Disabling "incompatible pointer type" warning.

**Bob Hairgrove** · Mar 29 '06, 12:05 PM

Re: Disabling "incompati ble pointer type" warning.

On 29 Mar 2006 01:55:06 -0800, "PraZ" <prasanna.sethu raman@patni.com >
wrote:
[color=blue]
>Hi all.
>
>Here is a simple code, which when compiled with gcc results in the
>warning "incompatib le pointer type" for arg 1, as expected. But this is
>just what I want to do, because it makes it easy for me to handle the
>single dimensional stream I have as a multidimensiona l array inside the
>function func(). Now, I am just wondering if there is a way by which I
>can disable this incompatible pointer type warning in gcc. Any
>suggestions?
>
>Best regards,
>Prasanna.
>
>/*The Code*/
>#include <stdio.h>
>
>void func(char [2][2]);
>
>int main()
>{
> char* src;
> int i;
>
> src = (char*)malloc(1 6);
>
> for (i=0; i<16; i++)
> src[i] = i;
>
> func(src+4);
>
>return 1;
>}
>
>void func(char src[2][2])
>{
> int r, c;
>
> for (r = 0; r < 2; r++)
> {
> for (c = 0; c < 2; c++)
> {
> printf("%d, ", src[r][c]);
> }
> printf("\n");
> }
>}[/color]

A cast is necessary:
func((char (*)[2])src+4);

But why do you need to do it this way? Also, you leak memory in main.
And it's not C++ code if you are using malloc and free.

--
Bob Hairgrove
NoSpamPlease@Ho me.com

**Ivan Vecerina** · Mar 29 '06, 12:05 PM

Re: Disabling "incompati ble pointer type" warning.

"PraZ" <prasanna.sethu raman@patni.com > wrote in message
news:1143626106 .137290.294600@ e56g2000cwe.goo glegroups.com.. .
: Here is a simple code, which when compiled with gcc results in the
: warning "incompatib le pointer type" for arg 1, as expected. But this is
: just what I want to do, because it makes it easy for me to handle the
: single dimensional stream I have as a multidimensiona l array inside the
: function func(). Now, I am just wondering if there is a way by which I
: can disable this incompatible pointer type warning in gcc. Any
: suggestions?
....
: void func(char [2][2]);
....
: char* src;
....
: func(src+4);

You can explicitly cast the parameter to the appropriate type:
func( (char(*)[2]) (src+4) );

But to me, it looks like func() might well be inapropriately
exposing implementation details in its interface.
If any 4-char sequence is acceptable as an input, the interface
should not expose a [2][2] bidimensional array to its callers.

If it is really useful for the implementation, func() could
create a local variable of the desired type:
void func( char const src[4] ) // include const if not modified
{
char (*tab)[2] = (char(*)[2])src;
...use tab as needed...
}

Regards -Ivan
--
http://ivan.vecerina.com/contact/?subject=NG_POST <- email contact form
Brainbench MVP for C++ <> http://www.brainbench.com

Me · Mar 29 '06, 01:45 PM

Re: Disabling "incompati ble pointer type" warning.

PraZ wrote:[color=blue]
> Here is a simple code, which when compiled with gcc results in the
> warning "incompatib le pointer type" for arg 1, as expected. But this is
> just what I want to do, because it makes it easy for me to handle the
> single dimensional stream I have as a multidimensiona l array inside the
> function func(). Now, I am just wondering if there is a way by which I
> can disable this incompatible pointer type warning in gcc. Any
> suggestions?
>
> void func(char src[2][2])
> {
> for (int r = 0; r < 2; r++) {
> for (int c = 0; c < 2; c++) {
> printf("%d, ", src[r][c]);
> }
> printf("\n");
> }
> }
>
> char *src = (char*)malloc(1 6);
> func(src+4);[/color]

Working with multidimentiona l arrays in C/C++ really sucks and it's
made worse by the fact that doing a cast like what people in this
thread suggest leads to undefined behavior due to aliasing. The most
flexible and least annoying way to work with multidimentiona l arrays is
to just bite the bullet and do it yourself:

void func(T *arr, size_t width, size_t height, size_t pitch)
{
for (size_t h = 0; h < height; ++h) {
for (size_t w = 0; w < width; ++w) {
do_stuff(arr[w]);
}
arr += pitch;
}
}

Or a few variations:
- you can get rid of pitch and just use the width, I don't recommend
this because it's extremely useful for handling smaller slices of a
larger matrix.
- you can make the pitch variable signed (like ptrdiff_t) so you can
pass it negative values for it to travel upwards instead of downwards.
I don't really like this but I have seen people use it when working
with bitmaps with the y-axis pointing the opposite direction than what
they were expecting.

