structs and malloc

[Lars Tackmann]

Hi, i have the following type and struct defined

typedef struct hNode {
unsigned char name[1024];
unsigned long hVal;
struct hNode *next;
} HashNode;

what i want, is to ensure that when doing:

HashNode aNode = malloc(sizeof(H ashNode));

i can be sure of the value of "next" in the aNode (eg. later in the
program i want to compare the value of "next" in aNode to see if it has
been linked to another HashNode) --- can i do this in
some easy way or do i need to write my own memory allocator ???.
Thanks.

[Alexander Bartolich]

begin followup to Lars Tackmann:

what i want, is to ensure that when doing:

HashNode aNode = malloc(sizeof(H ashNode)); ^
Something is missing here.
i can be sure of the value of "next" in the aNode
Well, you can use calloc instead of malloc to set all bytes of the
allocated memory to zero. On most platforms that will get you a
null-pointer (and on the one that uses a different representation
you will get a horrible, hard to track runtime error).
[...] can i do this in some easy way or do i need to write my own
memory allocator ???.

Well, you need to do two things:

HashNode* aNode = malloc(sizeof(* aNode));
aNode->next = 0;

If you want to group these two lines into one function, fine with me.

HashNode* new_HashNode(vo id)
{
HashNode* aNode = malloc(sizeof(* aNode));
aNode->next = 0;
return aNode;
}

--
Für Google, Tux und GPL!

[nrk]

Alexander Bartolich wrote:

begin followup to Lars Tackmann:

what i want, is to ensure that when doing:

HashNode aNode = malloc(sizeof(H ashNode)); ^
Something is missing here.

i can be sure of the value of "next" in the aNode

Well, you can use calloc instead of malloc to set all bytes of the
allocated memory to zero. On most platforms that will get you a
null-pointer (and on the one that uses a different representation
you will get a horrible, hard to track runtime error).

[...] can i do this in some easy way or do i need to write my own
memory allocator ???.

Well, you need to do two things:

HashNode* aNode = malloc(sizeof(* aNode));
aNode->next = 0;

If you want to group these two lines into one function, fine with me.

HashNode* new_HashNode(vo id)
{
HashNode* aNode = malloc(sizeof(* aNode));
aNode->next = 0;

ITYM:
if ( aNode ) aNode->next = 0;
return aNode;
}

-nrk.

[Lars Tackmann]

I think that my example was bad - i need a to be able to allocate a entire
array of hash nodes and be sure that all of the nodes has a sane value
from the start.

eg.

HashNode *hashTable = malloc(sizeof(h ashNode) * 100);

Thanks

[Tobias Oed]

Lars Tackmann wrote:

I think that my example was bad - i need a to be able to allocate a entire
array of hash nodes and be sure that all of the nodes has a sane value
from the start.

eg.

HashNode *hashTable = malloc(sizeof(h ashNode) * 100);

I'm confused. If you want an array why do you use a linked list?
Tobias.

[Malcolm]

"Lars Tackmann" <ro****@diku.dk > wrote in message

i need a to be able to allocate a entire array of hash nodes and be sure
that all of the nodes has a sane value from the start.

eg.

HashNode *hashTable = malloc(sizeof(h ashNode) * 100);

malloc() returns a pointer to memory containing garbage. To help makes bugs
more trackable, some implementations set this to a definite bit-pattern,
whilst others don't.

calloc() returns memory set to all bits zero. It is a false friend. On 99%
of systems a pointer with all bits zero is NULL, but there are a few
implementations where this is not true.

What you need to do is to call malloc(), then iterate over the array. For
instance, to create a linked list.

for(i=0;i<99;i+ +)
hashTable[i].next = &hashtable[i+1];
hashTable[99].next = NULL;

[Lars Tackmann]

Yep this is what i need - thanks.
On Sun, 21 Dec 2003, Malcolm wrote:

"Lars Tackmann" <ro****@diku.dk > wrote in message

i need a to be able to allocate a entire array of hash nodes and be sure
that all of the nodes has a sane value from the start.

eg.

HashNode *hashTable = malloc(sizeof(h ashNode) * 100);

malloc() returns a pointer to memory containing garbage. To help makes bugs
more trackable, some implementations set this to a definite bit-pattern,
whilst others don't.

calloc() returns memory set to all bits zero. It is a false friend. On 99%
of systems a pointer with all bits zero is NULL, but there are a few
implementations where this is not true.

What you need to do is to call malloc(), then iterate over the array. For
instance, to create a linked list.

for(i=0;i<99;i+ +)
hashTable[i].next = &hashtable[i+1];
hashTable[99].next = NULL;

[Lars Tackmann]

On Sun, 21 Dec 2003, Tobias Oed wrote:

Lars Tackmann wrote:

I'm confused. If you want an array why do you use a linked list?
Tobias.

In a hash table you usaly have the problem that more than one items hashes
to the same value -. If this happens you link the ones toghter that have
the same hash value (creating a bucket - that you end up searching linary).

That is why i need an array of items that also can be linked to other items.

[The Real OS/2 Guy]

On Sun, 21 Dec 2003 19:32:18 UTC, Lars Tackmann <ro****@diku.dk >
wrote:

Hi, i have the following type and struct defined

typedef struct hNode {
unsigned char name[1024];
unsigned long hVal;
struct hNode *next;
} HashNode;

what i want, is to ensure that when doing:

HashNode aNode = malloc(sizeof(H ashNode));

change to

if (!(aNode = malloc(sizeof(* aNode)) {
/* as malloc returns a pointer to undetermied memory */
/* initialise the struct members to meaningfull values */
/* to prevent unspezified or in worst case undefined behavior */
aNode->name[0] = 0; /* name is empty */
aNode->hVal = 0ul; /* no value */
aNode->next = NULL /* no pointer assigned */
} else {
/* malloc was unable to give enough memory for a new HashNode */
/* do the needed error handling */
}
--
Tschau/Bye
Herbert

Visit http://www.ecomstation.de the home of german eComStation

[Toni Uusitalo]

"Lars Tackmann" <ro****@diku.dk > wrote in message
news:Pi******** *************** *******@brok.di ku.dk...

On Sun, 21 Dec 2003, Tobias Oed wrote:

Lars Tackmann wrote:

I'm confused. If you want an array why do you use a linked list?
Tobias.
In a hash table you usaly have the problem that more than one items hashes
to the same value -. If this happens you link the ones toghter that have
the same hash value (creating a bucket - that you end up searching

linary).
That is why i need an array of items that also can be linked to other items.

You may have misunderstood how hashtable is usually put together:

horisontally it's an array, vertically items (duplicate hash values/keys)
are
usually implemented as linked list.

See my post in thread "Hash Function". It contains a link to snippets
collection
hashtable implementation - you can take some ideas from there I guess.

with respect,
Toni Uusitalo