Struct assignment

**Malcolm McLean** · Jun 30 '07, 05:15 PM

Re: Struct assignment

"Grey Alien" <grey@andromeda .comwrote in message
news:erydna87m6 gBFRvbRVnyjwA@b t.com...

If I have the ff struct:
>
struct A
{
unsigned int i;
char s[LONG_ENOUGH];
} a, b;
>
>
And use them in code like this:
>
a.i = 42 ;
strcpy(a.s,"tes t");
>
b.i = 100 ;
>
b = a ;
>
at this point, a (bitwise?) copy of a is made to b. Question is:
>
1). is b.s now ptr to a.s ? (I think so)
If so, what happens if for instance variable 'a' goes out of scope (?)
>
2). Does the compiler generate an implicit "memcpy" or "memmove" behind
the scenes when it sees an assignment like this (to avoid dangling ptrs)?
>

The answer is 2. A structure assignment will cause a call to memcpy to be
made, or equivalent code emitted if the structure is small enough to make
this wasteful and the compiler is clever.
However if structures contain pointers then the values of the pointers are
overwritten. So you have to be extremely careful not to orphan memory or
create peculiar bugs with aliases.

--
Free games and programming goodies.

http://www.personal.leeds.ac.uk/~bgy1mm

**Thomas Lumley** · Jun 30 '07, 05:15 PM

Re: Struct assignment

On Jun 30, 9:43 am, Grey Alien <g...@andromeda .comwrote:

If I have the ff struct:
>
struct A
{
unsigned int i;
char s[LONG_ENOUGH];
>
} a, b;
>
And use them in code like this:
>
a.i = 42 ;
strcpy(a.s,"tes t");
>
b.i = 100 ;
>
b = a ;
>
at this point, a (bitwise?) copy of a is made to b.
Question is:
>
1). is b.s now ptr to a.s ? (I think so)

I think you are confusing arrays and pointers. Since a.s is an array,
a.s[0] to a.s[LONG_ENOUGH-1] are actually stored in the structure and
are copied by the assignment.

If a.s were just a pointer to memory allocated elsewhere the
assignment would just copy the pointer.

2). Does the compiler generate an implicit "memcpy"
or "memmove" behind the scenes when it sees an
assignment like this (to avoid dangling ptrs)?

There is an explicit copy since a.s is an array. If a.s
were a pointer there would not be an implicit copy, and
Bad Things such as dangling pointers could result.

-thomas

**Flash Gordon** · Jun 30 '07, 06:05 PM

Re: Struct assignment

Grey Alien wrote, On 30/06/07 17:43:

If I have the ff struct:
>
struct A
{
unsigned int i;
char s[LONG_ENOUGH];
} a, b;
>
>
And use them in code like this:
>
a.i = 42 ;
strcpy(a.s,"tes t");
>
b.i = 100 ;
>
b = a ;
>
at this point, a (bitwise?) copy of a is made to b.

It is not a bitwise copy, it is a copy of all the elements in the struct.

Question is:
>
1). is b.s now ptr to a.s ? (I think so)

Since b.s was not defined as a pointer, what makes you think an
assignment could magically transform it from being an array in to being
a pointer? You need to read section 6 of the comp.lang.c FAQ at
http://c-faq.com/ specifically the questions dealing with whether
pointers and arrays are the same thing.

If so, what happens if for instance variable 'a' goes out of scope (?)
>
2). Does the compiler generate an implicit "memcpy" or "memmove" behind
the scenes when it sees an assignment like this (to avoid dangling ptrs)?

It is very rare for C to do things behind your back. Had there been
pointers in your struct (which there were not) then after the assignment
they would point to the same place as they point in the original struct,
and when that place is no longer valid (an automatic that goes out of
scope, for example) the pointers are no longer valid.
--
Flash Gordon

**Serve Lau** · Jun 30 '07, 06:55 PM

Re: Struct assignment

"Flash Gordon" <spam@flash-gordon.me.ukwro te in message
news:hhfil4xnvp .ln2@news.flash-gordon.me.uk...

Grey Alien wrote, On 30/06/07 17:43:

>If I have the ff struct:
>>
>struct A
>{
> unsigned int i;
> char s[LONG_ENOUGH];
>} a, b;
>>
>>
>And use them in code like this:
>>
>a.i = 42 ;
>strcpy(a.s,"te st");
>>
>b.i = 100 ;
>>
>b = a ;
>>
>at this point, a (bitwise?) copy of a is made to b.

>
It is not a bitwise copy, it is a copy of all the elements in the struct.
>

Question is:
>>
>1). is b.s now ptr to a.s ? (I think so)

>
Since b.s was not defined as a pointer, what makes you think an assignment
could magically transform it from being an array in to being a pointer?
You need to read section 6 of the comp.lang.c FAQ at http://c-faq.com/
specifically the questions dealing with whether pointers and arrays are
the same thing.

