I've been playing with C today, and something I never had the chance to play with, that is use a struct with pointers to functions...well all went good, until I started to get some strange bug, when I was cleaning the whole thing (BTW: I was compiling in x86_64 arch, in a Mac) Looking and looking I figured out that is the memory alignment, in the node_vtable
struct.
In i386, it works fine..no issues whatsoever. However, as I said in x86_64, it doesn't work.
/* NOT WORKING */
struct node_vtable {
void (*add_node) (linked_list *, node *);
node * (*create_node) (linked_list *, float, int, int );
void (*print) (linked_list *llist);
};
/* WORKING */
struct node_vtable {
void (*print) (linked_list *llist);
void (*add_node) (linked_list *, node *);
node * (*create_node) (linked_list *, float, int, int );
};
Now, I fixed this moving the pointer node * (*create_node) (linked_list *, float, int, int );
to the end of the struct, as that one has a size of 24 bytes, which is the largest in that struct. However, I really think that there is for sure a more elegant solution, and also I am looking for a clear explanation. Hence, if someone can give a hint or some explanation, that would be great ;) ....as my brain now is really stuck :)
The whole code:
#include <stdio.h>
#include <stdlib.h>
struct node {
int id;
int var;
float f_var;
struct node *next;
};
typedef struct node node;
typedef struct linked_list linked_list;
/* Structs */
struct node_vtable {
void (*add_node) (linked_list *, node *);
node * (*create_node) (linked_list *, float, int, int );
void (*print) (linked_list *llist);
};
struct linked_list {
/************************/
node *head;
node *tail;
/************************/
node *data;
/* VTable to Methods*/
struct node_vtable *method;
};
/*Prototypes*/
linked_list *constructor_linked_list();
void print(linked_list *llist);
void add_node(linked_list *this, node *node);
node *create_node(linked_list *this, float f_var, int var, int _id);
/***************/
linked_list *constructor_linked_list() {
printf("calling constructor_linked_list...\n");
linked_list *this = (linked_list *)malloc(sizeof(linked_list));
this->head = NULL;
this->tail = NULL;
this->method = NULL;
this->method = (struct node_vtable *)malloc(sizeof(struct node_vtable *));
this->method->print = &print;
this->method->add_node = &add_node;
this->method->create_node = &create_node;
return this;
}
void print(linked_list *llist) {
printf("calling print ...\n");
node *iter = llist->head;
while (iter){
printf("\tnode %d\n", iter->id);
iter = iter->next;
}
}
void add_node(linked_list *this, node *node) {
printf("calling add_node_...\n");
if (this->head == NULL) {
node->next = NULL;
this->head = node;
this->tail = node;
} else {
node->next = NULL;
this->tail->next = node;
this->tail = node;
}
}
node *create_node(linked_list *this, float f_var, int var, int _id) {
printf("calling create_node ...%d\n",(int)sizeof(struct node_vtable));
node *ret_node = (node *)malloc(sizeof(struct node));
ret_node->var = var;
ret_node->id = _id;
ret_node->f_var = f_var;
ret_node->next = NULL;
return ret_node;
}
int main(int argc, char *argv[]) {
linked_list *obj = constructor_linked_list();
int i;
for (i = 0; i < 10; i++) {
obj->method->add_node(obj, create_node(obj, 5.0, 3, i));
}
obj->method->print(obj);
return EXIT_SUCCESS;
}
Cheers
Alignment shouldn't matter at all, so it definitely has one or more bugs.
This line:
this->method = (struct node_vtable *)malloc(sizeof(struct node_vtable *));
should be:
this->method = (struct node_vtable *)malloc(sizeof(struct node_vtable));
Unfortuntaly I cannot check whether it fixes the problem (plain cygwin x86 here), if not then definitely check it with valgrind
.
You seem to have a typo :
this->method = (struct node_vtable *)malloc(sizeof(struct node_vtable *));
should be :
this->method = (struct node_vtable *)malloc(sizeof(struct node_vtable));
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