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Array definition and extern declaration with different size.

Hi,

I have an array defined in one file with an intializer as follows:

int arr[] = {0, 1, 2, 3};

I have a declaration of the array in another file as follows:

extern int arr[10];

This compiles without a problem on my implementation and arr ends up
being of size 10 on my implementation. Is this legal in "standard C"? I
was reading question 1.24 of the FAQ but that didn't seem to answer my
question in this case. Is the array definition with the initializer
somehow an incomplete definition?

Thanks for the help,

Josh

Jul 31 '06 #1
9 8416
I think it compiles OK, but may cause segmentation fault when run.
"joshc" <jo********@gmail.com>
??????:11*********************@m79g2000cwm.googleg roups.com...
Hi,

I have an array defined in one file with an intializer as follows:

int arr[] = {0, 1, 2, 3};

I have a declaration of the array in another file as follows:

extern int arr[10];

This compiles without a problem on my implementation and arr ends up
being of size 10 on my implementation. Is this legal in "standard C"? I
was reading question 1.24 of the FAQ but that didn't seem to answer my
question in this case. Is the array definition with the initializer
somehow an incomplete definition?

Thanks for the help,

Josh

Jul 31 '06 #2
On 30 Jul 2006 20:48:10 -0700, "joshc" <jo********@gmail.comwrote in
comp.lang.c:
Hi,

I have an array defined in one file with an intializer as follows:

int arr[] = {0, 1, 2, 3};

I have a declaration of the array in another file as follows:

extern int arr[10];

This compiles without a problem on my implementation and arr ends up
being of size 10 on my implementation. Is this legal in "standard C"? I
was reading question 1.24 of the FAQ but that didn't seem to answer my
question in this case. Is the array definition with the initializer
somehow an incomplete definition?

Thanks for the help,

Josh
Any size specified in the external declaration is ignored by the
compiler. The array contains four ints. Any attempt to access past
arr [3] produces undefined behavior.

This is the same as using a size in an array style parameter
definition (an unfortunate choice anyway), with the exception of the
C99 feature where the "static" keyword is used.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html
Jul 31 '06 #3
Jack Klein wrote:
On 30 Jul 2006 20:48:10 -0700, "joshc" <jo********@gmail.comwrote in
comp.lang.c:
Hi,

I have an array defined in one file with an intializer as follows:

int arr[] = {0, 1, 2, 3};

I have a declaration of the array in another file as follows:

extern int arr[10];

This compiles without a problem on my implementation and arr ends up
being of size 10 on my implementation. Is this legal in "standard C"? I
was reading question 1.24 of the FAQ but that didn't seem to answer my
question in this case. Is the array definition with the initializer
somehow an incomplete definition?

Thanks for the help,

Josh

Any size specified in the external declaration is ignored by the
compiler. The array contains four ints. Any attempt to access past
arr [3] produces undefined behavior.

This is the same as using a size in an array style parameter
definition (an unfortunate choice anyway), with the exception of the
C99 feature where the "static" keyword is used.
That may be what happens for some compilers, but it is not what the
standard says, and if I recall correctly, there are implementations
that would display a warning or error message for such code during
linking.

n1124 6.2.7#2:
"All declarations that refer to the same object or function shall have
compatible type; otherwise, the behavior is undefined."

n1124 6.7.5.2#6:
"For two array types to be compatible, both shall have compatible
element types, and if both size specifiers are present, and are
integer constant expressions, then both size specifiers shall have the
same constant value. If the two array types are used in a context which
requires them to be compatible, it is undefined behavior if the two
size specifiers evaluate to unequal values."

Jul 31 '06 #4
Harald van Dijk wrote:
n1124 6.2.7#2:
"All declarations that refer to the same object or function shall have
compatible type; otherwise, the behavior is undefined."

n1124 6.7.5.2#6:
"For two array types to be compatible, both shall have compatible
element types, and if both size specifiers are present, and are
integer constant expressions, then both size specifiers shall havethe
same constant value. If the two array types are used in a context which
requires them to be compatible, it is undefined behavior if the two
size specifiers evaluate to unequal values."
What does this tell us for the case where one of the size specifiers
is not present ?

Jul 31 '06 #5
Harald van Dijk wrote:
Jack Klein wrote:
On 30 Jul 2006 20:48:10 -0700, "joshc" <jo********@gmail.comwrote in
comp.lang.c:
Hi,
>
I have an array defined in one file with an intializer as follows:
>
int arr[] = {0, 1, 2, 3};
>
I have a declaration of the array in another file as follows:
>
extern int arr[10];
>
This compiles without a problem on my implementation and arr ends up
being of size 10 on my implementation. Is this legal in "standard C"?I
was reading question 1.24 of the FAQ but that didn't seem to answer my
question in this case. Is the array definition with the initializer
somehow an incomplete definition?
>
Thanks for the help,
>
Josh
Any size specified in the external declaration is ignored by the
compiler. The array contains four ints. Any attempt to access past
arr [3] produces undefined behavior.

This is the same as using a size in an array style parameter
definition (an unfortunate choice anyway), with the exception of the
C99 feature where the "static" keyword is used.

