Thread: Dynamic Memory Allocation

I am creating my first program using dynamic memory allocation, I have started it but can't get past how far I am now. I was just looking for advice on how to fix up what I have, don't need any tips for the file part yet, just looking to understand what I have now. Yes this is homework but I have honestly tried and done a lot myself, I just need a push in the right direction. This is my code:

Code:

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

struct point
{
	int x; //point x
	int y; //point y
	char label[21]; //label of 2 points
	struct point *ptrNext;		// self referential pointer
};

int isEmptyList(struct point *ptrF);
void PrintList(struct point *ptrF);
void ResetList(struct point *ptrF, struct point *ptrL);
void AddToEnd(struct point *ptrF, struct point *ptrL);
void AddToBeginning(struct point *ptrF, struct point *ptrL);
void InputRecord(struct point *ptrNew); // used by Add to interactively get the values from the user
//emptylist will determine if the list is empty
int isEmptyList(struct point *ptrF)
{
	if(ptrF=NULL)
		return 0;
	else
		return 1;
}
//print list will print out everything
void PrintList(struct point *ptrF)
{
	if(ptrF!=NULL)
	{
		printf("label: %s\nx: %d y: %d\n",ptrF->label,ptrF->x,ptrF->y);
		PrintList(ptrF->ptrNext);
	}	
}
//reset list will make everything equal to NULL
void ResetList(struct point *ptrF, struct point *ptrL)
{
	ptrF= NULL;
	ptrL= NULL;
}
//addtobeginning will add a number to the beginning
void AddToBeginning(struct point *ptrF, struct point *ptrL)
{
	struct point ptrNew;
	InputRecord(&ptrNew);
	ptrNew.ptrNext=ptrF;
	ptrF=&ptrNew;
}
//addtoend will add a number to the end
void AddToEnd(struct point *ptrF, struct point *ptrL)
{
	if(ptrF==NULL)
		InputRecord(ptrF);
	else
	{
		struct point New;
		InputRecord(&New);
		ptrL=&New;
	}	
}
//reads in input
void InputRecord(struct point *ptrNew)
{
	printf("Please enter the 2 Data points(x and y)");
	scanf("%d%d", ptrNew->x, ptrNew->y);
	printf("Please enter the label");
	scanf("%s", ptrNew->label);
	ptrNew->ptrNext=NULL;
}

struct point *ptrFirst = NULL;
struct point *ptrLast = NULL;

void main()
{
	int empty=0;//helps determine if list is empty
	int input=1;//reads in user input
	while(input!=0)
	{
		printf(
		"1. Add a point at the END of the list.\n"
		"2. Add a point at the BEGINNING of the list.\n"
		"3. Is the list empty?\n"
		"4. Erase all points from the list (reset).\n"
		"5. Display the list.\n"
		"6. Save the list to a sequential file (reset/replace file contents)\n"
		"7. Read the list back from a sequential file (reset/replace current memory content)\n"
		"0. Exit\n"
		);
		scanf("%d", &input);
		if(input==1)
		{
			AddToEnd(ptrFirst,ptrLast);
		}
		if(input==2)
		{
			AddToBeginning(ptrFirst,ptrLast);
		}
		if(input==3)
		{
			empty=isEmptyList(ptrFirst);
			if(empty==1)
			{
				printf("The list is not empty");
			}
			else
			{
				printf("The list is empty");
			}
		}
		if(input==4)
		{
			ResetList(ptrFirst,ptrLast);
		}
		if(input==5)
		{
			PrintList(ptrFirst);
		}
		if(input==6)
		{
		}
		if(input==7)
		{
		}
		if(input==0)
		{
			input=0;
		}
		else
			printf("Invalid entry, try again\n");
	}
}

My first problem is that when asking to scan in the first "input" from the user it gives segmentation fault(core dumped)

Really?! You can enter any number e.g. 99 and it seg faults?

I find this bit rather amusing:

Code:

		if(input==0)
		{
			input=0;
		}

Nothing you do there is dynamic.


Quzah.

Code:

void AddToBeginning(struct point *ptrF, struct point *ptrL)
{
	struct point ptrNew;
	InputRecord(&ptrNew);
	ptrNew.ptrNext=ptrF;
	ptrF=&ptrNew;
}

No, that's not dynamic. You are creating a local structure, which is destroyed as soon as that function returns. Therefore, anything that points to it, now points to an invalid memory location.

Dynamic memory allocation usually involves something like malloc setting aside a spot in memory for you to treat as a specific type of data. Then if you need more, you use malloc again to get more memory. When you're all done, you use something like free to get rid of it all.


Quzah.

Code:

struct foo *newnumber( int v )
{
    struct foo *n = malloc( sizeof *n );
    if( n != NULL )
    {
        n->value = v;
    }

    return n;
}
...

struct foo *p = NULL;

p = newnumber( 4 );
if( p != NULL )
{
    printf( "p->value = %d", p->value );
    free( p );
}

Quzah.

Yea saw the tutorial after I posted, I am reading through it thanks Adak and thanks for the code too quzah. I'm gonna read through some stuff and re-write this whole thing tomorrow hopefully, I will re-post my new code tomorrow night hopefully around the same time.