how to stop cycle that creates a linked list to create one more empty link?

Question

struct data {
  int date;
  int temperature;
  char day[11];
  struct data *next;
}; 
typedef struct data Data;

int from_file_to_list(File *fp, Data **list) {
  Data *p;
  int cnt = 0;
  *list = p;
  p = malloc(sizeof(Data));
  while (fscanf(fp, "%d" "%s" "%d", &(p->date), p->day, &(p->temperature)) == 3) {
    p->next =  malloc(sizeof(Data));
    p = p->next;
    cnt++;
  }
  return cnt;
}

I have this function that creates a linked list with the data of a text file and it returns the number of links in the list and it works fine, but it allocates memory for an additional empty link at the end, is there any way to stop that? Because if i would have a function that adds a link in the end there could be problems right?

Answer 1

For starters it seems there is a typo

int from_file_to_list(File *fp, Data **list) {
                      ^^^^

There should be

int from_file_to_list(FILE *fp, Data **list) {
                      ^^^^

I have this function that creates a linked list with the data of a text file and it returns the number of links in the list and it works fine,

You are mistaken. The function has undefined behavior. In this statement

int from_file_to_list(File *fp, Data **list) {
  Data *p;
  int cnt = 0;
  *list = p;
  //.. 

you assigned uninitialized pointer p to the pointer *list. That is after exiting the function the pointer list in main will have this indeterminate value.

The function can be defined for example at least the following way

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

//... 

size_t from_file_to_list( FILE *fp, Data **list ) 
{
    size_t cnt = 0;

    int date;
    int temperature;
    char day[11];
    
    while ( fscanf( fp, "%d" "%s" "%d", &date, day, &temperature ) == 3 && 
            ( *list = malloc( sizeof( Data ) ) ) != NULL ) 
    {
        ( *list )->date = date;
        ( *list )->temperature = temperature;
        strcpy( ( *list )->day, day );
        ( *list )->next = NULL;
        
        list = &( *list )->next;
        ++cnt;
    }
    
    return cnt;
}

Or even it will be better to free the list before creating a new list from a file

size_t from_file_to_list( FILE *fp, Data **list ) 
{
    while ( *list )
    {
        Data *tmp = *list;
        *list = ( *list )->next;
        free( tmp );
    }
    
    size_t cnt = 0;

    int date;
    int temperature;
    char day[11];
    
    while ( fscanf( fp, "%d" "%s" "%d", &date, day, &temperature ) == 3 && 
            ( *list = malloc( sizeof( Data ) ) ) != NULL ) 
    {
        ( *list )->date = date;
        ( *list )->temperature = temperature;
        strcpy( ( *list )->day, day );
        ( *list )->next = NULL;
        
        list = &( *list )->next;
        ++cnt;
    }
    
    return cnt;
}

If you will write a separate function that clears a list like

void clear( Data **list )
{
    while ( *list )
    {
        Data *tmp = *list;
        *list = ( *list )->next;
        free( tmp );
    }
}    

then the above function can be written like

size_t from_file_to_list( FILE *fp, Data **list ) 
{
    clear ( list );
    
    size_t cnt = 0;

    int date;
    int temperature;
    char day[11];
    
    while ( fscanf( fp, "%d" "%s" "%d", &date, day, &temperature ) == 3 && 
            ( *list = malloc( sizeof( Data ) ) ) != NULL ) 
    {
        ( *list )->date = date;
        ( *list )->temperature = temperature;
        strcpy( ( *list )->day, day );
        ( *list )->next = NULL;
        
        list = &( *list )->next;
        ++cnt;
    }
    
    return cnt;
}