need help with: pointer to pointer to an array of struct

Hey guys, I have a little code like this:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <string.h>

#define  KH_SUCCESS				0x00
#define EKH_MAX_PERSON_EXCEED	0x01

struct _KPerson {
	char *name; // person name
	int age; // person age
};
typedef struct _KPerson KPerson;

struct _KHome {
	KPerson **plist; // pointer to pointer of struct array
	int msize; // max size alloweed
	int csize; // current size
};
typedef struct _KHome KHome;

KPerson* kperson_new(const char* _n, const int _a)
{
	KPerson *kp;
	kp = (KPerson *) calloc( 1, sizeof( KPerson ) );
	
	kp->name = (char *) calloc( strlen(_n) + 1, sizeof( char ) );
	strncpy( kp-> name, _n, strlen(_n) + 1 );
	
	kp->age = _a;
	
	return kp;
};

KHome* khome_new( const int _ms )
{
	KHome *kh;
	int i = 0;
	kh = (KHome *) calloc( 1, sizeof( KHome ) );
	

	kh->plist = (KPerson **) calloc( _ms, sizeof( KPerson * ) );	
	for( ; i < _ms; ++i )
		kh->plist[i] = NULL;
	/* ??? [1] - how to initialize all persons in list to NULL */
	
	kh->msize = _ms;
	kh->csize = 0;
	
	return kh;
};

int khome_insert( KHome *kh, const KPerson* kp )
{
	if( kh->csize >= kh->msize ) 
		return EKH_MAX_PERSON_EXCEED;

	/* ??? [2] - How to insert a new person value into home person list */
	return KH_SUCCESS;
};

int main( int argc, char **argv )
{
	/* testing code here */

	return 0;
}

As you can see above, I inserted my 2 questions at place

??? [1] - how to initialize all persons in list to NULL
??? [2] - How to insert a new person value into home person list

I wonder of how to access each element in this kind of pointer.

Code:

struct _KHome {
	KPerson **plist; // pointer to pointer of struct array
	int msize; // max size alloweed
	int csize; // current size
};
KHome *kh;

How to access each element in plist ?

Note: i don't want to use linked list here, just raw dynamicaly allocated pointer style.

As for question 1, you put the question directly after the answer -- use a for-loop to assign each kh->plist[i] to be NULL. Probably, you don't have to do that, since calloc will fill each of your kh->plist[i] with all-bits-zero, which is usually the same deal.

As to 2, you have to decide where to put the guy, then use malloc to give you a piece of memory, and assign the return value of malloc to kh->plist[appropriate_number].

Originally Posted by tabstop

As for question 1, you put the question directly after the answer -- use a for-loop to assign each kh->plist[i] to be NULL. Probably, you don't have to do that, since calloc will fill each of your kh->plist[i] with all-bits-zero, which is usually the same deal.

As to 2, you have to decide where to put the guy, then use malloc to give you a piece of memory, and assign the return value of malloc to kh->plist[appropriate_number].

tks for helping me, however, this is what I implemented for question 2.

I commented on the code below

Code:

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#include <string.h>

#define  KH_SUCCESS				0x00
#define EKH_MAX_PERSON_EXCEED	0x01

struct _KPerson {
	char *name; // person name
	int age; // person age
};
typedef struct _KPerson KPerson;

struct _KHome {
	KPerson **plist; // pointer to pointer of struct array
	int msize; // max size alloweed
	int csize; // current size
};
typedef struct _KHome KHome;

KPerson* kperson_new(const char* _n, const int _a)
{
	KPerson *kp;
	kp = (KPerson *) calloc( 1, sizeof( KPerson ) );
	
	kp->name = (char *) calloc( strlen(_n) + 1, sizeof( char ) );
	strncpy( kp-> name, _n, strlen(_n) + 1 );
	
	kp->age = _a;
	
	return kp;
};

KHome* khome_new( const int _ms )
{
	KHome *kh;
	int i = 0;
	kh = (KHome *) calloc( 1, sizeof( KHome ) );	

	kh->plist = (KPerson **) calloc( _ms, sizeof( KPerson * ) );	
	
	kh->msize = _ms;
	kh->csize = 0;
	
	return kh;
};

int khome_insert( KHome *kh, const KPerson* kp )
{
	if( kh->csize >= kh->msize ) 
		return EKH_MAX_PERSON_EXCEED;

	/* ??? [1] - What's wrong with this part of code? */

	kh->plist[kh->csize]->name = (char *) calloc( strlen( kp->name ) + 1, sizeof( char ) );
	strncpy( kh->plist[kh->csize]->name, kp->name, strlen( kp->name ) + 1 );
	
	/* ============================================== */
	
	kh->plist[kh->csize]->age = kp->age;
	++kh->csize;
	
	return KH_SUCCESS;
};

int main( int argc, char **argv )
{
	/* testing code here */
	KHome *kh = khome_new(3);
	KPerson *kp = kperson_new( "Pete Houston", 22 );
	int res = khome_insert(kh, kp); // segmentation fault
	if( res == KH_SUCCESS ) 
		printf(" %s \n ", kh->plist[0]->name);

	free( kh );
	return 0;
}

If kh->plist[kh->csize] is NULL, then trying to follow that pointer (by doing kh->plist[kh->csize]->name) is going to crash. kh->plist[kh->csize] needs to point to valid memory first, and in order to get that memory you have to ask for it (with malloc). Hint: You've already done this in the "new" function.

Note that your calloc at the first only got you a bunch of pointers; it didn't give you any actual objects.

Originally Posted by tabstop

If kh->plist[kh->csize] is NULL, then trying to follow that pointer (by doing kh->plist[kh->csize]->name) is going to crash. kh->plist[kh->csize] needs to point to valid memory first, and in order to get that memory you have to ask for it (with malloc). Hint: You've already done this in the "new" function.

Note that your calloc at the first only got you a bunch of pointers; it didn't give you any actual objects.

i've already allocated before copying the content.
segmentation fault !

Originally Posted by bvkim

i've already allocated before copying the content.
segmentation fault !

That's true. So then why aren't you using that pointer? Set the kh->whatever equal to the pointer to the allocated object and you're done. The kh->whatever pointer equals NULL, so you can't follow it or use it until you assign it a valid pointer.

Originally Posted by tabstop

That's true. So then why aren't you using that pointer? Set the kh->whatever equal to the pointer to the allocated object and you're done. The kh->whatever pointer equals NULL, so you can't follow it or use it until you assign it a valid pointer.

oh dear ^^!
fixed !

tks so much buddy !