Thread: Read/Write linked lists from/to file

Hi, I'm trying to make a simple database, where I write a linked list to a file and then read it out again.

I've been working on this for the last week and I've been stuck most of the time hehe But anywho, this is the code;

Code:

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
#define TRUE 1

void newname(void);
void listfromfile(void);

struct medlem
{
	char name[255];
	char medlem[255];
	struct medlem *ptrnext;
};

struct medlem *ptrfirst = NULL;

void main(void)
{
char ch;

while(TRUE)
	{
	printf("\nTryk 'e' for at indtast medlem\n");
	printf("\nTryk 'p' for at laese fra fil\n");
	printf("\nTryk 'q' for at quitte ;>\n");
	ch = getche();
	switch(ch)
		{
		case 'e':
			newname(); break;
		case 'p':
			listfromfile(); break;
		case 'q':
			printf("\n");
			exit(1);
		default:
			printf("\nForkert valg. \n");
		}
	}
}


void newname(void)
{
	FILE *fp;
	struct medlem *ptrthis;
	struct medlem *ptrfirst;

	ptrthis = malloc(sizeof(struct medlem));
	ptrfirst = malloc(sizeof(struct medlem));

	if(ptrthis == NULL)
	{
		printf("Allocation failed.\n");
		return;
	}
	ptrthis->ptrnext = ptrfirst;

	if( (fp = fopen("c:\\database.txt", "a")) == NULL)
		{
		printf("\nCan't open file\n");
		exit(1);
		}
	printf("\nEnter name: ");
	gets(ptrthis->name);
	printf("\nEnter number: ");
	gets(ptrthis->medlem);
        //write to file
	fwrite(ptrthis, sizeof(*ptrthis), 1, fp);
	fclose(fp);

}
void listfromfile(void)
{
	struct medlem *ptrthis;
	FILE *fp;
	ptrthis = malloc(sizeof(struct medlem));
	ptrfirst = ptrthis;
	if( (fp = fopen("c:\\database.txt", "r")) == NULL)
	{
		printf("No such file\n");
		exit(1);
	}
	if(ptrthis == NULL)
	{
		printf("NULL\n");
		exit(1);
	}
	while(fread(ptrthis, sizeof(*ptrthis), 1, fp) ==1)
	{
		printf("\nNavn: %s\n", ptrthis->name);
		printf("\nNumber: %s\n", ptrthis->medlem);
		ptrthis = ptrthis->ptrnext;
	}
}

Nothing happens really.. Something gets written to the file, but it aint' clear text :| And when I call the "listfromfile" function, nothing happens.

Where did I fail?

Okay, int main it is then :P

But how should I link them together?

Something like this?

Code:

	while(fread(ptrthis, sizeof(*ptrthis), 1, fp) ==1)
	{
		printf("\nNavn: %s\n", ptrthis->name);
		printf("\nNumber: %s\n", ptrthis->medlem);
                ptrthis->ptrnext = malloc(sizeof(struct medlem));
		ptrthis = ptrthis->ptrnext;
	}

Or?

Hmm I can't find anything about gets() in the FAQ. Perhaps I'm not looking the right place :|

Anywho, how do you do #1?

something like;

Code:

while(fread(ptrthis, sizeof(*ptrthis), 1, fp) ==1)
	{
        nametemp =  ptrthis->name;
        numbertemp = ptrthis->number;
        printf("%s", nametemp);
        printf("%d", numbertemp);
        }

Or how do you mean? Sorry.. I'm not too good in C So any chance you could be a bit more specific?

Hi again

I think I understand the basics of Linked Lists.. main problem for me, is how to write them to a file and read them out again.

I've been to google, alltheweb and msn search. And I can't find anything that describes just that :/

What I don't understand is why fread returns 0..

I've tried singlestepping my program and either I get an access violation, when I call the readfromfile() function or fread returns 0.

Here is my struct;

Code:

struct prs
{
	char name[255];
	int number;
	struct prs *ptrnext;
};

struct prs *ptrfirst = NULL;

Here I enter a new member;

Code:

void newname()
{
	struct prs *ptrthis;						
	char numstr[81];					
	ptrthis = malloc(sizeof(struct prs));		
	
	if(ptrthis == NULL)
	{
		printf("Allocation failed\n");
		return;
	}

	ptrthis->ptrnext = ptrfirst
	ptrfirst = ptrthis;							

	printf("\n\nEnter name: \n");
	gets(ptrthis->name);
	printf("\n\nEnter number: \n");
	gets(numstr);
	ptrthis->number = atoi(numstr);
}

Here I save my data to a file

Code:

void savetofile()
{
	FILE *fp;
	struct prs *ptrthis;

	if(ptrfirst == NULL)
	{
		printf("Empty.\n");
		return;
	}

	ptrthis = ptrfirst;
	if((fp=fopen("c:\\database.txt", "wb")) == NULL)
	{
		printf("No such file.\n");
		return;
	}
	else
	{
		fwrite(&ptrthis, sizeof(*ptrthis), 1, fp);
		fclose(fp);
		printf("Saved to disk.\n");
		system("PAUSE");
	}
}

Here I read from the file and print. And it's here that fread returns 0 :|

Code:

void readfromfile()
{
	int nametemp;
	int numbertemp = 0;
	FILE *fp;
	struct prs *ptrthis = malloc(sizeof(*ptrthis));
	ptrthis = ptrfirst;

	fp=fopen("c:\\database.txt", "ab");

	while(fread(ptrthis, sizeof(*ptrthis), 1, fp) ==1)
	{
		printf("%s\n", ptrthis->name);
		printf("%d\n", ptrthis->number);
		ptrthis = ptrthis->ptrnext;
	}

	system("PAUSE");
}

Hopefully it's not to bad

Sorry Salem, but as I already said, I'm not that good at C

And fread still returns 0.. so maybe it's my savetofile() function thats not working? Just wondering though, as I really have no idea

That seemed to help. Now it's writing fine, afaik

But fread still returns 0..

Thank you for the help so far!