**Rolf Magnus** · Mar 29 '06, 06:05 PM

Re: Disabling "incompati ble pointer type" warning.

PraZ wrote:
[color=blue]
> Hi all.
>
> Here is a simple code, which when compiled with gcc results in the
> warning "incompatib le pointer type" for arg 1, as expected. But this is
> just what I want to do, because it makes it easy for me to handle the
> single dimensional stream I have as a multidimensiona l array inside the
> function func(). Now, I am just wondering if there is a way by which I
> can disable this incompatible pointer type warning in gcc.[/color]

By writing correct code?
[color=blue]
> Any suggestions?
>
> Best regards,
> Prasanna.
>
> /*The Code*/
> #include <stdio.h>
>
> void func(char [2][2]);[/color]

This is equivalent to:

void func(char (*)[2]);

i.e. a function that takes a pointer to an array of 2 chars.
[color=blue]
> int main()
> {
> char* src;
> int i;
>
> src = (char*)malloc(1 6);[/color]

Don't use malloc. Use new[].

Btw: you forgot to #include the header that contains the declaration for
malloc(): stdlib.h
[color=blue]
>
> for (i=0; i<16; i++)
> src[i] = i;
>
> func(src+4);[/color]

Your dynamically allocated memory is never freed.
[color=blue]
> return 1;[/color]

Why 1? Standard C++ defines three possible return values, which are 0,
EXIT_SUCCESS (being equivalent to 0) and EXIT_FAILURE. Most systems support
more than that, but typically, anything except 0 means failure.
[color=blue]
> }[/color]

I'd probably do:

int main()
{
char (*src)[2] = new char[8][2];

for (int i=0; i<8; i++)
{
src[i][0] = i*2;
src[i][1] = i*2+1;
}

func(src+2);

delete [] src;

return 0;
}
[color=blue]
> void func(char src[2][2])
> {
> int r, c;
>
> for (r = 0; r < 2; r++)
> {
> for (c = 0; c < 2; c++)
> {
> printf("%d, ", src[r][c]);
> }
> printf("\n");
> }
> }[/color]

**Fei Liu** · Mar 30 '06, 01:15 AM

Re: Disabling "incompati ble pointer type" warning.

"Me" <anti_spam_emai l2003@yahoo.com > wrote in message[color=blue]
> Working with multidimentiona l arrays in C/C++ really sucks and it's
> made worse by the fact that doing a cast like what people in this
> thread suggest leads to undefined behavior due to aliasing. The most
> flexible and least annoying way to work with multidimentiona l arrays is
> to just bite the bullet and do it yourself:
>
> void func(T *arr, size_t width, size_t height, size_t pitch)
> {
> for (size_t h = 0; h < height; ++h) {
> for (size_t w = 0; w < width; ++w) {
> do_stuff(arr[w]);
> }
> arr += pitch;
> }
> }
>
> Or a few variations:
> - you can get rid of pitch and just use the width, I don't recommend
> this because it's extremely useful for handling smaller slices of a
> larger matrix.
> - you can make the pitch variable signed (like ptrdiff_t) so you can
> pass it negative values for it to travel upwards instead of downwards.
> I don't really like this but I have seen people use it when working
> with bitmaps with the y-axis pointing the opposite direction than what
> they were expecting.
>[/color]

Use std::valarray, boost::multiarr ay, blitz::array. These implementations
all support multi dimensional arrays.

**Gernot Frisch** · Mar 30 '06, 09:45 AM

Re: Disabling "incompati ble pointer type" warning.

[color=blue]
> Why 1? Standard C++ defines three possible return values, which are
> 0,
> EXIT_SUCCESS (being equivalent to 0) and EXIT_FAILURE. Most systems
> support
> more than that, but typically, anything except 0 means failure.[/color]

Because of the memory leak? ;)

Disabling "incompatible pointer type" warning.

Disabling "incompatible pointer type" warning.

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