Allocating space for array of pointers?

[Paminu]

If I have this struct:

#include <stdlib.h>
#include <stdio.h>
#define KIDS 4

typedef struct test {
int x;
int y;
} container;


I would now like to make an array containing 4 pointers to this struct. But
I would also like to allocate space for this array:

int main(void)
{
container *new[4];
new=malloc(sizeof(container *) * 4); // THIS LINE GIVES AN ERROR!

return 0;
}

Do I have to allocate space for each pointer at a time?

 

[Artie Gold]

If I have this struct:

#include <stdlib.h>
#include <stdio.h>
#define KIDS 4

typedef struct test {
int x;
int y;
} container;


I would now like to make an array containing 4 pointers to this struct. But
I would also like to allocate space for this array:

int main(void)
{
container *new[4];
new=malloc(sizeof(container *) * 4); // THIS LINE GIVES AN ERROR!

return 0;
}

And what might that error be? (Looks OK to me...)

Do I have to allocate space for each pointer at a time?

No.

HTH,
--ag

 

[Paminu]

If I have this struct:

#include <stdlib.h>
#include <stdio.h>
#define KIDS 4

typedef struct test {
int x;
int y;
} container;


I would now like to make an array containing 4 pointers to this struct.
But I would also like to allocate space for this array:

int main(void)
{
container *new[4];
new=malloc(sizeof(container *) * 4); // THIS LINE GIVES AN ERROR!

return 0;
}

And what might that error be? (Looks OK to me...)

test4.c:13: error: incompatible types in assignment

where line 13 contains:
new=malloc(sizeof(container *) * 4); // THIS LINE GIVES AN ERROR!



I have instead tried:

container *new[4];
int j;
for (j = 0; j < 4; j++)
{
new[j]=malloc(sizeof(container));
new[j]=NULL;

}

Then I get no error!

 

[Martin Ambuhl]

If I have this struct:

#include <stdlib.h>
#include <stdio.h>
#define KIDS 4

typedef struct test {
int x;
int y;
} container;


I would now like to make an array containing 4 pointers to this struct. But
I would also like to allocate space for this array:

int main(void)
{
container *new[4];
new=malloc(sizeof(container *) * 4); // THIS LINE GIVES AN ERROR!

As it should. You have already declared new as an array[4] of pointers
to container.

{
size_t i;
for (i = 0; i < 4; i++)
if (!(new = malloc(sizeof *new)))
{ /* handle error */ }
}

return 0;
}

Do I have to allocate space for each pointer at a time?

No, you need to allocate space for what each pointer points to.

 

[Kenneth Brody]

Artie Gold wrote:
[...]

container *new[4];
new=malloc(sizeof(container *) * 4); // THIS LINE GIVES AN ERROR!

[...]

And what might that error be? (Looks OK to me...)

"new" is an array of 4 pointers to "container", not a pointer to an
array of 4 "container"s.

No.

It depends. If you want "new" to be an array of 4 pointers, then yes:

container *new[4]; /* As in the original code. */

/* Allocate each pointer. This can also be in a for-loop */
new[0] = malloc(sizeof(container));
new[1] = malloc(sizeof(container));
new[2] = malloc(sizeof(container));
new[3] = malloc(sizeof(container));

If you want it to point to an array of 4 containers:

container *new;
new = malloc(sizeof(container)*4);


BTW, this is a perfect example of how C++ is not a "superset" of C.

--
+-------------------------+--------------------+-----------------------------+
| Kenneth J. Brody | www.hvcomputer.com | |
| kenbrody/at\spamcop.net | www.fptech.com | #include <std_disclaimer.h> |
+-------------------------+--------------------+-----------------------------+
Don't e-mail me at: <mailto:[email protected]>

 

[Artie Gold]

Artie Gold wrote:
[...]

container *new[4];
new=malloc(sizeof(container *) * 4); // THIS LINE GIVES AN ERROR!

[...]

And what might that error be? (Looks OK to me...)

Of course, a ludicrous example of cerebral flatulence on m part...
[snip]

--ag

 

[Mark McIntyre]

container *new[4];
new=malloc(sizeof(container *) * 4); // THIS LINE GIVES AN ERROR!

test4.c:13: error: incompatible types in assignment

"new" is an array of four pointers. You can't assign directly to the
array.

I have instead tried:
new[j]=malloc(sizeof(container));

yes, thats ok, because new[j] is a pointer.
Mark McIntyre