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C Pointer problem

Hi,

I can't understand why this code causes a "memory read exception" at
int x=**a;

void pass(int** a)
{
int x=**a;
}
void main()
{
int arr[2][2]={{1,2},{3,4}};
pass(arr);
}
The assignment of int int x=**a in the main function works.

Tanks for your help,
Markus

May 31 '06 #1
73 3822
Markus said:
Hi,

I can't understand why this code causes a "memory read exception" at
int x=**a;

void pass(int** a)
{
int x=**a;
}
void main()
main returns int.

If you can't even get the entry point right, what chance do you stand with
pointers?
{
int arr[2][2]={{1,2},{3,4}};
pass(arr);


pass() takes int **.

arr's value is taken as the address of its first element. Its first element
is an int[2] array, so the address of its first element has type int
(*)[2], which is not the same as int **.
--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)
May 31 '06 #2
Markus wrote:
Hi,

I can't understand why this code causes a "memory read exception" at
int x=**a;

void pass(int** a)
{
int x=**a;
}
void main()
{
int arr[2][2]={{1,2},{3,4}};
pass(arr);
}
The assignment of int int x=**a in the main function works.


Did this really compile without any warnings or errors?

Brian
May 31 '06 #3
Default User said:
Markus wrote:
Hi,

I can't understand why this code causes a "memory read exception" at
int x=**a;

void pass(int** a)
{
int x=**a;
}
void main()
{
int arr[2][2]={{1,2},{3,4}};
pass(arr);
}
The assignment of int int x=**a in the main function works.


Did this really compile without any warnings or errors?


No.

foo.c:2: warning: no previous prototype for `pass'
foo.c: In function `pass':
foo.c:3: warning: unused variable `x'
foo.c: At top level:
foo.c:6: warning: function declaration isn't a prototype
foo.c:6: warning: return type of `main' is not `int'
foo.c: In function `main':
foo.c:8: warning: passing arg 1 of `pass' from incompatible pointer type

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)
May 31 '06 #4
Richard Heathfield a écrit :
foo.c:2: warning: no previous prototype for `pass'
foo.c: In function `pass':
foo.c:3: warning: unused variable `x'
foo.c: At top level:
foo.c:6: warning: function declaration isn't a prototype
foo.c:6: warning: return type of `main' is not `int'
foo.c: In function `main':
foo.c:8: warning: passing arg 1 of `pass' from incompatible pointer type


lcc-win32 has:
D:\lcc\mc66\tes t>lcc -A tw2.c
Warning tw2.c: 3 x is assigned a value that is never used
Warning tw2.c: 6 old-style function definition for 'main'
Warning tw2.c: 6 missing prototype for 'main'
Warning tw2.c: 6 'void main()' is a non-ANSI definition
Warning tw2.c: 8 assignment of pointer to array 2 of int to pointer to
pointer to int
0 errors, 5 warnings

Basically all warnings are the same, with wording differences.
The first warning of gcc however, is not clear to me:

foo.c:2: warning: no previous prototype for `pass'

Why is that an eror?

The function definition is correct, and it wasn't
previously used. That looks correct to me.
May 31 '06 #5
Richard Heathfield wrote:
Default User said:
Markus wrote:
The assignment of int int x=**a in the main function works.


Did this really compile without any warnings or errors?


No.


Of course. It was a nudge to the OP to let him know that the compiler
doesn't issue diagnostics (just) because it doesn't like you.


Brian
May 31 '06 #6
2006-05-31 <44************ ***********@new s.wanadoo.fr>, jacob navia wrote:
Richard Heathfield a écrit :
foo.c:2: warning: no previous prototype for `pass'
foo.c: In function `pass':
foo.c:3: warning: unused variable `x'
foo.c: At top level:
foo.c:6: warning: function declaration isn't a prototype
foo.c:6: warning: return type of `main' is not `int'
foo.c: In function `main':
foo.c:8: warning: passing arg 1 of `pass' from incompatible pointer type

lcc-win32 has:
D:\lcc\mc66\tes t>lcc -A tw2.c
Warning tw2.c: 3 x is assigned a value that is never used
Warning tw2.c: 6 old-style function definition for 'main'


While void main() is a lot of things (none of them good), it is not an
old-style definition. Warning on empty brackets might make sense for
a declaration without a definition, it does NOT make sense for
a definition.
Warning tw2.c: 6 missing prototype for 'main'
Warning tw2.c: 6 'void main()' is a non-ANSI definition
Warning tw2.c: 8 assignment of pointer to array 2 of int to pointer to
pointer to int
0 errors, 5 warnings

Basically all warnings are the same, with wording differences.
The first warning of gcc however, is not clear to me:

foo.c:2: warning: no previous prototype for `pass'

Why is that an eror?

