## Doubly-Linked Lists

Doubly-Linked Lists — linked lists that can be iterated over in both directions

## Synopsis

    #include <glib.h>

    struct              GList;

    GList *             g_list_append                       (GList *list,
                                                             gpointer data);
    GList *             g_list_prepend                      (GList *list,
                                                             gpointer data);
    GList *             g_list_insert                       (GList *list,
                                                             gpointer data,
                                                             gint position);
    GList *             g_list_insert_before                (GList *list,
                                                             GList *sibling,
                                                             gpointer data);
    GList *             g_list_insert_sorted                (GList *list,
                                                             gpointer data,
                                                             GCompareFunc func);
    GList *             g_list_remove                       (GList *list,
                                                             gconstpointer data);
    GList *             g_list_remove_link                  (GList *list,
                                                             GList *llink);
    GList *             g_list_delete_link                  (GList *list,
                                                             GList *link_);
    GList *             g_list_remove_all                   (GList *list,
                                                             gconstpointer data);
    void                g_list_free                         (GList *list);
    void                g_list_free_full                    (GList *list,
                                                             GDestroyNotify free_func);

    GList *             g_list_alloc                        (void);
    void                g_list_free_1                       (GList *list);
    #define             g_list_free1

    guint               g_list_length                       (GList *list);
    GList *             g_list_copy                         (GList *list);
    GList *             g_list_copy_deep                    (GList *list,
                                                             GCopyFunc func,
                                                             gpointer user_data);
    GList *             g_list_reverse                      (GList *list);
    GList *             g_list_sort                         (GList *list,
                                                             GCompareFunc compare_func);
    gint                (*GCompareFunc)                     (gconstpointer a,
                                                             gconstpointer b);
    GList *             g_list_insert_sorted_with_data      (GList *list,
                                                             gpointer data,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);
    GList *             g_list_sort_with_data               (GList *list,
                                                             GCompareDataFunc compare_func,
                                                             gpointer user_data);
    gint                (*GCompareDataFunc)                 (gconstpointer a,
                                                             gconstpointer b,
                                                             gpointer user_data);
    GList *             g_list_concat                       (GList *list1,
                                                             GList *list2);
    void                g_list_foreach                      (GList *list,
                                                             GFunc func,
                                                             gpointer user_data);
    void                (*GFunc)                            (gpointer data,
                                                             gpointer user_data);

    GList *             g_list_first                        (GList *list);
    GList *             g_list_last                         (GList *list);
    #define             g_list_previous                     (list)
    #define             g_list_next                         (list)
    GList *             g_list_nth                          (GList *list,
                                                             guint n);
    gpointer            g_list_nth_data                     (GList *list,
                                                             guint n);
    GList *             g_list_nth_prev                     (GList *list,
                                                             guint n);

    GList *             g_list_find                         (GList *list,
                                                             gconstpointer data);
    GList *             g_list_find_custom                  (GList *list,
                                                             gconstpointer data,
                                                             GCompareFunc func);
    gint                g_list_position                     (GList *list,
                                                             GList *llink);
    gint                g_list_index                        (GList *list,
                                                             gconstpointer data);

## Description

The GList structure and its associated functions provide a standard doubly-linked list data structure.

Each element in the list contains a piece of data, together with pointers which link to the previous and next elements in the list. Using these pointers it is possible to move through the list in both directions (unlike the Singly-Linked Lists which only allows movement through the list in the forward direction).

The data contained in each element can be either integer values, by using one of the Type Conversion Macros, or simply pointers to any type of data.

List elements are allocated from the slice allocator, which is more efficient than allocating elements individually.

Note that most of the GList functions expect to be passed a pointer to the first element in the list. The functions which insert elements return the new start of the list, which may have changed.

There is no function to create a GList. `NULL` is considered to be the empty list so you simply set a GList* to `NULL`.

To add elements, use `g_list_append()`, `g_list_prepend()`, `g_list_insert()` and `g_list_insert_sorted()`.

To remove elements, use `g_list_remove()`.

To find elements in the list use `g_list_first()`, `g_list_last()`, `g_list_next()`, `g_list_previous()`, `g_list_nth()`, `g_list_nth_data()`, `g_list_find()` and `g_list_find_custom()`.

To find the index of an element use `g_list_position()` and `g_list_index()`.

To call a function for each element in the list use `g_list_foreach()`.

To free the entire list, use `g_list_free()`.

