Thread: Problems with linked lists from *.dat

The problem is that you are trying to use a pointer before it has been assigned an address. In other words, it's NULL.

This is because the variable you pass to check_open is just a pointer, that hasn't been assigned yet. You then copy the value of this pointer (NULL) to a local variable, and try to use it in the fscanf call.

The prog actually terminates on line
curptr->nextptr = newptr ;
because curptr is NULL.

In order to get around this, I'd suggest you use a temporary struct to load the data into, then once you are happy the read was good, proceed to malloc up some memory for the data, and assign the pointer the return from malloc. Then copy in the data from the temporary struct to the new memory area. (This is just one way of doing it, you will find other methods if you look around).

You could still use the pointer method you have already, but you would need to malloc memory before trying to use the pointer. In this case though, if the read of the data fails, you wasted time malloc'ing memory that'd you'd have to free up straight away.

There's plenty of examples of link list code about here. If you get stuck though, post back here with your changes and we'll have another go.

This is my version, based on your original code:

Code:

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

struct checkingnode
{
	char num[8] ;
	char comment[32] ;
	char deb_cred[8] ;
	float amt ;
	char date[12] ;
	float bal ;
	struct checkingnode *nextptr ;
} ;

typedef struct checkingnode Checking ;
typedef Checking *checkptr ;

void check_open( checkptr * ) ;

int main(void)
{
	checkptr firstptr = NULL, tmpptr;

	check_open( &firstptr ) ;
	
	tmpptr = firstptr;

	// print the list to prove it worked.	
	while (tmpptr)
	{
		printf("%s %s %.2f\n", tmpptr->num, tmpptr->comment, tmpptr->amt);
		tmpptr = tmpptr->nextptr;
	}
	
	// next job, write function to free all nodes in the list!
	return (0);
}

/*****************************************
Function: check_open
Purpose: Opens the checking.dat file and
creates the checking linked list.
*****************************************/
void check_open( checkptr *sptr)
{
	FILE *ptrReadDat ;
	char buffer[200];
	checkptr curptr = NULL, newptr;
	Checking tmpNode;

	if ((ptrReadDat = fopen( "checking.dat", "r" )) == NULL)
	{
		printf( "\nFile could not be opened.\n" ) ;
		return;
	}
	
	while (fgets(buffer, 200, ptrReadDat))	// read a line
	{	
		// sscanf is lame, 'cos it can't handle input errors.
		// Also, strings are terminated when a white space character is read (eg space).
		// Search the board for better ways of doing this type of thing.
		if (sscanf( buffer, "%s\t%s\t%s\t%f\t%s\t%f\n", tmpNode.num, tmpNode.comment,
			tmpNode.deb_cred, &tmpNode.amt, tmpNode.date, &tmpNode.bal ) == 6)
		{	// line split up and stored into temp struct OK, so proceed
			
			if ((newptr = malloc( sizeof( Checking ) )) == NULL)
			{	// malloc up some memory
				perror("out of memory");
				fclose( ptrReadDat );
				return;  // do something better here
			}
			*newptr = tmpNode;	// copy the tmp struct to the new memory
			if (curptr == NULL)
			{	// if first time...
				curptr = newptr;	//  pointer to new memory
				*sptr = curptr;		//	also update the calling pointer (from function arg)
			}
			else
			{	// pointer already been used, so ...
				curptr->nextptr = newptr;	//	make the link from one node to the next
				curptr = newptr;			//	move the pointer to the next node
			}
			curptr->nextptr = NULL;		// force the new nodes nextptr to NULL to prevent errors.
		}
	}
	fclose(ptrReadDat);
}