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I'm working on a project that strictly requires to realize two set of functions in C with same signature that can be used from a sigle .c test file. one set is for a data structure, the other one for a different and incompatible data structure.
Since in C there is no polymorphism is not possible to call a function that has two implementation with same signature in two different headers (.h) files and taking for granted that the call will be referred to the right implementation of the function that is actually capable of managing the right data structure.
Ok I know it seems impossible and contradictory but..that is it...
I have to merge two generic items that can be list or dynamic array
Update:
on List.h (dynamicArray is in another .h)
typedef struct Node{
void *data;
struct Node *next, *prevNode;
} Node;
//typedef struct declaration List
typedef struct List {
struct Node *top, *bot, *prev;
int size;
} List;
//in the dynamicarray.h file:
typedef struct dynamicArray{
void **array;
size_t size;
size_t capacity;
}dynArray;
//in the dynamicarray.h file:
void* merge(void *element1,void *element2, int parameters){
void * returner;
if (parameters==ARRAY) {
returner= Array_Merge(element1,element2); // expected to receive two arrays
}
else {
returner= List_Merge(element1,element2); // expected to reveice two lists
}
return returner;
}
Do you have any suggestion to accomplish this request?
Thanks.
You need to pass both, a pointer to your function and some handler function to the test, along with argument(s). In 'c' void * can be use in place of any pointer. Something like the following might work for you:
int mytest(void*(*function)(void *), int(*handler)(void *), void *arg) {
if (handler(function(arg)))
return OK;
return FAIL;
}
So, you just need to have separate handler functions for arrays and lists and pass them to the test function along with other params.
Answering your last comment
I can imagine some scheme as the following.
List list1;
dyArray array1;
MergedList outList;
MergedArray outArray;
...
void *getNextArrayElement(dynArray *array){...}
void *getNextListElement(List *list){...}
int mergeAsList(void* el, void *list){
if (el == NULL)
return 0;
ListMember *mmb = malloc(sizeof(ListMember));
mmb->el = el;
mmb->next = ((MergeList*)list)->head;
(MergeList*)mergeList->head = mmb;
return 1;
}
int mergeAsArray(void *el, void *array) {
if (el == NULL)
return 0;
if (((MergeArray *)array)->index) >= MAX)
return 0;
((MergeArray *)array)[((MergeArray *)array)->index++] = el;
return 1;
}
int mergeAsSortedArray(void *el, void *array){...}
...
test(getNextArrayEelement, mergeAsList, &arraty1, &outList);
test(getNextListEelement, mergeAsList, &list1, &outArray);
...
int test (void *(get*)(void*),
int (merge*)(void *m1, void *result),
void *in,
void *out) {
void *el = get(in);
int res = merge(el, out);
return res;
}
Function pointers are the means in which you accomplish this.
Function pointers are what you would use if, for example, you wanted to pass a function to a sort function that told the sort function how to compare two adjacent members. Such a comparison function allows you to provide a generalized sort function that will work on a collection of any struct, since you can change out the comparison function to accommodate any struct.
Consider the following sort code:
typedef struct node{
void* item;
struct node* next;
} Node;
// Just an ordinary bubble sort
void sort(Node *start, bool greaterThan(void* a, void* b))
{
int swapped, i;
Node *ptr1;
Node *lptr = NULL;
/* Checking for empty list */
if (start == NULL)
return;
do
{
swapped = 0;
ptr1 = start;
while (ptr1->next != lptr)
{
if (greaterThan(ptr1->item, ptr1->next->item))
{
swap(ptr1, ptr1->next);
swapped = 1;
}
ptr1 = ptr1->next;
}
lptr = ptr1;
}
while (swapped);
}
// Swap function used above
void swap(Node *a, Node *b)
{
void* temp = a->item;
a->item = b->item;
b->item = temp;
}
To use it, we just need to define a payload to put into Node* item and a sort function to tell it how to order the items:
typedef struct {
int book_id;
char title[50];
char author[50];
char subject[100];
char ISBN[13];
} Book;
// Comparison function.
bool bookGreaterThan(void* left, void* right)
{
Book* a = (Book*)left;
Book* b = (Book*)right;
return strcmp(a->title, b->title) > 0;
}
Finally, you would sort your list like so:
// Pass a pointer to the first node in your list, and a function pointer to your comparer.
sort(pointerToMyList, bookGreaterThan);
A complete example can be found here.
See also Is it possible to achieve runtime polymorphism in C?