## Details

### struct GList

    struct GList {
      gpointer data;
      GList *next;
      GList *prev;
    };

The GList struct is used for each element in a doubly-linked list.

gpointer `data`; | holds the element's data, which can be a pointer to any kind of data, or any integer value using the Type Conversion Macros.
GList *`next`; | contains the link to the next element in the list.
GList *`prev`; | contains the link to the previous element in the list.

### g_list_append ()

    GList *             g_list_append                       (GList *list,
                                                             gpointer data);

Adds a new element on to the end of the list.

### Note

The return value is the new start of the list, which may have changed, so make sure you store the new value.

### Note

Note that `g_list_append()` has to traverse the entire list to find the end, which is inefficient when adding multiple elements. A common idiom to avoid the inefficiency is to prepend the elements and reverse the list when all elements have been added.

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    /* Notice that these are initialized to the empty list. */
    GList *list = NULL, *number_list = NULL;

    /* This is a list of strings. */
    list = g_list_append (list, "first");
    list = g_list_append (list, "second");

    /* This is a list of integers. */
    number_list = g_list_append (number_list, GINT_TO_POINTER (27));
    number_list = g_list_append (number_list, GINT_TO_POINTER (14));

`list` : | a pointer to a GList
`data` : | the data for the new element
Returns : | the new start of the GList

### g_list_prepend ()

    GList *             g_list_prepend                      (GList *list,
                                                             gpointer data);

Adds a new element on to the start of the list.

### Note

The return value is the new start of the list, which may have changed, so make sure you store the new value.

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    /* Notice that it is initialized to the empty list. */
    GList *list = NULL;
    list = g_list_prepend (list, "last");
    list = g_list_prepend (list, "first");

`list` : | a pointer to a GList
`data` : | the data for the new element
Returns : | the new start of the GList

### g_list_insert ()

    GList *             g_list_insert                       (GList *list,
                                                             gpointer data,
                                                             gint position);

Inserts a new element into the list at the given position.

`list` : | a pointer to a GList
`data` : | the data for the new element
`position` : | the position to insert the element. If this is negative, or is larger than the number of elements in the list, the new element is added on to the end of the list.
Returns : | the new start of the GList

### g_list_insert_before ()

    GList *             g_list_insert_before                (GList *list,
                                                             GList *sibling,
                                                             gpointer data);

Inserts a new element into the list before the given position.

`list` : | a pointer to a GList
`sibling` : | the list element before which the new element is inserted or `NULL` to insert at the end of the list
`data` : | the data for the new element
Returns : | the new start of the GList

### g_list_insert_sorted ()

    GList *             g_list_insert_sorted                (GList *list,
                                                             gpointer data,
                                                             GCompareFunc func);

Inserts a new element into the list, using the given comparison function to determine its position.

`list` : | a pointer to a GList
`data` : | the data for the new element
`func` : | the function to compare elements in the list. It should return a number > 0 if the first parameter comes after the second parameter in the sort order.
Returns : | the new start of the GList

### g_list_remove ()

    GList *             g_list_remove                       (GList *list,
                                                             gconstpointer data);

Removes an element from a GList. If two elements contain the same data, only the first is removed. If none of the elements contain the data, the GList is unchanged.

`list` : | a GList
`data` : | the data of the element to remove
Returns : | the new start of the GList

### g_list_remove_link ()

    GList *             g_list_remove_link                  (GList *list,
                                                             GList *llink);

Removes an element from a GList, without freeing the element. The removed element's prev and next links are set to `NULL`, so that it becomes a self-contained list with one element.

`list` : | a GList
`llink` : | an element in the GList
Returns : | the new start of the GList, without the element

### g_list_delete_link ()

    GList *             g_list_delete_link                  (GList *list,
                                                             GList *link_);

Removes the node link_ from the list and frees it. Compare this to `g_list_remove_link()` which removes the node without freeing it.

`list` : | a GList
`link_` : | node to delete from `list`
Returns : | the new head of `list`

### g_list_remove_all ()

    GList *             g_list_remove_all                   (GList *list,
                                                             gconstpointer data);

Removes all list nodes with data equal to `data`. Returns the new head of the list. Contrast with `g_list_remove()` which removes only the first node matching the given data.

`list` : | a GList
`data` : | data to remove
Returns : | new head of `list`

### g_list_free ()

    void                g_list_free                         (GList *list);

Frees all of the memory used by a GList. The freed elements are returned to the slice allocator.

