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Date: Tue, 05 Jul 2011 00:13:14 -0400
From: pete <pfiland@mindspring.com>
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Subject: Re: Linked-list problem - compiles but segfaults
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.c:7639

Andy Elvey wrote:
> 
> Hi all -
>   I've been poking around with C for a while, but I have big problems
> with linked lists. This seems to be mainly around the "declaration,
> initialisation, iterating" area.  Anyway, I have the code below, which
> compiles but gives a segfault (and I have no idea why), so hoping that
> someone may be able to help.
> ( I have three puts statements at the end because I'm unsure how to
> iterate along a list. )
> [code]
> /* This code is released to the public domain  */
> #include <ctype.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <sys/stat.h>
> 
> /* Linked list implementation */
> typedef struct _List List;
> 
> struct _List {
>         void *data;
>         List *next;
> };
> 
> static List *list_malloc (void)
> {
>    return (List *) malloc (sizeof (List));
> }
> 
> List *list_prepend (List *list, void *data)
> {
>         List *ret = list_malloc ();
>         ret->data = data;
>         ret->next = list;
>         return ret;
> }
> 
> List *list_append (List *list, void *data)
> {
>         List *ret;
>         if (list == NULL) return list_prepend (list, data);
>         ret = list;
>         for (; list->next; list = list->next);
>         list->next = list_malloc ();
>         list->next->data = data;
>         list->next->next = NULL;
>         return ret;
> }
> 
> void list_free (List *list)
> {
>         List *next;
>         while (list)
>         {
>                 next = list->next;
>                 free (list);
>                 list = next;
>         }
> }
> 
> /* End linked list implementation */
> 
> int main()
> {
> /* Declare a list */
> List *mylist = NULL;
> list_append(mylist, "foo");
> puts(mylist->data);
> list_append(mylist, (int*) 42);
> puts(mylist->data);
> list_append(mylist, "baz");
> puts(mylist->data);
> return 0;
> }
> [/code]
> Very many thanks if you can get this code working...... :)
> - Andy

/* BEGIN new.c */

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

/* Linked list implementation */
typedef struct LIST {
    struct LIST *next;
    void *data;
} List;

List *
list_append(List **head,
            List *tail,
            const void *data,
            size_t size)
{
    List *node;

    node = malloc(sizeof *node);
    if (node != NULL) {
        node -> next = NULL;
        node -> data = malloc(size);
        if (node -> data != NULL) {
            memcpy(node -> data, data, size);
            if (*head != NULL) {
                tail -> next = node;  
            } else {
                *head = node;
            }
        } else {
            free(node);
            node = NULL;
        }
    }
    return node;
}

void 
list_free(List *node, void (*free_data)(void *))
{
    List *next_node;

    while (node != NULL) {
        next_node = node -> next;
        free_data(node -> data);
        free(node);
        node = next_node;
    }
}

int 
list_fputs(List *node, FILE *stream)
{
    int rc = 0;
    
    while (node != NULL
        && (rc = fputs(node -> data, stream)) != EOF
        && (rc =  putc(        '\n', stream)) != EOF) 
    {
        node = node -> next; 
    }
    return rc;
}

/* End linked list implementation */

int 
main(void)
{
    /* Declare a list */
    List *mylist = NULL;
    List *tail = NULL;
    char *w[] = {"foo", "bar", "bazooka"};
    char **p = w;

    do {
        tail = list_append(&mylist, tail, *p, 1 + strlen(*p));
    } while(tail != NULL && ++p != w + sizeof w / sizeof *w);
    puts("/* BEGIN new.c output */\n");
    list_fputs(mylist, stdout);
    list_free(mylist, free);
    puts("\n/* END new.c output */");
    return 0;
}

/* END new.c */

-- 
pete