Embed a shared memory segment using the a_list structure (linked list in shared memory) in C

Question

I need to make a shared memory segment that contains the nodes using this structure that I created

struct a_list
{
  //The head of the list that acts as a node, this list has a next* and a prev*
  struct list_head list;
  unsigned long*   val;
  char* str;
  char state;
};

I tried to find answers on the stack, but they implemented Node, unlike my structure, which is a list with nodes inside

I also have this buffer and the add method that I used to add to this list (using malloc), but I know that for shared memory I cannot do this

struct buffer
{
    struct a_list* buffer[BUFFER_SIZE];
    int in;
    int out;
};
//Method to append a file to the end of the queue (Most recently used files are at the tail)
static void append(struct a_list* ptr,const char* str, unsigned long val)           
{
  struct a_list* tmp;
  tmp = (struct a_list*)malloc(sizeof(struct a_list));

 // tmp->str = str;
  tmp->str = (char *)malloc(strlen(str)+1);
  strcpy(tmp->str, str);
  tmp->val = (unsigned long*)malloc(sizeof(unsigned long));
  memcpy(tmp->val, &val, sizeof(unsigned long));
  tmp->index = (int *)malloc(sizeof(int));
  memcpy(tmp->index, &buffer_p->in, sizeof(int));
  tmp->address = (struct a_list*)malloc(sizeof(struct a_list*));
  memcpy(tmp->address, &fileQueue, sizeof(struct a_list*));
  if(list_empty(&fileQueue.list)){ //If this is the first element to be inserted into the fileQueue, initialize the head
    list_add_tail( &(tmp->list), &(ptr->list) );
    head = list_entry(fileQueue.list.next,struct a_list, list);
  }
  else{ //Else just add it to the tail of the list
    list_add_tail( &(tmp->list), &(ptr->list) );
  }
 // while(((buffer_p->in+1)%BUFFER_SIZE) == buffer_p->out)
    //      ;

  buffer_p->buffer[buffer_p->in] = tmp->address;
        //printf("Placed: %d\n", counter);

  buffer_p->in = (buffer_p->in+1) % BUFFER_SIZE;
}

and basically I'm trying to initialize everything with

if ((shmid = shmget(key, (BUFFER_SIZE+2)*sizeof(int), IPC_CREAT | 0666)) < 0) {
        perror("shmget");
        exit(1);
    }

    if ((shm = shmat(shmid, NULL, 0)) < 0) {
        perror("shmat");
        exit(1);
    }

    buffer_p = (struct buffer*) shm;
    buffer_p->in = 0;
    buffer_p->out = 0;

, , im , malloc'd ( ), , , Beej Guide , , (, , ). , , , . list_head a_list (fileQueue) , .

EDIT: list_head, ,

/**
 * 
 * I grub it from linux kernel source code and fix it for user space
 * program. Of course, this is a GPL licensed header file.
 *
 * Here is a recipe to cook list.h for user space program
 *
 * 1. copy list.h from linux/include/list.h
 * 2. remove 
 *     - #ifdef __KERNE__ and its #endif
 *     - all #include line
 *     - prefetch() and rcu related functions
 * 3. add macro offsetof() and container_of
 *
 * - [e-mail]
 */
#ifndef _LINUX_LIST_H
#define _LINUX_LIST_H

/**
 * @name from other kernel headers
 */
/*@{*/

/**
 * Get offset of a member
 */
#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)

/**
 * Casts a member of a structure out to the containing structure
 * @param ptr        the pointer to the member.
 * @param type       the type of the container struct this is embedded in.
 * @param member     the name of the member within the struct.
 *
 */
#define container_of(ptr, type, member) ({                      \
        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
        (type *)( (char *)__mptr - offsetof(type,member) );})
/*@}*/


/*
 * These are non-NULL pointers that will result in page faults
 * under normal circumstances, used to verify that nobody uses
 * non-initialized list entries.
 */
#define LIST_POISON1  ((void *) 0x00100100)
#define LIST_POISON2  ((void *) 0x00200200)