### Note

If list elements contain dynamically-allocated memory, you should either use `g_list_free_full()` or free them manually first.

`list` : | a GList

### g_list_free_full ()

    void                g_list_free_full                    (GList *list,
                                                             GDestroyNotify free_func);

Convenience method, which frees all the memory used by a GList, and calls the specified destroy function on every element's data.

`list` : | a pointer to a GList
`free_func` : | the function to be called to free each element's data

Since 2.28

### g_list_alloc ()

    GList *             g_list_alloc                        (void);

Allocates space for one GList element. It is called by `g_list_append()`, `g_list_prepend()`, `g_list_insert()` and `g_list_insert_sorted()` and so is rarely used on its own.

Returns : | a pointer to the newly-allocated GList element.

### g_list_free_1 ()

    void                g_list_free_1                       (GList *list);

Frees one GList element. It is usually used after `g_list_remove_link()`.

`list` : | a GList element

### g_list_free1

    #define             g_list_free1

Another name for `g_list_free_1()`.

### g_list_length ()

    guint               g_list_length                       (GList *list);

Gets the number of elements in a GList.

### Note

This function iterates over the whole list to count its elements.

`list` : | a GList
Returns : | the number of elements in the GList

### g_list_copy ()

    GList *             g_list_copy                         (GList *list);

Copies a GList.

### Note

Note that this is a "shallow" copy. If the list elements consist of pointers to data, the pointers are copied but the actual data is not. See `g_list_copy_deep()` if you need to copy the data as well.

`list` : | a GList
Returns : | a copy of `list`

### g_list_copy_deep ()

    GList *             g_list_copy_deep                    (GList *list,
                                                             GCopyFunc func,
                                                             gpointer user_data);

Makes a full (deep) copy of a GList.

In contrast with `g_list_copy()`, this function uses `func` to make a copy of each list element, in addition to copying the list container itself.

`func`, as a GCopyFunc, takes two arguments, the data to be copied and a user pointer. It's safe to pass NULL as user_data, if the copy function takes only one argument.

For instance, if `list` holds a list of GObjects, you can do:

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    another_list = g_list_copy_deep (list, (GCopyFunc) g_object_ref, NULL);

And, to entirely free the new list, you could do:

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    g_list_free_full (another_list, g_object_unref);

`list` : | a GList
`func` : | a copy function used to copy every element in the list
`user_data` : | user data passed to the copy function `func`, or NULL
Returns : | a full copy of `list`, use g_list_free_full to free it

Since 2.34

### g_list_reverse ()

    GList *             g_list_reverse                      (GList *list);

Reverses a GList. It simply switches the next and prev pointers of each element.

`list` : | a GList
Returns : | the start of the reversed GList

### g_list_sort ()

    GList *             g_list_sort                         (GList *list,
                                                             GCompareFunc compare_func);

Sorts a GList using the given comparison function. The algorithm used is a stable sort.

`list` : | a GList
`compare_func` : | the comparison function used to sort the GList. This function is passed the data from 2 elements of the GList and should return 0 if they are equal, a negative value if the first element comes before the second, or a positive value if the first element comes after the second.
Returns : | the start of the sorted GList

### GCompareFunc ()

    gint                (*GCompareFunc)                     (gconstpointer a,
                                                             gconstpointer b);

Specifies the type of a comparison function used to compare two values. The function should return a negative integer if the first value comes before the second, 0 if they are equal, or a positive integer if the first value comes after the second.

`a` : | a value.
`b` : | a value to compare with.
Returns : | negative value if `a` < `b`; zero if `a` = `b`; positive value if `a` > `b`.

### g_list_insert_sorted_with_data ()

    GList *             g_list_insert_sorted_with_data      (GList *list,
                                                             gpointer data,
                                                             GCompareDataFunc func,
                                                             gpointer user_data);

Inserts a new element into the list, using the given comparison function to determine its position.

`list` : | a pointer to a GList
`data` : | the data for the new element
`func` : | the function to compare elements in the list. It should return a number > 0 if the first parameter comes after the second parameter in the sort order.
`user_data` : | user data to pass to comparison function.
Returns : | the new start of the GList

Since 2.10

### g_list_sort_with_data ()

    GList *             g_list_sort_with_data               (GList *list,
                                                             GCompareDataFunc compare_func,
                                                             gpointer user_data);

Like `g_list_sort()`, but the comparison function accepts a user data argument.