The function definition is correct, and it wasn't
previously used. That looks correct to me.

May 31 '06 #7
Jordan Abel wrote:
2006-05-31 <44************ ***********@new s.wanadoo.fr>, jacob navia wrote:

lcc-win32 has:
D:\lcc\mc66\tes t>lcc -A tw2.c
Warning tw2.c: 3 x is assigned a value that is never used
Warning tw2.c: 6 old-style function definition for 'main'
While void main() is a lot of things (none of them good), it is not an
old-style definition.


int main() is part of an an old style definition, or at least a
definition
that's been in style for a long time. ;)
Warning on empty brackets might make sense for
a declaration without a definition, it does NOT make sense for
a definition.


Compare...

int foo() { }

int main()
{
foo(42); /* no diagnostic required */
return 0;
}

With...

int foo(void) { }

int main()
{
foo(42); /* diagnostic required */
return 0;
}

--
Peter

May 31 '06 #8
On 31 May 2006 09:42:51 -0700, "Markus" <ma***@punjabi. net> wrote:
Hi,

I can't understand why this code causes a "memory read exception" at
int x=**a;
Because you lied to the compiler. Arrays are not pointers and
pointers are not arrays.

void pass(int** a)
{
int x=**a;
a is a pointer to pointer to int. To do determine the value of the
int itself, the following steps are required:

1 - Determine the value of a. It was passed "by value" to the
function using whatever calling convention is appropriate for your
system so this pretty straight forward.

2 - Use this value as the address of a pointer to int.

3 - Determine the value of this pointer.

4 - Use this value as the address of the int.

5 - Determine the value of the int.

6 - Store this value in x. (Not part of determining the value
but the concluding step in the initialization. )
}
void main()
int main(void) please.{
int arr[2][2]={{1,2},{3,4}};
pass(arr);
Since this statement has a syntax error and will not compile cleanly,
why did you bother to execute the code at all. If you did not see the
mandatory diagnostic, you need to up the warning level on your
compiler.

You pass a 2D array to the function. In this context, the array name
evaluates to the address of the first element with type pointer to
first element. In other words, this is identical to coding
pass(&arr[0]);

arr[0] is itself an array of 2 int. A pointer to arr[0] has type
pointer to array of 2 int, written as int (*)[2].

The syntax error is because an int(*)[2] is incompatible with an
int**. There is no implicit conversion between the two.

Apparently on your system the two pointer types have similar
representations (not uncommon). pass will take the value and treat it
as the address of a pointer to int (steps 1 and 2 above).

Undefined behavior #1. Since the address is actually the
address of arr[0] which is an array of 2 int, it is really the address
of arr[0][0], a normal int. You have no idea if the alignment of this
int is suitable for a pointer to int.

Undefined behavior #2. pass will attempt to use the value at
this location as the address of an int (steps 3 and 4 above). You
have no idea if the value of the int that is actually at that location
is a valid address. It could be an invalid address or even a trap
representation.

Undefined behavior #3. pass will attempt to extract the int
value at this "address" (step 5 above). Since the value at this new
address was an int and not an address to begin with, attempting to
dereference it should be an obvious no-no. This is probably where
your system decided enough was enough.
}
The assignment of int int x=**a in the main function works.
There is no a in main. Perhaps you meant
int x = **arr;

This would work because the compiler know arr is a 2D array. *arr is
identical to arr[0] which is a1D array. **arr is identical to *arr[0]
which is identical to arr[0][0] which is the first int in the 1D array
and a perfectly valid value to assign to the int x.

Tanks for your help,
Markus

Remove del for email
Jun 1 '06 #9
2006-05-31 <11************ *********@c74g2 000cwc.googlegr oups.com>, Peter Nilsson wrote:
Jordan Abel wrote:
2006-05-31 <44************ ***********@new s.wanadoo.fr>, jacob navia wrote:
>
> lcc-win32 has:
> D:\lcc\mc66\tes t>lcc -A tw2.c
> Warning tw2.c: 3 x is assigned a value that is never used
> Warning tw2.c: 6 old-style function definition for 'main'


While void main() is a lot of things (none of them good), it is not an
old-style definition.


int main() is part of an an old style definition, or at least a
definition
that's been in style for a long time. ;)
Warning on empty brackets might make sense for
a declaration without a definition, it does NOT make sense for
a definition.


Compare...

int foo() { }

int main()
{
foo(42); /* no diagnostic required */


False. The standard makes a very clear distinction between empty
brackets in a declaration not part of a definition, and empty brackets
that _are_ in part of a definition. And the latter is EXACTLY equivalent
to (void).
Jun 1 '06 #10

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