I cannot figure out how to run this correctly, gives segmentation error. A piece of code is below. Can you look at head too , i am not sure if it is right way of initialising head to null in another file , it is run as follows :
Table tb ;
tb= initialise_table (table_size);
tb = insert(text_words,tb);
//these 3 typedef declarations are in a "some.h" file
typedef struct node * tree_ptr;
typedef char* Key_Type;
typedef struct table* Table;
struct node {
Key_Type element;
tree_ptr left;
tree_ptr right;
};
struct table {
tree_ptr head;
};
Table init_table() {
Table head = NULL;
}
Table insert(Key_Type key ,Table temp ) {
tree_ptr t = (tree_ptr)malloc(sizeof(tree_ptr));
t->element = key;
// t->left = t->right = NULL;
if (temp->head==NULL) {
temp = (Table)malloc (sizeof (Table));
temp->head = t;
printf("empty tree ");
}
else {
temp = insert(t->element,temp);
printf("inserted into ");
}
return temp;
printf("wowo!");
}
The primary issue is in the code which, you say, is used to invoke the functions:
Table tb;
tb = insert(text_words, tb);
You have an uninitialized pointer, tb, which you pass to the function. Inside the function, you have:
Table insert(Key_Type key, Table temp)
{
tree_ptr t = (tree_ptr)malloc(sizeof(*t)); // Fixed size
t->element = key;
// t->left = t->right = NULL;
if (temp->head==NULL)
{
You're therefore accessing (dereferencing) the undefined pointer, and your program is crashing.
You should, I assume, be initializing your table with table_init(), but that function is actually no help whatsoever. It defines and initializes a local variable, but doesn't return anything even though it promises to do so.
Please see Is it a good idea to typedef pointers? The short answer is 'No, it usually isn't a good idea'.
You still have problems even if you fix the calling code like this (a necessary but not sufficient step):
Table tb = NULL;
tb = insert(text_words, tb);
or maybe:
Table tb = init_table();
tb = insert(text_words, tb);
but you need a seriously upgraded version of init_table(), such as:
Table init_table(void)
{
Table root = malloc(sizeof(*head));
root->head = NULL;
return root;
}
Your code in insert() needs to ensure that it does not dereference a null pointer (instead of an indeterminate pointer).
Table insert(Key_Type key, Table root)
{
tree_ptr t = (tree_ptr)malloc(sizeof(*t)); // Fixed size
t->element = key;
t->left = t->right = NULL;
if (root == NULL)
{
root = init_table();
root->head = t;
}
else
{
…
}
return root;
}
Given the Key_Type is a char * in disguise, you may need to review how you save the keys in the tree structure; you may need to use strdup() to copy the data. It is impossible to say for sure without seeing how you are managing the strings that you pass to the insert() function. It could be OK to just save the pointer if the calling code ensures that a new pointer is passed each time. OTOH, if the same pointer is passed each time, you definitely need to copy the data, and using strdup() is a sensible way of doing that. Note that strdup() is standard on POSIX; it is not part of standard C.
Here's one major problem:
tree_ptr t = (tree_ptr) malloc(sizeof(tree_ptr));
should be:
tree_ptr t = (tree_ptr) malloc(sizeof(struct node));
Your code doesn't actually do any binary search. Indeed, it just infinitely recurses creating new nodes. Try something more like this:
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
typedef struct Node
{
char *element;
struct Node *left;
struct Node *right;
} Node;
typedef struct
{
Node *root;
size_t size;
} Tree;
void Tree_init(Tree *t);
Node *Tree_insert(Tree *t, const char *key);
void Tree_insert_r(Node *subtree, Node *n, size_t size);
void Tree_pre_order_r(Node *subtree);
void Tree_init(Tree *t)
{
t->root = NULL;
t->size = 0;
}
Node *Tree_insert(Tree *t, const char *key)
{
Node *ret = (Node*) malloc(sizeof(Node));
if (ret)
{
ret->left = ret->right = NULL;
if ((ret->element = strdup(key))) /* make a copy of key */
{
if (NULL != t->root)
Tree_insert_r(t->root, ret, t->size);
else
t->root = ret;
++t->size;
}
else
{
free(ret);
ret = NULL;
}
}
return ret;
}
void Tree_insert_r(Node *subtree, Node *n, size_t size)
{
int cmp = strcmp(n->element, subtree->element);
if (cmp < 0 || (cmp == 0 && size % 2 == 0))
{
if (NULL != subtree->left)
subtree = subtree->left;
else
{
subtree->left = n;
return;
}
}
else
{
if (NULL != subtree->right)
subtree = subtree->right;
else
{
subtree->right = n;
return;
}
}
Tree_insert_r(subtree, n, size);
}
void Tree_pre_order_r(Node *subtree)
{
if (NULL == subtree)
return;
fprintf(stdout, "'%s'\n", subtree->element);
Tree_pre_order_r(subtree->left);
Tree_pre_order_r(subtree->right);
}
int main()
{
Tree t;
Tree_init(&t);
Tree_insert(&t, "Hello");
Tree_insert(&t, "World!");
Tree_insert(&t, "etc.");
Tree_pre_order(t.root);
return 0;
}
I have written the following code, and it prints the root value correctly, but not the ret value. Here a memory address is potentially printed (1707388). I believe that ret could now be modified and the result would be seen in main. Any help is appreciated.
