the union.
If you want the union aligned for type int, just give it an int member.

--
pete