Its easy to see where the confusion comes from. There are situations where
pointers degenerate into pointers , the OP probably had that in mind

**Richard Heathfield** · Jun 30 '07, 07:55 PM

Re: Struct assignment

Serve Lau said:

<snip>

Its easy to see where the confusion comes from. There are situations
where pointers degenerate into pointers ,

Pointers always degenerate into pointers.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

**Flash Gordon** · Jun 30 '07, 08:05 PM

Re: Struct assignment

Serve Lau wrote, On 30/06/07 19:53:

"Flash Gordon" <spam@flash-gordon.me.ukwro te in message
news:hhfil4xnvp .ln2@news.flash-gordon.me.uk...

>Grey Alien wrote, On 30/06/07 17:43:

<snip>

>>1). is b.s now ptr to a.s ? (I think so)

>Since b.s was not defined as a pointer, what makes you think an assignment
>could magically transform it from being an array in to being a pointer?
>You need to read section 6 of the comp.lang.c FAQ at http://c-faq.com/
>specifically the questions dealing with whether pointers and arrays are
>the same thing.

>
Its easy to see where the confusion comes from. There are situations where
pointers degenerate into pointers , the OP probably had that in mind

In my opinion it is only easy to confuse arrays and pointers if it is
badly taught. If arrays and pointers are taught as fundamentally
different concepts and *then* the way array names degenerate to pointers
to the first element is explained there will not be anything like the
problems. In one-to-one sessions I've been able to explain the basics of
arrays pointers to non-computer people in minutes (I needed to so I
could explain approximately what was the cause of a problem), although I
did not go on to how things are done in C.
--
Flash Gordon

**Keith Thompson** · Jun 30 '07, 08:35 PM

Re: Struct assignment

Flash Gordon <spam@flash-gordon.me.ukwri tes:

Grey Alien wrote, On 30/06/07 17:43:

>If I have the ff struct:
>struct A
>{
> unsigned int i;
> char s[LONG_ENOUGH];
>} a, b;
>And use them in code like this:
>a.i = 42 ;
>strcpy(a.s,"te st");
>b.i = 100 ;
>b = a ;
>at this point, a (bitwise?) copy of a is made to b.

>
It is not a bitwise copy, it is a copy of all the elements in the struct.

Yes -- and that can be, and commonly is, implemented as a bitwise copy
of the struct.

Suppose there's a gap between the members "i" and "s". After the
assignment "b = a;", the gaps in "a" and "b" may or may not have the
same contents. The assignment can be done either as a bitwise copy or
by copying the members one-by-one, leaving any gaps alone.

99% of the type, this doesn't matter because you're never going to
look at what's in the gaps anyway.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

**Flash Gordon** · Jun 30 '07, 09:05 PM

Re: Struct assignment

Richard Heathfield wrote, On 30/06/07 20:56:

Serve Lau said:
>
<snip>
>

>Its easy to see where the confusion comes from. There are situations
>where pointers degenerate into pointers ,

>
Pointers always degenerate into pointers.

No they don't, they stay as pointers ;-)

Knowing what Serve Lau must have intended I read it as "where arrays
degenerate...".
--
Flash Gordon

**Richard Heathfield** · Jun 30 '07, 10:35 PM

Re: Struct assignment

Flash Gordon said:

Richard Heathfield wrote, On 30/06/07 20:56:

>Serve Lau said:
>>
><snip>
>>

>>Its easy to see where the confusion comes from. There are situations
>>where pointers degenerate into pointers ,

>>
>Pointers always degenerate into pointers.

>
No they don't, they stay as pointers ;-)
>
Knowing what Serve Lau must have intended I read it as "where arrays
degenerate...".

I know. Actually, arrays never actually do that. We often say they do,
but we're just being lazy. What we really mean is that the name of an
array, when used in an expression, is often (indeed, *usually*) treated
as if it were a pointer to the array's first element. The array itself
never "degenerate s" (or "decays", as it is usually put) at all.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

**Flash Gordon** · Jun 30 '07, 10:35 PM

Re: Struct assignment

Keith Thompson wrote, On 30/06/07 21:30:

Flash Gordon <spam@flash-gordon.me.ukwri tes:

>Grey Alien wrote, On 30/06/07 17:43:

>>If I have the ff struct:
>>struct A
>>{
>> unsigned int i;
>> char s[LONG_ENOUGH];
>>} a, b;
>>And use them in code like this:
>>a.i = 42 ;
>>strcpy(a.s,"t est");
>>b.i = 100 ;
>>b = a ;
>>at this point, a (bitwise?) copy of a is made to b.

>It is not a bitwise copy, it is a copy of all the elements in the struct.