#include <stdlib.h>
struct node{
int value;
int order;
struct node *left;
struct node *right;
};
typedef struct node node_t;
node_t array[10];
void createTree(node_t *p, int order){
p->value = rand()%10;
p->order = order;
printf("%i", p->value);
printf(" ");
printf("%i\n", p->order);
if (!order){
p->left = NULL;
p->right = NULL;
return;
}
order--;
createTree(&p->left, order);
createTree(&p->right, order);
}
void traverse(node_t *current, node_t *ret, int size){
printf("%i\n", current->value);
if (current->value > size){
ret = current;
traverse(¤t->left, &ret, size);
traverse(¤t->right, &ret, size);
}
return;
}
int main(void){
node_t *root = &array[0];
node_t *ret;
srand(time(NULL));
createTree(root, 4);
int i = 3;
printf("%s", "root-value: ");
printf("%i\n", root->value);
traverse(root, ret, i);
printf("%s", "root-value: ");
printf("%i\n", root->value);
printf("%i\n", ret->value);
return 1;
}
This:
void createTree(node_t *p, int order)
Should be
void createTree(node_t **p, int order)
Otherwise you are modifying a local node_t pointer, instead of the one outside the function. Your tree isn't being built properly either.
You are passing ret by value to
void traverse(node_t *current, node_t *ret, int size){
When the function changes ret, the changes do not propagate back to the caller.
This means that ret in main() remains uninitialized, and the behaviour of your code is undefined.
To fix this, make traverse either return ret, or take it as node_t**.
There are few issues with the code.
First, you don't correctly allocate the memory for nodes. In your code, you are passing wrong pointer type, futhermore, pointer to uninitialized area.
Here, how it can be used differently:
node_t *createTree(int order)
{
node_t *result = malloc(sizeof(*result));
result->value = rand() % 10;
result->order = order;
if (order)
{
result->left = createTree(order - 1);
result->right = createTree(order - 1);
}
else
{
result->left = result->right = 0;
}
return result;
}
Then, your traverse function need some block to restrict agains failed search:
node_t *traverse(node_t *current, int size)
{
node_t *ret = NULL;
if (current->value > size)
{
// assuming current node fit - stops the search
ret = current;
}
if (!ret && current->left)
{
// try left node
ret = traverse(current->left, size);
}
if (!ret && current->right)
{
// try right node
ret = traverse(current->right, size);
}
return ret;
}
In case you need (usually you do), here is a destroyTree:
void destroyTree(node_t *node)
{
if (!node) return; // we treat NULL as a valid pointer for simplicity
destroyTree(node->left);
destroyTree(node->right);
free(node);
}
And here is a usage example:
node_t *root, *found;
root = createTree(4);
found = traverse(root, 3);
if (found)
{
printf("Found!");
}
destroyTree(root);
In traverse(node_t *current, node_t *ret, int size), ret is a stack variable. In other words, you are passing the pointer by value, instead of passing it by reference.
What have you done at the moment is essentially the same as:
int f(int i) {
...
i = <any value>;
...
}
In this case you are modifying only a copy of the value.
In your program, you are also modifying a copy of the pointer. Outside of the function the pointer stays not modified.
If you want to modify it, you need to pass a pointer to it:
void traverse(node_t *current, node_t **ret, int size){
...
*ret = current;
...
return;
}
The same for createTree().
