C: Output error linked-lists and writing to and reading from file

Question

Im trying to create a linked list containing numbers and write those numbers to a file, and then read the same file and read the data within the file and print those numbers.

What I think the problem is, is that there's something wrong when reading the file.

I've added som print statements for debugging, and when printing what I'm writing to the file, it looks ok. But when I'm reading the file and printing I get the first number the user entered printed twice. ex:

input: 1,2,3
output:3,2,1,1

I don't really know if there's a problem with my linked list, writing to file or if it's the reading. So I would appreciate any kind of input that would help me to better understand.

Thanks

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


typedef struct postTyp
{
    int num;
    struct postTyp *next;
}postTyp;

FILE *fp;

int main()
{
    postTyp *l, *p; //l=list , p=pointer

    l = NULL;
    p=malloc(sizeof(postTyp));


    //Creates linked list until user enters 0
    printf("Enter a number, 0 to exit: ");
    scanf("%i", &p->num);

    while (p->num != 0)
    {
        p->next=l;
        l=p; 
        p=malloc(sizeof(postTyp)); 
        printf("Enter a number, 0 to exit: ");
        scanf("%i", &p->num);

    }
    free(p);
    p=l;

    //write the linked list to file 
    fp = fopen("test.txt", "w");
    while(p->next != NULL)
    {
        printf("%2i", p->num);
        fwrite(p, 1, sizeof(postTyp), fp);
        p=p->next;
    }
    printf("%2i", p->num);
    fwrite(p, 1, sizeof(postTyp), fp);
    fclose(fp);

    printf("\n");

    //Code below to read the file content and print the numbers 
    fp = fopen("test.txt", "r");

    fread(p,sizeof(postTyp),1,fp);
    fseek(fp,0,SEEK_SET);

    //The first number entered at the beginning, will be printed twice here.
    while(!feof(fp))
    {
       fread(p,sizeof(postTyp),1,fp);
        printf("-----\n");
        printf("%i\n", p->num); 
    }

    fclose(fp);


    return 0;


}

Your thought about something being wrong while reading the file is correct: Possible duplicate of [Why is “while ( !feof (file) )” always wrong?](http://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong). — Jabberwocky, Jan 03 '17 at 12:54
thanks guys, all comments helped when understand the problem with !feof — Pa_clae, Jan 03 '17 at 13:17

score 2 · Accepted Answer · answered Jan 03 '17 at 13:08

From the fread manual (https://linux.die.net/man/3/fread):

fread() does not distinguish between end-of-file and error, and callers must use feof(3) and ferror(3) to determine which occurred.

So, you have to check the fread return value before printing p->num.

C: Output error linked-lists and writing to and reading from file

1 Answers1