The malloc works sometimes and sometimes not (generic linked list)

Question

I'm creating generic linked list. Data to the list is loading from the .txt file (loadClients function). The problem is, when I push second element to the list (push_back function), malloc() doesn't work sometimes and I have an error 0xC0000005. For example, I run the program 10 times and it works well but for the 11th time I have an error. I can't even check what malloc() returns.

Here is the client struct:

typedef struct client
{
    int card_no;
    char last_name[30];
    char first_name[30];
    int phone_no;
    char email[60];
    char login[18];
    char password[20];
    bool admin;
}client_struct;

Here is the generic list:

typedef struct List
{
    void *data;
    struct List *next;
}List;

Here is the push_back function:

void push_back(List **head,void *data,size_t size)
{
    if(*head==NULL)
    {
        *head = (List *)malloc(sizeof(List));
        (*head)->data = malloc(size);
        (*head)->data = data;
        (*head)->next = NULL;
    }else
    {
        List *current = *head;
        while(current->next != NULL)
            current = current->next;
        printf("PUSH BACK1!\n");
        current->next = (List *)malloc(sizeof(List)); //HERE IS THE PROBLEM
        printf("PUSH BACK2!\n");
        current->next->data = malloc(size);
        current->next->data = data;
        current->next->next = NULL;
    }
}

Here is the loadClients function. This function returns the pointer to the created list.

List *loadClients()
{
    List *client_head = NULL;
    FILE *users = fopen("users.txt","r");
    if(users == NULL)
        return NULL;
    while(!feof(users))
    {
        client_struct *cl = malloc(sizeof(client_struct));
        memset(cl,0,sizeof(client_struct));
        char tab[200];
        memset(tab,0,200);
        fgets(tab,200,users);
        sscanf(tab,"%d %s %s %d %s %s %s %d",
    &cl->card_no,cl->last_name,cl->first_name,&cl->phone_no,
    cl->email,cl->login,cl->password,&cl->admin);
        printf("ADD\n");
        if(cl->card_no != 0)
            push_back(&client_head,(client_struct *)cl,sizeof(client_struct));
    }
    fclose(users);
    return client_head;
}

How can I fix this problem?

Not your main problem, but please read [Why is “while (!feof(file))” always wrong?](https://stackoverflow.com/questions/5431941/why-is-while-feoffile-always-wrong). — unwind, Apr 08 '19 at 11:34
`(*head)->data = malloc(size);` `(*head)->data = data;` looks wrong and is a memory leak. I think you intended to copy the data. For that, use `(*head)->data = malloc(size);` `memcpy((*head)->data, data, size);`. Similarly for the other branch that uses `current->next->data`. — Ian Abbott, Apr 08 '19 at 11:37
Regarding my earlier comment, since `push_back` is being called with a pointer to a `malloc`d block anyway (from the `cl` variable in `loadClients()`), perhaps you do not need to copy the data and can just get rid of the `(*head)->data = malloc(size);` and `current->next->data = `malloc(size);` lines, and keep the `(*head)->data = data;` and `current->next->data = data;` lines. Then there is no need to pass `size` to the `push_back` function unless you want to store the size in additional member of `List`. — Ian Abbott, Apr 08 '19 at 11:52
Thank you for pointing out, I changed to memcpy() and it still worked bad, but I also changed loadClients() function inside the while loop. This time I used `client_struct cl = EmptyStruct;` and `push_back(&client_head,(client_struct *)&cl,sizeof(client_struct));` and it works well (EmptyStruct is `static const client_struct EmptyStruct;`). Thank you very much for help! — Damian, Apr 08 '19 at 11:58
`current->next` This will be always NULL after `while(current->next != NULL) {...}`. Result will be `NULL = malloc`, but wait this is illegal. — Igor Galczak, Apr 08 '19 at 12:10

score -1 · Answer 1 · answered Apr 08 '19 at 12:35

-1

Compile with debugging info (-g flag):

gcc -g code.c

Then run it through valgrind:

valgrind ./a.out

Then valgrind will give you lots of useful info about what it is that goes wrong, on what line in the code and so on.

answered Apr 08 '19 at 12:35

Elias

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The malloc works sometimes and sometimes not (generic linked list)

1 Answers1