Question

I just wrote a simple linked list, however when iterating through the list via add() and display() the program seg faults.

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

typedef struct entry {
    void *value;
    struct entry *next;
} entry;

typedef struct list {
    entry *items;
} list;

list *create(void) {
    list *l;

    l = malloc (sizeof(list));
    l->items = malloc(sizeof(entry*));
    l->items->next = NULL;

    return l;
}

void add(list *l, void *value) {
    entry *temp, *last, *new;

    for (temp = l->items; temp != NULL; temp = temp->next) {
        last = temp;
    }

    new = malloc(sizeof(*new));

    new->value = value;
    new->next = NULL;

    last->next = new;
}

void display(list *l) {
    entry *temp;

    for (temp = l->items; temp != NULL; temp = temp->next) {
        printf("%s\n", temp->value);
    }
}

int main(void) {
    list *l = create();

    add(l, "item1");
    add(l, "item2");
    add(l, "item3");
    add(l, "item4");

    display(l);

    return 0;
}

I have tested the code on a few machines and it works on a few and doesn’t on others. I’m clueless about the source of the error.

Answer 1

In addition to the wrong size passed to malloc that FatalError mentioned, you allocate memory for l->items

list *create(void) {
    list *l;

    l = malloc (sizeof(list));
    l->items = malloc(sizeof(entry*));
    l->items->next = NULL;

    return l;
}

but don’t ever set l->items->value to anything, so it’s an uninitialised pointer and when you try to dereference it when printing

void display(list *l) {
    entry *temp;

    for (temp = l->items; temp != NULL; temp = temp->next) {
        printf("%s\n", temp->value);
    }
}

in the first iteration of the loop, that can easily lead to a segfault even if the insufficient allocation size didn’t cause one before.