>
Yes -- and that can be, and commonly is, implemented as a bitwise copy
of the struct.
>
Suppose there's a gap between the members "i" and "s". After the
assignment "b = a;", the gaps in "a" and "b" may or may not have the
same contents. The assignment can be done either as a bitwise copy or
by copying the members one-by-one, leaving any gaps alone.
>
99% of the type, this doesn't matter because you're never going to
look at what's in the gaps anyway.

Where there are multiple representations for the same value the
representation could change. It is important for the OP to know this
IMHO so that s/he does not assume that memcmp can be used safely.
--
Flash Gordon

**Army1987** · Jul 1 '07, 08:05 AM

Re: Struct assignment

"Grey Alien" <grey@andromeda .comha scritto nel messaggio news:erydna87m6 gBFRvbRVnyjwA@b t.com...

If I have the ff struct:
>
struct A
{
unsigned int i;
char s[LONG_ENOUGH];
} a, b;
>
>
And use them in code like this:
>
a.i = 42 ;
strcpy(a.s,"tes t");
>
b.i = 100 ;
>
b = a ;
>
at this point, a (bitwise?) copy of a is made to b. Question is:
>
1). is b.s now ptr to a.s ? (I think so)

No, since it is an array, not a pointer.

If so, what happens if for instance variable 'a' goes out of scope (?)
>
2). Does the compiler generate an implicit "memcpy" or "memmove" behind the scenes when it sees an assignment like this?

It could, but it needn't. For example, padding bits needn't be
copied.

**Eric Sosman** · Jul 2 '07, 03:05 PM

Re: Struct assignment

Malcolm McLean wrote On 06/30/07 13:05,:

"Grey Alien" <grey@andromeda .comwrote in message
news:erydna87m6 gBFRvbRVnyjwA@b t.com...
>

>>If I have the ff struct:
>>
>>struct A
>>{
> unsigned int i;
> char s[LONG_ENOUGH];
>>} a, b;
>>
>>
>>And use them in code like this:
>>
>>a.i = 42 ;
>>strcpy(a.s,"t est");
>>
>>b.i = 100 ;
>>
>>b = a ;
>>
>>at this point, a (bitwise?) copy of a is made to b. Question is:
>>
>>1). is b.s now ptr to a.s ? (I think so)
>>If so, what happens if for instance variable 'a' goes out of scope (?)
>>
>>2). Does the compiler generate an implicit "memcpy" or "memmove" behind
>>the scenes when it sees an assignment like this (to avoid dangling ptrs)?
>>

>
The answer is 2. A structure assignment will cause a call to memcpy to be
made, or equivalent code emitted if the structure is small enough to make
this wasteful and the compiler is clever. [...]

The compiler *may* do this, but it is not required
to copy padding bytes and padding bits. Assignment only
copies the value; padding bytes and bits are not part of
the value of a struct, and might not be copied -- as they
would be if memcpy() or an equivalent were used.

--
Eric.Sosman@sun .com

**Old Wolf** · Jul 3 '07, 12:05 AM

Re: Struct assignment

On Jul 3, 2:59 am, Eric Sosman <Eric.Sos...@su n.comwrote:

The compiler *may* do this, but it is not required
to copy padding bytes and padding bits. Assignment only
copies the value; padding bytes and bits are not part of
the value of a struct, and might not be copied -- as they
would be if memcpy() or an equivalent were used.

Something that just occurred to me. Say we
have:
struct S
{
char s[10];
};

struct S s = { "hello" };
struct S t;
t = s;

Initializing an automatic array from a string
literal is only guaranteed to initialize the
characters of the string and the null terminator
- it could leave subsequent array entries (in
this case, indices 6 through 9) uninitialized.

So does the assignment 't = s' cause undefined
behaviour by copying the values of uninitialized
array members?

**Robert Gamble** · Jul 3 '07, 12:25 AM

Re: Struct assignment

On Jul 2, 8:02 pm, Old Wolf <oldw...@inspir e.net.nzwrote:

On Jul 3, 2:59 am, Eric Sosman <Eric.Sos...@su n.comwrote:
>

The compiler *may* do this, but it is not required
to copy padding bytes and padding bits. Assignment only
copies the value; padding bytes and bits are not part of
the value of a struct, and might not be copied -- as they
would be if memcpy() or an equivalent were used.

>
Something that just occurred to me. Say we
have:
struct S
{
char s[10];
};
>
struct S s = { "hello" };
struct S t;
t = s;
>
Initializing an automatic array from a string
literal is only guaranteed to initialize the
characters of the string and the null terminator
- it could leave subsequent array entries (in
this case, indices 6 through 9) uninitialized.

Actually you are wrong about that, 9899:1999 §6.7.8p21:

"If there are fewer initializers in a brace-enclosed list than there
are elements or members
of an aggregate, or fewer characters in a string literal used to
initialize an array of known
size than there are elements in the array, the remainder of the
aggregate shall be
initialized implicitly the same as objects that have static storage
duration."

Robert Gamble

Struct assignment

Struct assignment

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