How to load data from file into linked list c - c

Question

The program should read a simple text file and store the data ( name and id) into a linked list.
This is the linked list:
struct Prova
{
char nome[16];
int id;
};
typedef struct Node {
struct Prova struttura;
struct Node * next;
}TNodo;
typedef TNodo* Nodo;
This function creates the linked list:
void NewList(struct Prova p, Nodo * pp)
{
Nodo temp;
temp = (Nodo)malloc(sizeof(struct Node));
temp->struttura = p;
temp->next = *pp;
*pp = temp;
}
This is the function i wrote to read the file:
void Load(Nodo *pp)
{
FILE *f;
struct Prova p;
char * buffer;
if(!(f = fopen(PATH, "r")))
{
perror("Error");
exit(-1);
}
buffer = malloc(sizeof(struct Prova));
while(fgets(buffer, sizeof(buffer), f))
{
if(sscanf(buffer, "%s%d", p.nome, &p.id) > 0)
{
NewList(p, pp);
}
}
free(buffer);
fclose(f);
}
The text file i'm trying to read is this:
Stefano 31
Paperino 23
Pippo 1
Pluto 14
The functions for displaying the list are these:
void View(struct Prova p)
{
printf("%s %d\n", p.nome, p.id);
}
void ViewList(Nodo nodo)
{
while(nodo != NULL)
{
View(nodo->struttura);
nodo = nodo->next;
}
The program compiles fine, but it outputs the data in a strange order.
Let me know if you need more info, i think it's all related to the function Load() which is the newest.
Main function is this:
int main()
{
int scelta;
struct Prova test;
Nodo lista = NULL;
do {
scelta = Menu();
switch (scelta)
{
case 1: Load(&lista); break
case 2: ViewList(lista); break;
default: scelta = 0;
}
} while (scelta != 0);
return 0;
}
For now the output is this:

Answer 1

The problem is occurring because of passing sizeof(buffer) to fgets(). buffer is a pointer and sizeof(buffer) will return either 4 or 8 based on underlying architecture 32 bit or 64 bit. Change this to:
while(fgets(buffer, sizeof(struct Prova), f))
as you are allocating sizeof(struct Prova) size to buffer.
This is also not the correct solution of the problem. Reason is-
Say, your file is having this data:
abcdefghijklmno 123456789
The name part is 15 character long which nome member can hold and id member can also hold the number 123456789 as it is less than INT_MAX. fgets read characters from the stream and the size of above-given data is 25 characters. In your code, you are allocating sizeof(struct Prova) size memory to buffer and size of struct Prova is 20 byte. Hence for the above-given data, buffer is not allocated enough memory to read the whole line in one go because the fgets reads until size-1 characters have been read or either a newline or the end-of-file is reached, whichever happens first. So, in this case, the partial line will be read and passed to sscanf() and rest of the line will be read in next iteration and passed to sscanf() which will give the incorrect results.
You should not allocate the memory to buffer based on the size of struct Prova. Instead, I would suggest taking buffer of some bigger size, like this (no need to allocate it dynamically):
char buffer[100];
ensure it should be bigger enough to accommodate a line of the file in one read of fgets or modify your code to not to fill p.nome and p.id until fgets hits newline character or EOF i.e. to make sure to read the whole line and then only sscanf() the p.nome and p.id.