/**
 * Simple doubly linked list implementation.
 *
 * Some of the internal functions ("__xxx") are useful when
 * manipulating whole lists rather than single entries, as
 * sometimes we already know the next/prev entries and we can
 * generate better code by using them directly rather than
 * using the generic single-entry routines.
 */
struct list_head {
    struct list_head *next, *prev;
};

#define LIST_HEAD_INIT(name) { &(name), &(name) }

#define LIST_HEAD(name) \
    struct list_head name = LIST_HEAD_INIT(name)

#define INIT_LIST_HEAD(ptr) do { \
    (ptr)->next = (ptr); (ptr)->prev = (ptr); \
} while (0)

/*
 * Insert a new entry between two known consecutive entries.
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
static inline void __list_add(struct list_head *new,
                  struct list_head *prev,
                  struct list_head *next)
{
    next->prev = new;
    new->next = next;
    new->prev = prev;
    prev->next = new;
}

/**
 * list_add - add a new entry
 * @new: new entry to be added
 * @head: list head to add it after
 *
 * Insert a new entry after the specified head.
 * This is good for implementing stacks.
 */
static inline void list_add(struct list_head *new, struct list_head *head)
{
    __list_add(new, head, head->next);
}

/**
 * list_add_tail - add a new entry
 * @new: new entry to be added
 * @head: list head to add it before
 *
 * Insert a new entry before the specified head.
 * This is useful for implementing queues.
 */
static inline void list_add_tail(struct list_head *new, struct list_head *head)
{
    __list_add(new, head->prev, head);
}


/*
 * Delete a list entry by making the prev/next entries
 * point to each other.
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
static inline void __list_del(struct list_head * prev, struct list_head * next)
{
    next->prev = prev;
    prev->next = next;
}

/**
 * list_del - deletes entry from list.
 * @entry: the element to delete from the list.
 * Note: list_empty on entry does not return true after this, the entry is
 * in an undefined state.
 */
static inline void list_del(struct list_head *entry)
{
    __list_del(entry->prev, entry->next);
    entry->next = LIST_POISON1;
    entry->prev = LIST_POISON2;
}



/**
 * list_del_init - deletes entry from list and reinitialize it.
 * @entry: the element to delete from the list.
 */
static inline void list_del_init(struct list_head *entry)
{
    __list_del(entry->prev, entry->next);
    INIT_LIST_HEAD(entry);
}

/**
 * list_move - delete from one list and add as another head
 * @list: the entry to move
 * @head: the head that will precede our entry
 */
static inline void list_move(struct list_head *list, struct list_head *head)
{
        __list_del(list->prev, list->next);
        list_add(list, head);
}

/**
 * list_move_tail - delete from one list and add as another tail
 * @list: the entry to move
 * @head: the head that will follow our entry
 */
static inline void list_move_tail(struct list_head *list,
                  struct list_head *head)
{
        __list_del(list->prev, list->next);
        list_add_tail(list, head);
}

/**
 * list_empty - tests whether a list is empty
 * @head: the list to test.
 */
static inline int list_empty(const struct list_head *head)
{
    return head->next == head;
}

static inline void __list_splice(struct list_head *list,
                 struct list_head *head)
{
    struct list_head *first = list->next;
    struct list_head *last = list->prev;
    struct list_head *at = head->next;

    first->prev = head;
    head->next = first;

    last->next = at;
    at->prev = last;
}

/**
 * list_splice - join two lists
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 */
static inline void list_splice(struct list_head *list, struct list_head *head)
{
    if (!list_empty(list))
        __list_splice(list, head);
}

/**
 * list_splice_init - join two lists and reinitialise the emptied list.
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 *
 * The list at @list is reinitialised
 */
static inline void list_splice_init(struct list_head *list,
                    struct list_head *head)
{
    if (!list_empty(list)) {
        __list_splice(list, head);
        INIT_LIST_HEAD(list);
    }
}

/**
 * list_entry - get the struct for this entry
 * @ptr:    the &struct list_head pointer.
 * @type:   the type of the struct this is embedded in.
 * @member: the name of the list_struct within the struct.
 */
#define list_entry(ptr, type, member) \
    container_of(ptr, type, member)

