Dynamic array - malloc

**erasm** · 10-04-2009

I would like to implement a dynamic array buffer_array for this tutorial. A hint for the tutorial suggests using malloc. How can malloc be used to implement this dynamic array for my tutorial?

Code:

/*
Create a data type (e.g. a struct) for the buffer
(queue) with an appropriate constructor (intialiser) and
destructor (cleanup function).  The constructor should
take at least two (integer) parameters: a (maximum)
buffer size, and a minimum fill level.
*/

typedef struct bufferQueue
{	
	int *buffer_array;	
	int min_buffer_size;
	int max_buffer_size;	
} bufferQueue;

void init_bufferQueue(int min, int max)
{
	
}

destroy_bufferQueue
{

}

/*
Create a put_buffer() function that puts an
integer into the buffer so that it can be read later.
For the moment, you can ignore the maximum buffer size.
*/

/*
Create a get_buffer() function that reads the
oldest integer that was put into the buffer, removes that
integer from the buffer and returns it. 
For the moment, you can ignore the minimum fill level.
*/

/*
Create a test program that spawns a child thread
that acts as the producer. I.e. the child should
use the function you created earlier to read integers from
/dev/random as fast as it can and immediately put each integer into the
buffer. The main (parent) thread should act as the consumer 
(i.e. read and remove the numbers
from the buffer, and print them to stdout).
*/

**MK27** · 10-04-2009

I would say the init function should return a pointer so you can use it like this:

Code:

bufferqueue *init_buffer(int max, int min); /* func prototype */
bufferqueue *example = init_buffer(30, 5);   /* example call */

Does that give you a clue? The destructor would then accept a bufferqueue* as a parameter.

**erasm** · 10-04-2009

Should i use max - min to allocate memory for the buffer array?

If so how would this be implemented, i am having difficulty getting my head around the manual for malloc

**MK27** · 10-04-2009

Originally Posted by erasm

Should i use max - min to allocate memory for the buffer array?

If so how would this be implemented, i am having difficulty getting my head around the manual for malloc

Malloc is fairly simple:

Code:

bufferqueue *init_buffer(int max, int min) {
         bufferqueue *ptr = malloc(sizeof(bufferqueue));
         ptr->buffer_array = malloc(max*sizeof(int));
         [....]
         return ptr;
}

The most important thing about using malloc is understanding free(), and using it where appropriate to prevent wasted memory and memory leaks. In fact this is almost THE MOST fundamental aspect of C low level programming.

In this case, free() is the essential element of your destructor.

If your code needs to compile as C++, you need to cast malloc() calls. Otherwise that is not necessary.

Some people will stress the importance of checking to make sure malloc returns a non-NULL pointer, but this is debatable. If you are submitting this for an assignment, make sure you know your prof's opinion.

**erasm** · 10-04-2009

I am using a C++ tutorial as a reference on circular buffers and have a problem understanding the logic behind part of it.

Shouldn't remove() return an array excluding the oldest element in the array?

I don't see how this code from the tutorial is doing so.

Code:

    void insert (Node node) {
        array[next] = node;
        next++;
    }

    Node remove () {
        first++;
        return array[first-1];
    }

From: C++Course: Circular Buffer

b->buffer_array[out] = NULL; <-- Would this be the best way to remove the array element?

Code:

#include "circbuffer.h"

bufferQueue *init_buffer(int min, int max)
{
        bufferQueue *b = malloc(sizeof(bufferQueue));
        b->buffer_array = malloc(max*sizeof(int));
        b->in = 0;
        b->out = 0;
        return b;
}

bufferQueue *destroy_buffer(bufferQueue *b)
{
        free(b);
        return b;
}

void put_buffer(bufferQueue *b, int v)
{
        b->buffer_array[b->in] = v;
        b->in++;
}

void get_buffer(bufferQueue *b)
{
        //b->buffer_array[out] = NULL;
        b->out++;        
}

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