returning an array of pointers to structures

[dharh]

This is mostely a problem ive made for myself. I have:

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

#define MAXLIST 10

struct ipList {
  int id;
  char name[ 35 ];
  char ip[ 15 ];
};

void getList( struct ipList *list[ ] );
struct ipList *makeIP( void );
void printList( struct ipList *list[ ] );

int main( void ) { 
  struct ipList *theList[ MAXLIST ];
  
  getList( theList );

  printList( theList );

  return 0;
}

void getList( struct ipList *list[ ] ) {
  int i = 0;
  
  while ( i < MAXLIST ) {
    list[ i ] = makeIP( );
    list[ i ]->id = i;
    strcpy( ( list[ i ]->name ), "citibank.co.cx" );
    strcpy( ( list[ i ]->ip ), "209.249.147.15" );
    i++;
  }
}

struct ipList *makeIP( void ) {
  struct ipList *temp;
  temp = malloc( sizeof( struct ipList ) );
  return ( temp );
}


void printList( struct ipList *list[ ] ) {
  int i = 0;
  
  while ( i < MAXLIST ) {
    printf( "%s - %s\n", ( list[ i ]->name ), ( list[ i ]->ip ) );
    i++;
  }
}

Which works, however ive been trying to do this without passing parameters to getList( ), instead I've been trying to have getList( ) return the address to an array of pointers to structures. I can make the array, but seem to be unable to have it be returned.

[quzah]

I can make the array, but seem to be unable to have it be returned.

How are you creating your array? Like this:

struct something * getList( void )
{
    struct something *list[ SIZE ];
    ... do stuff ...

    return list;
}

That won't work because the list is a local variable. You could "fix" this by making it a static variable. Or you could dynamicly create the "array" and return it.

struct something * getList( void )
{
    struct something ** list;
    int x;
    list = malloc ( SIZE * sizeof(struct something*) );
    for( x = 0; x < SIZE; x++ )
        list[x] = malloc( sizeof( struct something ) );

    ... do stuff ...

    return list;
}

There's probably another way or so to do this, but that should suffice.

Quzah.

[dharh]

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

#define MAXLIST 10

struct ipList {
  int id;
  char *name;
  char *ip;
};

struct ipList *getList( void );
void printList( struct ipList *list );

int main( void ) { 
  struct ipList **theList;
  int i;

  theList = malloc( MAXLIST * sizeof( struct ipList ) );

  *theList = getList( );

  i = 0;
  while ( i < MAXLIST ) {
    printf( "%s - %s\n", ( theList[ i ]->name ), ( theList[ i ]->ip ) );
    i++;
  }

  return 0;
}

struct ipList *getList( void ) {
  struct ipList **list;
  int i;
  
  list = malloc( MAXLIST * sizeof( struct ipList ) );
  i = 0;
  while ( i < MAXLIST ) {
  	list[ i ] = malloc( sizeof( struct ipList ) );
    list[ i ]->id = i;
    list[ i ]->name = "citibank.co.cx";
    list[ i ]->ip = "209.249.147.15";
    i++;
  }

  return ( *list );
}

void printList( struct ipList *list ) {
  int i = 0;
  
  while ( i < MAXLIST ) {
    printf( "%s - %s\n", ( list[ i ].name ), ( list[ i ].ip ) );
    i++;
  }
}

I run into one of problems I had before when I was messing around trying to get it to work where it seems only the first address remains in sequence the others are dropped off. I essentialy get an invalid access in memory when the second struct in the array is called.

[Hammer]

At a very quick glance, I'd guess this is wrong:
>>list = malloc( MAXLIST * sizeof( struct ipList ) );
I think you mean:
>>list = malloc( MAXLIST * sizeof( struct ipList *) );

[Cela]

The problem is that you're trying to be fancy with the return value of getList(). You allocate memory to theList, then assign the one element that getList() returns. Instead, have getList() return a double pointer and don't worry about memory in main except for freeing when you're done :-) You have the same problem with printList(), you're trying to use a single pointer when you really want and need a double pointer

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

#define MAXLIST 10

struct ipList {
  int id;
  char *name;
  char *ip;
};

struct ipList **getList( void );
void printList( struct ipList **list );

int main( void ) {
  struct ipList **theList;
  
  theList = getList( );
  printList( theList );
  
  return 0;
}

struct ipList **getList( void ) {
  struct ipList **list;
  int i;
  
  list = malloc( MAXLIST * sizeof( struct ipList * ) );
  i = 0;
  while ( i < MAXLIST ) {
    list[ i ] = malloc( sizeof( struct ipList ) );
    list[ i ]->id = i;
    list[ i ]->name = "citibank.co.cx";
    list[ i ]->ip = "209.249.147.15";
    i++;
  }
  
  return ( list );
}

void printList( struct ipList **list ) {
  int i = 0;
  
  while ( i < MAXLIST ) {
    printf( "%s - %s\n", ( list[ i ]->name ), ( list[ i ]->ip ) );
    i++;
  }
}

[dharh]

Though im still not sure how:
malloc( MAXLIST * sizeof( struct ipList * ) );
differs from:
malloc( MAXLIST * sizeof( struct ipList ) );
exactly. I do understand what I was doing wrong with the rest. Thanks, it works now.

[Dave_Sinkula]

Originally posted by dharh
Though im still not sure how:
malloc( MAXLIST * sizeof( struct ipList * ) );
differs from:
malloc( MAXLIST * sizeof( struct ipList ) );
exactly.

The first requests memory for MAXLIST pointers to objects of type struct ipList; the second requests memory for MAXLIST objects of type struct ipList. Maybe the following will help clarify.

#include <stdio.h>

#define MAXLIST 10

struct ipList
{
    int id;
    char *name;
    char *ip;
};

int main(void)
{
    printf("MAXLIST = %d\n", MAXLIST);
    printf("sizeof(struct ipList) = %lu\n", (unsigned long)sizeof(struct ipList));
    printf("sizeof(struct ipList *) = %lu\n", (unsigned long)sizeof(struct ipList *));
    printf("MAXLIST * sizeof(struct ipList) = %lu\n", (unsigned long)(MAXLIST * sizeof(struct ipList)));
    printf("MAXLIST * sizeof(struct ipList *) = %lu\n", (unsigned long)(MAXLIST * sizeof(struct ipList *)));
    return 0;
}

/* my output
MAXLIST = 10
sizeof(struct ipList) = 12
sizeof(struct ipList *) = 4
MAXLIST * sizeof(struct ipList) = 120
MAXLIST * sizeof(struct ipList *) = 40
*/

[dharh]

That explains some of the problems ive had in the past.

I guess the final question I have is:

struct ipList **getList( void ) {
  static struct ipList *list[ MAXLIST ];
  int i;
  
  //list = malloc( MAXLIST * sizeof( struct ipList * ) );
  i = 0;
  while ( i < MAXLIST ) {
  	list[ i ] = malloc( sizeof( struct ipList ) );
    list[ i ]->id = i;
    list[ i ]->name = "citibank.co.cx";
    list[ i ]->ip = "209.249.147.15";
    i++;
  }

  return ( list );
}

Is there a way to not use static in this case:
struct ipList *list[ MAXLIST ];

[quzah]

Originally posted by dharh
That explains some of the problems ive had in the past.

I guess the final question I have is:

Is there a way to not use static in this case:
struct ipList *list[ MAXLIST ];

Sure, use a global variable.

Quzah.

[dharh]

No way inside the function?