Hi,

I want to create a container structure (array/list etc), that is expandable.

Currently, I have static arrays like this

SomeStruct *ptrArray[MAX_NUM_STRUCT] ;

What I want to do is to be able to use ptrs to ptrs like so:

SomeStruct **ptrArray ;

However, I am unsure as to how to do the ff after the declaration above:

1). Allocate memory (both initially and for subsequent data structure
increases)
2). Navigate the array - i.e. index to specific element in the array

mtia

Alfonso Morra wrote:
> Hi,
>
> I want to create a container structure (array/list etc), that is expandable.
>
> Currently, I have static arrays like this
>
> SomeStruct *ptrArray[MAX_NUM_STRUCT] ;
>
> What I want to do is to be able to use ptrs to ptrs like so:
>
> SomeStruct **ptrArray ;
>
> However, I am unsure as to how to do the ff after the declaration above:
>
> 1). Allocate memory (both initially and for subsequent data structure
> increases)
> 2). Navigate the array - i.e. index to specific element in the array
>
> mtia

Use realloc() to create and extend a dynamic buffer. You can access
individual elements as you would with a normal array. Here's an
untested example:

#include <stdlib.h>

typedef struct {...} SomeStruct;

SomeStruct **extendArray(SomeStruct **arr, size_t *cursize, size_t
extent)
{
SomeStruct **tmp;

/**
* realloc() will return NULL on failure, address of the
* first element in the buffer on success. In order to
* extend the buffer, realloc() may move it to a
* different location, so the base address may change.
*/
tmp = realloc(arr, sizeof arr[0] * (*cursize + extent));
if (tmp)
{
arr = tmp;
*cursize += extent;
}
return arr;
}

int main(void)
{
/**
* Make sure array pointer is initially NULL
*/
SomeStruct **myArr = NULL;
size_ t arrSize = 0;
size_t i;

/**
* Initially allocate 20 elements to myArr
*/
myArr = extendArray(myArr, &arrSize, 20);
if (arrSize != 20)
{
/**
* Allocation failed; fatal error
*/
return EXIT_FAILURE;
}

for (i = 0; i < arrSize; i++)
{
/**
* do something with myArr[i]
*/
}

/**
* Extend array by another 5 elements
*/
myArr = extendArray(myArr, &arrSize, 5);
if (arrSize != 25)
{
/**
* Attempt to extend existing array failed.
*/
return EXIT_FAILURE;
}

for (i = 20; i < arrSize; i++)
{
/**
* do something with myArr[i]
*/
}

return EXIT_SUCCESS;
}

There are probably a hundred better ways to indicate success or
failure, and you'll want to add some intelligent error handling, but
that should give you the basic idea.

John Bode wrote:

[..]
> */
> }
>
> return EXIT_SUCCESS;
> }
>
> There are probably a hundred better ways to indicate success or
> failure, and you'll want to add some intelligent error handling, but
> that should give you the basic idea.
>

many thanks John