/**
 * list_for_each    -   iterate over a list
 * @pos:    the &struct list_head to use as a loop counter.
 * @head:   the head for your list.
 */

#define list_for_each(pos, head) \
  for (pos = (head)->next; pos != (head);   \
       pos = pos->next)

/**
 * __list_for_each  -   iterate over a list
 * @pos:    the &struct list_head to use as a loop counter.
 * @head:   the head for your list.
 *
 * This variant differs from list_for_each() in that it the
 * simplest possible list iteration code, no prefetching is done.
 * Use this for code that knows the list to be very short (empty
 * or 1 entry) most of the time.
 */
#define __list_for_each(pos, head) \
    for (pos = (head)->next; pos != (head); pos = pos->next)

/**
 * list_for_each_prev   -   iterate over a list backwards
 * @pos:    the &struct list_head to use as a loop counter.
 * @head:   the head for your list.
 */
#define list_for_each_prev(pos, head) \
    for (pos = (head)->prev; prefetch(pos->prev), pos != (head); \
            pos = pos->prev)

/**
 * list_for_each_safe   -   iterate over a list safe against removal of list entry
 * @pos:    the &struct list_head to use as a loop counter.
 * @n:      another &struct list_head to use as temporary storage
 * @head:   the head for your list.
 */
#define list_for_each_safe(pos, n, head) \
    for (pos = (head)->next, n = pos->next; pos != (head); \
        pos = n, n = pos->next)

/**
 * list_for_each_entry  -   iterate over list of given type
 * @pos:    the type * to use as a loop counter.
 * @head:   the head for your list.
 * @member: the name of the list_struct within the struct.
 */
#define list_for_each_entry(pos, head, member)              \
    for (pos = list_entry((head)->next, typeof(*pos), member);  \
         pos->member != (head);                 \
         pos = list_entry(pos->member->next, typeof(*pos), member))

/**
 * list_for_each_entry_reverse - iterate backwards over list of given type.
 * @pos:    the type * to use as a loop counter.
 * @head:   the head for your list.
 * @member: the name of the list_struct within the struct.
 */
#define list_for_each_entry_reverse(pos, head, member)          \
    for (pos = list_entry((head)->prev, typeof(*pos), member);  \
         &pos->member != (head);    \
         pos = list_entry(pos->member.prev, typeof(*pos), member))

/**
 * list_prepare_entry - prepare a pos entry for use as a start point in
 *          list_for_each_entry_continue
 * @pos:    the type * to use as a start point
 * @head:   the head of the list
 * @member: the name of the list_struct within the struct.
 */
#define list_prepare_entry(pos, head, member) \
    ((pos) ? : list_entry(head, typeof(*pos), member))

/**
 * list_for_each_entry_continue -   iterate over list of given type
 *          continuing after existing point
 * @pos:    the type * to use as a loop counter.
 * @head:   the head for your list.
 * @member: the name of the list_struct within the struct.
 */
#define list_for_each_entry_continue(pos, head, member)         \
    for (pos = list_entry(pos->member.next, typeof(*pos), member);  \
         &pos->member != (head);    \
         pos = list_entry(pos->member.next, typeof(*pos), member))

/**
 * list_for_each_entry_safe - iterate over list of given type safe against removal of list entry
 * @pos:    the type * to use as a loop counter.
 * @n:      another type * to use as temporary storage
 * @head:   the head for your list.
 * @member: the name of the list_struct within the struct.
 */
#define list_for_each_entry_safe(pos, n, head, member)          \
    for (pos = list_entry((head)->next, typeof(*pos), member),  \
        n = list_entry(pos->member.next, typeof(*pos), member); \
         &pos->member != (head);                    \
         pos = n, n = list_entry(n->member.next, typeof(*n), member))

/**
 * list_for_each_entry_safe_continue -  iterate over list of given type
 *          continuing after existing point safe against removal of list entry
 * @pos:    the type * to use as a loop counter.
 * @n:      another type * to use as temporary storage
 * @head:   the head for your list.
 * @member: the name of the list_struct within the struct.
 */
#define list_for_each_entry_safe_continue(pos, n, head, member)         \
    for (pos = list_entry(pos->member.next, typeof(*pos), member),      \
        n = list_entry(pos->member.next, typeof(*pos), member);     \
         &pos->member != (head);                        \
         pos = n, n = list_entry(n->member.next, typeof(*n), member))

