LIST_FIRST_ENTRY(9) Data Types LIST_FIRST_ENTRY(9)NAME
list_first_entry - get the first element from a list
SYNOPSIS
list_first_entry(ptr, type, member);
ARGUMENTS
ptr
the list head to take the element from.
type
the type of the struct this is embedded in.
member
the name of the list_struct within the struct.
DESCRIPTION
Note, that list is expected to be not empty.
COPYRIGHT Kernel Hackers Manual 3.10 June 2014 LIST_FIRST_ENTRY(9)
Check Out this Related Man Page
libtalloc_destructors(3) talloc libtalloc_destructors(3)NAME
libtalloc_destructors - Chapter 4: Using destructors
Using destructors
Destructors are well known methods in the world of object oriented programming. A destructor is a method of an object that is automatically
run when the object is destroyed. It is usually used to return resources taken by the object back to the system (e.g. closing file
descriptors, terminating connection to a database, deallocating memory).
With talloc we can take the advantage of destructors even in C. We can easily attach our own destructor to a talloc context. When the
context is freed, the destructor will run automatically.
To attach/detach a destructor to a talloc context use: talloc_set_destructor().
Example
Imagine that we have a dynamically created linked list. Before we deallocate an element of the list, we need to make sure that we have
successfully removed it from the list. Normally, this would be done by two commands in the exact order: remove it from the list and then
free the element. With talloc, we can do this at once by setting a destructor on the element which will remove it from the list and
talloc_free() will do the rest.
The destructor would be:
int list_remove(void *ctx)
{
struct list_el *el = NULL;
el = talloc_get_type_abort(ctx, struct list_el);
/* remove element from the list */
}
GCC version 3 and newer can check for the types during the compilation. So if it is our major compiler, we can use a more advanced
destructor:
int list_remove(struct list_el *el)
{
/* remove element from the list */
}
Now we will assign the destructor to the list element. We can do this directly in the function that inserts it.
struct list_el* list_insert(TALLOC_CTX *mem_ctx,
struct list_el *where,
void *ptr)
{
struct list_el *el = talloc(mem_ctx, struct list_el);
el->data = ptr;
/* insert into list */
talloc_set_destructor(el, list_remove);
return el;
}
Because talloc is a hierarchical memory allocator, we can go a step further and free the data with the element as well:
struct list_el* list_insert_free(TALLOC_CTX *mem_ctx,
struct list_el *where,
void *ptr)
{
struct list_el *el = NULL;
el = list_insert(mem_ctx, where, ptr);
talloc_steal(el, ptr);
return el;
}
Version 2.0 Tue Jun 17 2014 libtalloc_destructors(3)
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