I am trying to traverse a binary tree using twalk() with <search.h>
#define _GNU_SOURCE /* Expose declaration of tdestroy() */
#include <search.h>
#include <stdlib.h>
#include <stdio.h>
#include <time.h>
void *root = NULL;
void *
xmalloc(unsigned n)
{
void *p;
p = malloc(n);
if (p)
return p;
fprintf(stderr, "insufficient memory\n");
exit(EXIT_FAILURE);
}
int
compare(const void *pa, const void *pb)
{
if (*(int *) pa < *(int *) pb)
return -1;
if (*(int *) pa > *(int *) pb)
return 1;
return 0;
}
void
action(const void *nodep, const VISIT which, const int depth)
{
int *datap;
switch (which) {
case preorder:
break;
case postorder:
datap = *(int **) nodep;
printf("%6d\n", *datap);
break;
case endorder:
break;
case leaf:
datap = *(int **) nodep;
printf("%6d\n", *datap);
break;
}
}
int
main(void)
{
int i, *ptr;
void *val;
srand(time(NULL));
for (i = 0; i < 12; i++) {
ptr = (int *) xmalloc(sizeof(int));
*ptr = rand() & 0xff;
val = tsearch((void *) ptr, &root, compare);
if (val == NULL)
exit(EXIT_FAILURE);
else if ((*(int **) val) != ptr)
free(ptr);
}
twalk(root, action);
tdestroy(root, free);
exit(EXIT_SUCCESS);
}
As you can see there is no way to pass to or return any variable from action().
why is so hermetic? I can't use any global because the program uses threads, my question is: how can I traverse (and share nodep with non-global variable) in thread-safe mode?
Excuse my poor english
EDIT:
As unwind said, the solution is to re-invent this particular wheel, redefine the structure used at tsearch.c solves the problem:
/* twalk() fake */
struct node_t
{
const void *key;
struct node_t *left;
struct node_t *right;
unsigned int red:1;
};
static void tmycallback(const xdata *data, const void *misc)
{
printf("%s %s\n", (const char *)misc, data->value);
}
static void tmywalk(const struct node_t *root, void (*callback)(const xdata *, const void *), const void *misc)
{
if (root->left == NULL && root->right == NULL) {
callback(*(xdata * const *)root, misc);
} else {
if (root->left != NULL) tmywalk(root->left, callback, misc);
callback(*(xdata * const *)root, misc);
if (root->right != NULL) tmywalk(root->right, callback, misc);
}
}
/* END twalk() fake */
if (root) tmywalk(root, tmycallback, "Hello walker");
I guess nobody can answer the "why" exactly, except those who specified and implemented the functions. I guess "shortsightedness", or maybe "historical reasons" (they did it before thread programming became a common thing).
Anyway, this API seems a bit "toyish" to me due to this limitation, as do in fact all APIs that fail to include a user-owned void * that is just opaquely passed around between API and any callbacks.
So, the solution I guess is to re-invent this particular wheel, and write your own functions to traverse a binary tree.
You can use thread-local storage to be able to use a global variable and still be thread-safe. Apparently you can use the __thread keyword for this purpose. Also check Using __thread in c99.
I need to write AVL-tree with generic type in C. The best way I know is to use [ void* ] and to write some functions for creating, copying, assignment and destruction. Please, tell me some better way.
I will give you an example on how you can achieve generics functionality in C. The example is on a linked list, but I am sure you can adapt it on your AVL tree if necessary.
First of all you will need to define a structure for list element. A possible (most simple implementation):
struct list_element_s {
void *data;
struct list_element_s *next;
};
typedef struct list_element_s list_element;
Where 'data' will act as the "container" where you are going to keep your information, and 'next' is the reference to the direct linked element. (NOTE: Your binary tree element should include a reference to the right / left children elements).
After you create you element structure, you will need to create your list structure. A good practice is to have some members that are pointing to functions: destructor (needed to free the memory being hold by 'data'), and comparator (to be able to compare two of your list elements).
A list structure implementation could look like this:
struct list_s {
void (*destructor)(void *data);
int (*cmp)(const void *e1, const void *e2);
unsigned int size;
list_element *head;
list_element *tail;
};
typedef struct list_s list;
After you design your data structure, you should design your data structure interface. Let's say our list will have the following, most simple, interface:
nmlist *list_alloc(void (*destructor)(void *data));
int list_free(list *l);
int list_insert_next(list *l, list_element *element, const void *data);
void *list_remove_next(list *l, list_element *element);
Where:
list_alloc : will alocate memory for your list.
list_free : will free memory allocated for list, and all 'data' being held by list_element(s).
list_insert_next : will insert a new element next to 'element' . If 'element' is NULL, the insertion will be made at the head of the list.
list_remove_next : will remove & return (void*)'data' being held by 'element->next' . If 'element' is NULL, it will perform "list->head removal".
