链接列表C与指针

Question

I'm trying to push an element in a linked list, then print it but the problem received is: segmentation fault .

I've created struct libro , cella and lista . Then in function insHead i've tried to insert an element to the list, taking input from user and then in function printList (and here i've got the segmentation fault) i would print list's elements.

The code is:

typedef struct libro {
    char titolo[64];
    char autore[32];
    short inLibreria;
    short fuoriLibreria;
    short id;
} Libro;

typedef struct cella {
    Libro libro;
    struct cella *pNext;
} Cella;

typedef Cella *Pcella;

typedef struct listaLibri{
    Cella *pFirst;
    Cella *pLast;
} ListaLibri;

void insHead(ListaLibri lista){
    Pcella cella;
    Libro libro;
    printf("Inserisci titolo libro: ");
    scanf("%s",  libro.titolo);
    printf("Inserisci autore libro: ");
    scanf("%s",  libro.autore);
    printf("Inserisci il numero di copie presenti in libreria: ");
    scanf("%d",&libro.inLibreria);
    if(lista.pFirst == NULL){
        cella = NULL;
        Pcella temp = cella;
        cella = malloc(sizeof(Cella));
        (*cella).libro = libro;
        (*cella).pNext = temp;
        lista.pFirst = cella;
        lista.pLast = cella;
    }
    printList(lista);
}

void printList(ListaLibri *lista){
    Pcella cella = lista->pFirst;
    Pcella temp = cella;
    while (temp != NULL){
        printf("%s", temp->libro.autore);
        temp = temp->pNext;
    }
    free(cella);
    free(temp);
}

void main(){
    ListaLibri lista;
    insHead(lista);
}

Answer 1

You are not assigning the pFirst value, so it is having garbage data, which is not equal to NULL . Modify your code like this;

void insHead(ListaLibri lista)
{
    // Your code goes here

    lista.pFirst = NULL;
    if(lista.pFirst == NULL)
    {
        // Your code goes here
    }
    printList(&lista); //Pass it as a reference, because printList() input parameter is a pointer type.
}

Hope this helps.

Answer 2

You declare lista in main, but you don't initialize any fields. My guess is that lista.pFirst is non-NULL garbage and you if is then skipped.

Answer 3

First of all you have to initialize lista.pFirst and lista.pLast with NULL .

int main()
{
    ListaLibri lista;
    lista.pFirst = NULL;  // <- init NULL
    lista.pLast = NULL;   // <- init NULL

    insHead( &lista );    // <- pass pointer to list to function insHead
    insHead( &lista );
    .....       

    deleteList( &lista ); 
    return 0;
}

The input paramter to your function insHead has to be an in and output paramter. Otherwise you would pass an copy of lista to function insHead and never get back its content.

void insHead( ListaLibri *lista )
                     // ^ input and outout paramter
{
    if ( lista == NULL )
        return;

    Cella *head = lista->pFirst;           // remember head of list
    lista->pFirst = malloc(sizeof(Cella)); // allocate new node right to target
    lista->pFirst->pNext = head;           // successor of new node is head of list
    if ( lista->pLast == NULL )            // if list was empty new node is tail of list
        lista->pLast = lista->pFirst;

    Libro libro;
    printf("Inserisci titolo libro: ");
    scanf("%s",  libro.titolo);
    printf("Inserisci autore libro: ");
    scanf("%s",  libro.autore);
    printf("Inserisci il numero di copie presenti in libreria: ");
    scanf("%d",&libro.inLibreria);
    lista->pFirst->libro = libro;

    printList( lista );
}

Don't delete nodes infunction print.

void printList(ListaLibri *lista)
{
    if ( lista == NULL )
        return;

    Pcella temp = lista->pFirst;
    while (temp != NULL)
    {
        printf("%s\n", temp->libro.autore);
        temp = temp->pNext;
    }
}

Write a function which deletes the list.

void deleteList(ListaLibri *lista)
{
    if ( lista == NULL )
        return;

    Pcella temp = lista->pFirst;
    while (temp != NULL)           // terminate if tail of list is reached
    {
        Pcella next = temp->pNext; // rmember successor of node
        free( temp );              // delete node
        temp = next;               // go to next node
    }
    lista->pFirst = NULL;
    lista->pLast = NULL;
}