/**
 * list_for_each_entry_safe_reverse - iterate backwards over list of given type safe against
 *                    removal of list entry
 * @pos:    the type * to use as a loop counter.
 * @n:      another type * to use as temporary storage
 * @head:   the head for your list.
 * @member: the name of the list_struct within the struct.
 */
#define list_for_each_entry_safe_reverse(pos, n, head, member)      \
    for (pos = list_entry((head)->prev, typeof(*pos), member),  \
        n = list_entry(pos->member.prev, typeof(*pos), member); \
         &pos->member != (head);                    \
         pos = n, n = list_entry(n->member.prev, typeof(*n), member))




/*
 * Double linked lists with a single pointer list head.
 * Mostly useful for hash tables where the two pointer list head is
 * too wasteful.
 * You lose the ability to access the tail in O(1).
 */

struct hlist_head {
    struct hlist_node *first;
};

struct hlist_node {
    struct hlist_node *next, **pprev;
};

#define HLIST_HEAD_INIT { .first = NULL }
#define HLIST_HEAD(name) struct hlist_head name = {  .first = NULL }
#define INIT_HLIST_HEAD(ptr) ((ptr)->first = NULL)
#define INIT_HLIST_NODE(ptr) ((ptr)->next = NULL, (ptr)->pprev = NULL)

static inline int hlist_unhashed(const struct hlist_node *h)
{
    return !h->pprev;
}

static inline int hlist_empty(const struct hlist_head *h)
{
    return !h->first;
}

static inline void __hlist_del(struct hlist_node *n)
{
    struct hlist_node *next = n->next;
    struct hlist_node **pprev = n->pprev;
    *pprev = next;
    if (next)
        next->pprev = pprev;
}

static inline void hlist_del(struct hlist_node *n)
{
    __hlist_del(n);
    n->next = LIST_POISON1;
    n->pprev = LIST_POISON2;
}


static inline void hlist_del_init(struct hlist_node *n)
{
    if (n->pprev)  {
        __hlist_del(n);
        INIT_HLIST_NODE(n);
    }
}

static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h)
{
    struct hlist_node *first = h->first;
    n->next = first;
    if (first)
        first->pprev = &n->next;
    h->first = n;
    n->pprev = &h->first;
}



/* next must be != NULL */
static inline void hlist_add_before(struct hlist_node *n,
                    struct hlist_node *next)
{
    n->pprev = next->pprev;
    n->next = next;
    next->pprev = &n->next;
    *(n->pprev) = n;
}

static inline void hlist_add_after(struct hlist_node *n,
                    struct hlist_node *next)
{
    next->next = n->next;
    n->next = next;
    next->pprev = &n->next;

    if(next->next)
        next->next->pprev  = &next->next;
}



#define hlist_entry(ptr, type, member) container_of(ptr,type,member)

#define hlist_for_each(pos, head) \
    for (pos = (head)->first; pos && ({ prefetch(pos->next); 1; }); \
         pos = pos->next)

#define hlist_for_each_safe(pos, n, head) \
    for (pos = (head)->first; pos && ({ n = pos->next; 1; }); \
         pos = n)

/**
 * hlist_for_each_entry - iterate over list of given type
 * @tpos:   the type * to use as a loop counter.
 * @pos:    the &struct hlist_node to use as a loop counter.
 * @head:   the head for your list.
 * @member: the name of the hlist_node within the struct.
 */
#define hlist_for_each_entry(tpos, pos, head, member)            \
    for (pos = (head)->first;                    \
         pos && ({ prefetch(pos->next); 1;}) &&          \
        ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
         pos = pos->next)

/**
 * hlist_for_each_entry_continue - iterate over a hlist continuing after existing point
 * @tpos:   the type * to use as a loop counter.
 * @pos:    the &struct hlist_node to use as a loop counter.
 * @member: the name of the hlist_node within the struct.
 */
#define hlist_for_each_entry_continue(tpos, pos, member)         \
    for (pos = (pos)->next;                      \
         pos && ({ prefetch(pos->next); 1;}) &&          \
        ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
         pos = pos->next)