`list` : | a GList
`compare_func` : | comparison function
`user_data` : | user data to pass to comparison function
Returns : | the new head of `list`

### GCompareDataFunc ()

    gint                (*GCompareDataFunc)                 (gconstpointer a,
                                                             gconstpointer b,
                                                             gpointer user_data);

Specifies the type of a comparison function used to compare two values. The function should return a negative integer if the first value comes before the second, 0 if they are equal, or a positive integer if the first value comes after the second.

`a` : | a value.
`b` : | a value to compare with.
`user_data` : | user data to pass to comparison function.
Returns : | negative value if `a` < `b`; zero if `a` = `b`; positive value if `a` > `b`.

### g_list_concat ()

    GList *             g_list_concat                       (GList *list1,
                                                             GList *list2);

Adds the second GList onto the end of the first GList. Note that the elements of the second GList are not copied. They are used directly.

`list1` : | a GList
`list2` : | the GList to add to the end of the first GList
Returns : | the start of the new GList

### g_list_foreach ()

    void                g_list_foreach                      (GList *list,
                                                             GFunc func,
                                                             gpointer user_data);

Calls a function for each element of a GList.

`list` : | a GList
`func` : | the function to call with each element's data
`user_data` : | user data to pass to the function

### GFunc ()

    void                (*GFunc)                            (gpointer data,
                                                             gpointer user_data);

Specifies the type of functions passed to `g_list_foreach()` and `g_slist_foreach()`.

`data` : | the element's data.
`user_data` : | user data passed to `g_list_foreach()` or `g_slist_foreach()`.

### g_list_first ()

    GList *             g_list_first                        (GList *list);

Gets the first element in a GList.

`list` : | a GList
Returns : | the first element in the GList, or `NULL` if the GList has no elements

### g_list_last ()

    GList *             g_list_last                         (GList *list);

Gets the last element in a GList.

`list` : | a GList
Returns : | the last element in the GList, or `NULL` if the GList has no elements

### g_list_previous()

    #define             g_list_previous(list)

A convenience macro to get the previous element in a GList.

`list` : | an element in a GList.
Returns : | the previous element, or `NULL` if there are no previous elements.

### g_list_next()

    #define             g_list_next(list)

A convenience macro to get the next element in a GList.

`list` : | an element in a GList.
Returns : | the next element, or `NULL` if there are no more elements.

### g_list_nth ()

    GList *             g_list_nth                          (GList *list,
                                                             guint n);

Gets the element at the given position in a GList.

`list` : | a GList
`n` : | the position of the element, counting from 0
Returns : | the element, or `NULL` if the position is off the end of the GList

### g_list_nth_data ()

    gpointer            g_list_nth_data                     (GList *list,
                                                             guint n);

Gets the data of the element at the given position.

`list` : | a GList
`n` : | the position of the element
Returns : | the element's data, or `NULL` if the position is off the end of the GList

### g_list_nth_prev ()

    GList *             g_list_nth_prev                     (GList *list,
                                                             guint n);

Gets the element `n` places before `list`.

`list` : | a GList
`n` : | the position of the element, counting from 0
Returns : | the element, or `NULL` if the position is off the end of the GList

### g_list_find ()

    GList *             g_list_find                         (GList *list,
                                                             gconstpointer data);

Finds the element in a GList which contains the given data.

`list` : | a GList
`data` : | the element data to find
Returns : | the found GList element, or `NULL` if it is not found

### g_list_find_custom ()

    GList *             g_list_find_custom                  (GList *list,
                                                             gconstpointer data,
                                                             GCompareFunc func);

Finds an element in a GList, using a supplied function to find the desired element. It iterates over the list, calling the given function which should return 0 when the desired element is found. The function takes two gconstpointer arguments, the GList element's data as the first argument and the given user data.

`list` : | a GList
`data` : | user data passed to the function
`func` : | the function to call for each element. It should return 0 when the desired element is found
Returns : | the found GList element, or `NULL` if it is not found

### g_list_position ()

    gint                g_list_position                     (GList *list,
                                                             GList *llink);

Gets the position of the given element in the GList (starting from 0).

`list` : | a GList
`llink` : | an element in the GList
Returns : | the position of the element in the GList, or -1 if the element is not found

### g_list_index ()

    gint                g_list_index                        (GList *list,
                                                             gconstpointer data);

Gets the position of the element containing the given data (starting from 0).

`list` : | a GList
`data` : | the data to find
Returns : | the index of the element containing the data, or -1 if the data is not found