Memory allocation problem

[ariko]

Hi all,

I have a little problem with memory allocation.

At first, just have a look in my code:

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

//Free memory
void end(int *a)
{
	if(a != NULL)
	{
		free(a);
		a = NULL;
	}
	
}

//Fill random number into array
void fill(int *a, int n)
{
	int i;	
	
	for ( i  = 0; i < n; i++)
	{
		a[i] = rand()%19 -9;
	}
}

//Display array
void display(int *a,int n)
{
	int i;
	for (i = 0; i < n; i++)
	{
		printf("%d\t", a[i]);
	}
}


void main()
{

	int *a = NULL;
	int n = 10;//Element size

	if ((a = calloc(n, sizeof(int))) == NULL) 
	{
		printf("Memory can not allocate");
		exit(-1);
	}
	fill(a,n);
	display(a,n);
	end(a);
}

You can see that it runs perfectly.
But if I make a little changes in fill function, my program will be crashed.

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

//Free memory
void end(int *a)
{
	if(a != NULL)
	{
		free(a);
		a = NULL;
	}
	
}

//Fill random number into array
void fill(int *a, int n)
{
	int i;	

	if ((a = calloc(n, sizeof(int))) == NULL) 
	{
		printf("Memory can not allocate");
		exit(-1);
	}
	
	for ( i  = 0; i < n; i++)
	{
		a[i] = rand()%19 -9;
	}
}

//Display array
void display(int *a,int n)
{
	int i;
	for (i = 0; i < n; i++)
	{
		printf("%d\t", a[i]);
	}
}


void main()
{

	int *a = NULL;
	int n = 10;//Element size


	fill(a,n);
	display(a,n);
	end(a);
}

My questions is:
1. Why is my program crashed ?
2. I see that many programs which use the global variables to store pointer, so they can write initial function and de-allocated function separately. If I do not want to use global variables, how can I initiate and free memory by passing the pointer ?

Thank you.

[Adak]

I believe Quzah's post has answered your questions, here:

Dynamic string input

If not, post back.

And Welcome to the forum, Ariko!

[ariko]

I believe Quzah's post has answered your questions, here:

Dynamic string input

If not, post back.

And Welcome to the forum, Ariko!

Thanks for your reply.

I solved my problems based on Quzah's post.

void init(int **a, int n)
{
	if ((*a = calloc(n, sizeof(int))) == NULL) 
	{
		printf("Memory can not allocate");
		exit(-1);
	}
}

void main()
{

	int *a = NULL;
	int n = 10;//Element size

	init(&a, n);
	fill(a,n);
	display(a,n);
	end(a);
}

[Elysia]

Do not ever use void main: SourceForge.net: Void main - cpwiki

[ariko]

Do not ever use void main: SourceForge.net: Void main - cpwiki

Thanks. I will avoid it.

[Bayint Naung]

You end() function will not set its argument to NULL.

void end(int *a)
{
	if(a != NULL)
	{
		free(a);
		a = NULL;         // not change in caller! 
	}
	
}

[laserlight]

You end() function will not set its argument to NULL.

Indeed. You might as well dispense with it, and just call free(a); in the main function. Checking for a null pointer is unnecessary here.

[ariko]

You end() function will not set its argument to NULL.

Thanks for your reply.
I forget to check this function.
I think this is better.

void end(int **a)
{
	if(*a != NULL)
	{
		free(*a);
		*a = NULL;   
	}
	
}

Indeed. You might as well dispense with it, and just call free(a); in the main function. Checking for a null pointer is unnecessary here.

Thanks for your suggestion. In some complicated programs, I realize that they write the initiate and free function separately, but they use global pointer. I can not use global variables in my program, I must included an unallocated output buffer in the prototype of my function. Then, malloc and free in another functions.

[Elysia]

In complicated programs, they most certainly would not use global variables. You are doing it the correct way.

[whiteflags]

Thanks for your suggestion. In some complicated programs, I realize that they write the initiate and free function separately, but they use global pointer. I can not use global variables in my program, I must included an unallocated output buffer in the prototype of my function. Then, malloc and free in another functions.

Laserlight did not mention anything about global variables. Your end() function is an abstraction of free, that does nothing other than free one thing. Just use free!

[ariko]

Laserlight did not mention anything about global variables. Your end() function is an abstraction of free, that does nothing other than free one thing. Just use free!

Laserlight suggest that:

just call free(a); in the main function

I don't want to do this because I have to separate malloc/calloc and free into different function.

But Laserlight is right about

Checking for a null pointer is unnecessary here.

This is my new code:

int end(int **a)
{
	free(*a);
	*a = NULL;
}

int main()
{

	int *a = NULL;
	int n = 10;//Element size

	init(&a, n);
...

	end(&a);
}