That may be what happens for some compilers, but it is not what the
standard says, and if I recall correctly, there are implementations
that would display a warning or error message for such code during
linking.

n1124 6.2.7#2:
"All declarations that refer to the same object or function shall have
compatible type; otherwise, the behavior is undefined."

n1124 6.7.5.2#6:
"For two array types to be compatible, both shall have compatible
element types, and if both size specifiers are present, and are
integer constant expressions, then both size specifiers shall havethe
same constant value. If the two array types are used in a context which
requires them to be compatible, it is undefined behavior if the two
size specifiers evaluate to unequal values."
Hmm. Re-reading it, though, I think you may technically be right for
this particular case, but not for "int arr[4] = {0, 1, 2, 3};"

Both 'int arr[]' and 'extern int arr[10]' have compatible (identical,
even) element type, and even though "int arr[] = {0, 1, 2, 3};"
declares arr as int [4] (per 6.7.8#22), it is not defined with a size
specifier (a syntactic construct), so it seems the behaviour is not
actually undefined. Maybe comp.std.c can help out.

Jul 31 '06 #6
sp****@gmail.com wrote:
Harald van Dijk wrote:
n1124 6.2.7#2:
"All declarations that refer to the same object or function shall have
compatible type; otherwise, the behavior is undefined."

n1124 6.7.5.2#6:
"For two array types to be compatible, both shall have compatible
element types, and if both size specifiers are present, and are
integer constant expressions, then both size specifiers shall have the
same constant value. If the two array types are used in a context which
requires them to be compatible, it is undefined behavior if the two
size specifiers evaluate to unequal values."

What does this tell us for the case where one of the size specifiers
is not present ?
Yeah, I noticed that too after posting. "int arr[] = { 0, 1, 2, 3 };"
defines arr as int[4], and (&arr) + 1 is allowed, though, so it's meant
to be undefined, I think, but it actually isn't in this case.

Jul 31 '06 #7

Jack Klein wrote:
On 30 Jul 2006 20:48:10 -0700, "joshc" <jo********@gmail.comwrote in
comp.lang.c:
Hi,

I have an array defined in one file with an intializer as follows:

int arr[] = {0, 1, 2, 3};

I have a declaration of the array in another file as follows:

extern int arr[10];

This compiles without a problem on my implementation and arr ends up
being of size 10 on my implementation. Is this legal in "standard C"? I
was reading question 1.24 of the FAQ but that didn't seem to answer my
question in this case. Is the array definition with the initializer
somehow an incomplete definition?

Thanks for the help,

Josh

Any size specified in the external declaration is ignored by the
compiler. The array contains four ints. Any attempt to access past
arr [3] produces undefined behavior.

This is the same as using a size in an array style parameter
definition (an unfortunate choice anyway), with the exception of the
C99 feature where the "static" keyword is used.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html
Thanks, Jack. I figured it was probably just how my compiler behaved
but I was surprised that the space allocated for the array was based on
the external declaration. I was expecting a warning but I guess that's
not required.

Thanks again.

Jul 31 '06 #8
On Mon, 31 Jul 2006 03:48:10 UTC, "joshc" <jo********@gmail.com>
wrote:
Hi,

I have an array defined in one file with an intializer as follows:

int arr[] = {0, 1, 2, 3};
This defines an array of 4 members. When the number of members defined
through the initialisers is too low you must define the real size to
get the list of initialisers expanded to the real number of members
you needs.
I have a declaration of the array in another file as follows:

extern int arr[10];
The best way would be:

insert the declaration into an header file and include it even on the
file you has to write the definition.

This will give the compiler the chance to extend the definition [] to
the size the declaration defines. On the other hand you can simply use
'extern int arr[]' in other translation units.
So 'int arr[ARR_SIZE] = { [incomplete] list of initialisers }; and
'extern arr[ARR_SIZE]' in a common header while ARR_SIZE describes the
size you needs really will give you in any place the chance to change
the arraysize only on one place.
This compiles without a problem on my implementation and arr ends up
being of size 10 on my implementation. Is this legal in "standard C"? I
was reading question 1.24 of the FAQ but that didn't seem to answer my
question in this case. Is the array definition with the initializer
somehow an incomplete definition?

Thanks for the help,

Josh

--
Tschau/Bye
Herbert

Visit http://www.ecomstation.de the home of german eComStation
eComStation 1.2 Deutsch ist da!
Jul 31 '06 #9
sp****@gmail.com wrote:
>
Harald van D??k wrote:
>>
n1124 6.7.5.2#6:
"For two array types to be compatible, both shall have compatible
element types, and if both size specifiers are present, and are
integer constant expressions, then both size specifiers shall have the
same constant value. If the two array types are used in a context which
requires them to be compatible, it is undefined behavior if the two
size specifiers evaluate to unequal values."

What does this tell us for the case where one of the size specifiers
is not present ?
It tells you that the committee isn't infallible (in case you hadn't
already discovered that). I'm sure the intent is that a size imputed
from an initialization behave just like a constant size, but the
document doesn't actually say so.

-Larry Jones

Don't you hate it when your boogers freeze? -- Calvin
Aug 1 '06 #10

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