And now the functions implementation:
list *list_alloc(void (*destructor)(void *data))
{
list *l = NULL;
if ((l = calloc(1,sizeof(*l))) != NULL) {
l->size = 0;
l->destructor = destructor;
l->head = NULL;
l->tail = NULL;
}
return l;
}
int list_free(list *l)
{
void *data;
if(l == NULL || l->destructor == NULL){
return (-1);
}
while(l->size>0){
if((data = list_remove_next(l, NULL)) != NULL){
list->destructor(data);
}
}
free(l);
return (0);
}
int list_insert_next(list *l, list_element *element, const void *data)
{
list_element *new_e = NULL;
new_e = calloc(1, sizeof(*new_e));
if (l == NULL || new_e == NULL) {
return (-1);
}
new_e->data = (void*) data;
new_e->next = NULL;
if (element == NULL) {
if (l->size == 0) {
l->tail = new_e;
}
new_e->next = l->head;
l->head = new_e;
} else {
if (element->next == NULL) {
l->tail = new_e;
}
new_e->next = element->next;
element->next = new_e;
}
l->size++;
return (0);
}
void *list_remove_next(list *l, list_element *element)
{
void *data = NULL;
list_element *old_e = NULL;
if (l == NULL || l->size == 0) {
return NULL;
}
if (element == NULL) {
data = l->head->data;
old_e = l->head;
l->head = l->head->next;
if (l->size == 1) {
l->tail = NULL;
}
} else {
if (element->next == NULL) {
return NULL;
}
data = element->next->data;
old_e = element->next;
element->next = old_e->next;
if (element->next == NULL) {
l->tail = element;
}
}
free(old_e);
l->size--;
return data;
}
And now, how to use your simple generic linked list implementation. In the following example the list is acting like a stack:
#include <stdlib.h>
#include <stdio.h>
#include "nmlist.h"
void simple_free(void *data){
free(data);
}
int main(int argc, char *argv[]){
list *l = NULL;
int i, *j;
l = list_alloc(simple_free);
for(i = 0; i < 10; i++){
j = calloc(1, sizeof(*j));
if(j != NULL){
*j = i;
list_insert_next(l, NULL, (void*) j);
}
}
for(i = 0; i < 10; i++){
j = (int*) list_remove_next(l, NULL);
if(j != NULL){
printf("%d \n", *j);
}
}
list_free(l);
return (0);
}
Note that instead of "int *j" you can use a pointer that references more complex structures. If you do, don't forget to modify your 'list->destructor' function accordingly.
What Alex said. In c, void * is what there is.
Assuming you must work in C, though... Why do you need to provide the create/copy/assignment/destruction functions to the library? Which features of this library require the AVL-tree code to use those operations?
The major operations on a search tree are insert, delete and lookup, correct? You will need to provide a comparison function for all of those operations, but you should let the clients of this library handle all of the other operations. Simple is probably better in this case.
To do true, performant generics in C, you hack with the preprocessor. This approach has many of the same disadvantages of the C++ template approach; namely that all (most, anyway) code must live in header files, and debugging and testing are a pain. The advantages are also there; that you can get superior performance and let the compiler do all sorts of inlining to speed things up, minimize allocations by reducing indirection, and a modicum of type safety.
The definition looks like (let's imagine we have a hash set)
int my_int_set(int x);
#define HASH_SET_CONTAINED_TYPE int
#define HASH_SET_TYPE my_int_set
#define HASH_SET_FUNC hash_int
#include "hash_set.h"
And then to use it, you simply use the type you created above:
my_int_set x;
my_int_set_init(&x);
my_int_set_add(&x, 7);
if (my_int_set_check(&x, 7)) printf("It worked!\n");
...
// or, if you prefer
my_int_set *x = my_int_set_create();
Internally, this is implemented by a whole bunch of token pasting, etc., and (as noted above) is a huge pain to test.
So something like:
#ifndef HASH_SET_CONTAINED_TYPE
#error Must define HASH_SET_CONTAINED_TYPE
#endif
... /// more parameter checking
#define HASH_SET_ENTRY_TYPE HASH_SET_TYPE ## _entry
typedef struct HASH_SET_ENTRY_TYPE ## _tag {
HASH_SET_CONTAINED_TYPE key;
bool present;
} HASH_SET_ENTRY_TYPE;
typedef struct HASH_SET_TYPE ## _tag {
HASH_SET_TYPE ## _entry data[];
size_t elements;
} HASH_SET_TYPE;
void HASH_SET_TYPE ## _add(HASH_SET_CONTAINED_TYPE value) {
...
}
...
#undef HASH_SET_CONTAINED_TYPE
... // remaining uninitialization
You can even add options; like #define HASH_SET_VALUE_TYPE or #define HASH_SET_DEBUG.