Segmentation Fault when removing list occurrences

Question

The following code aims to remove all occurrences of an integer in a list, (linked lists), returning at the end, the number of elements removed.
Give me the error of Segmentation fault. I appreciate any explanation.

typedef struct lligada {
    int valor;
    struct lligada *prox;
} *LInt;

int removeAll (LInt *l, int x){
    int r = 0;
    LInt pt = *l, ant = NULL;
    while (pt != NULL){
        if (pt -> valor == x){
            ant -> prox = pt -> prox;
            pt = pt -> prox;
            r++;
        }
        else {
            ant  = pt;
            pt = pt -> prox;
        }
    }

    return r;
}

Answer 1

You have 2 problems with your code:

• You’re not doing free of the nodes that are removed, and so has a memory leak. This problem despite not being notorious because it does not soon launches a Segmentation fault, ends up leaving its program incorrect, and in a case where it is used excessively and will probably continue.

• Are you accessing the ant without guarantee that it has been awarded:

  LInt pt = *l, ant = NULL; //ant null aqui
  while (pt != NULL){
      if (pt -> valor == x){
          ant -> prox = pt -> prox; //utilizado aqui
  

  Note that if the first node is to be removed, then the ant is still the NULL for there is no element before that. This causes ant->prox give at once Segmentation fault, as can not dereference null/0.

To fix both problems could rewrite your removeAll for:

int removeAll (LInt *l, int x){
    int r = 0;
    LInt pt = *l, ant = NULL;
    while (pt != NULL){
        if (pt -> valor == x){
            if (ant == NULL){ //se é o primeiro nó
                *l = pt->prox; //avança a cabeça da lista para o seguinte
            }
            else { //senão faz o que já tinha
                ant -> prox = pt -> prox;
            }
            LInt toDelete = pt; //guarda o nó a remover com free
            pt = pt -> prox;
            free(toDelete); //remove o nó que já não é necessário
            r++;
        }
        else {
            ant = pt;
            pt = pt -> prox;
        }
    }

    return r;
}

See this example working on Ideone

Answer 2

suggestion:

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

struct lligada
{
    int valor;
    struct lligada *next;
};
typedef struct lligada *LInt;

// protótipos
int removeAll (LInt *l, int x);


int removeAll (LInt *l, int x)
{
    int r = 0;
    LInt pt = *l;
    LInt ant = NULL;

    while (pt != NULL)
    {
        if (pt -> valor == x)
        {
            r++;

            ant->next = pt->next->next; 
            free( pt );
        }

        else
        {
            ant = pt;
            pt = pt -> next;
        }
    }

    return r;
}