/**
 * hlist_for_each_entry_from - iterate over a hlist continuing from existing point
 * @tpos:   the type * to use as a loop counter.
 * @pos:    the &struct hlist_node to use as a loop counter.
 * @member: the name of the hlist_node within the struct.
 */
#define hlist_for_each_entry_from(tpos, pos, member)             \
    for (; pos && ({ prefetch(pos->next); 1;}) &&            \
        ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
         pos = pos->next)

/**
 * hlist_for_each_entry_safe - iterate over list of given type safe against removal of list entry
 * @tpos:   the type * to use as a loop counter.
 * @pos:    the &struct hlist_node to use as a loop counter.
 * @n:      another &struct hlist_node to use as temporary storage
 * @head:   the head for your list.
 * @member: the name of the hlist_node within the struct.
 */
#define hlist_for_each_entry_safe(tpos, pos, n, head, member)        \
    for (pos = (head)->first;                    \
         pos && ({ n = pos->next; 1; }) &&               \
        ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
         pos = n)


#endif

Answer 1

:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/shm.h>
#define MAX_SHM_ID_NUM 100

typedef struct node
{
    int val;
    struct node * next;
} node;
int shm_id_arr[MAX_SHM_ID_NUM];
int current_index = -1 ;

void insert_at_tail (int num);
node * my_head = NULL;
int * num_of_elements = NULL ;

key_t key_first = 5681;
key_t key_current;

void *  my_malloc(int size)
{
    void * ptr = NULL;
    key_current = key_first ++;
    int shm_id;
    if ((shm_id = shmget(key_current, size , IPC_CREAT | 0666)) < 0) {
        perror("shmget error.");printf("errno= %d EINVAL=%d \n ", errno , EINVAL);
       return NULL;
    }

    if ((ptr = shmat(shm_id, NULL, 0)) == (void *) - 1) {
        perror("shmat error");
        //exit(1);
        return NULL;

    }
    current_index ++ ;
    shm_id_arr[current_index] = shm_id ;
    return ptr;

}

void insert_at_tail (int num)
{
    if(my_head == NULL)
    {
        my_head = my_malloc(sizeof(node));
        my_head->val = num;
        my_head->next = NULL;

    }else
    {
        node * tmp = my_head;
        while(tmp->next != NULL)
            tmp = tmp->next;
        tmp->next = my_malloc(sizeof(node));
        tmp->next->val = num;
        tmp->next->next = NULL;
    }
    (* num_of_elements) ++;
}
/* deAttach the shared memory without removing. */
void deattach_shared_mem()
{

   if (shmdt(num_of_elements) < 0) { /*  deAttach  num_of_elements */
       perror("shmdt error num_of_elements\n");

   }

   if (shmdt(my_head) < 0) {
       perror("shmdt error my_head\n");
   }


   //how to deattach all pointers in list?
}
void remove_shared_mem()
{
    int i;
    for(i = 0 ; i < current_index ; i ++)
    {
        if (shmctl(shm_id_arr[i], IPC_RMID, NULL) < 0) { /* remove the shared memory segment. */
            perror("shmctl error.\n");

        }
    }

}

void print_it()
{
    node * tmp = my_head;
    while(tmp != NULL)
    {
        printf("%d\n" , tmp->val);
        tmp = tmp->next;
    }
}

int main()
{
    num_of_elements = (int *)my_malloc(sizeof(int));
    (* num_of_elements) = 0 ;
    insert_at_tail(10);
    insert_at_tail(8);
    insert_at_tail(6);
    insert_at_tail(4);
    insert_at_tail(2);

    printf("we have %d elements.\n" , (*num_of_elements));

    print_it();
    deattach_shared_mem();
    remove_shared_mem();


    return 0 ;
}

Answer 2

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, , .

, unsigned long, val struct a_list. . , unsigned long, . , unsigned long, , .

index struct a_list, . struct list_head. , , , . - . MCVE ( , ?) SSCCE (Short, Self-Contained, ) - .