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To allocate an object dynamically, call the library function
malloc
(see The GNU C Library in The GNU C Library
Reference Manual). Here is how to allocate an object of type
struct intlistlink
. To make this code work, include the file
stdlib.h, like this:
#include <stddef.h> /* DefinesNULL
. */ #include <stdlib.h> /* Declaresmalloc
. */ … struct intlistlink * alloc_intlistlink () { struct intlistlink *p; p = malloc (sizeof (struct intlistlink)); if (p == NULL) fatal ("Ran out of storage"); /* Initialize the contents. */ p->datum = 0; p->next = NULL; return p; }
malloc
returns void *
, so the assignment to p
will automatically convert it to type struct intlistlink *
.
The return value of malloc
is always sufficiently aligned
(see Type Alignment) that it is valid for any data type.
The test for p == NULL
is necessary because malloc
returns a null pointer if it cannot get any storage. We assume that
the program defines the function fatal
to report a fatal error
to the user.
Here’s how to add one more integer to the front of such a list:
struct intlistlink *my_list = NULL; void add_to_mylist (int my_int) { struct intlistlink *p = alloc_intlistlink (); p->datum = my_int; p->next = mylist; mylist = p; }
The way to free the objects is by calling free
. Here’s
a function to free all the links in one of these lists:
void free_intlist (struct intlistlink *p) { while (p) { struct intlistlink *q = p; p = p->next; free (q); } }
We must extract the next
pointer from the object before freeing
it, because free
can clobber the data that was in the object.
For the same reason, the program must not use the list any more after
freeing its elements. To make sure it won’t, it is best to clear out
the variable where the list was stored, like this:
free_intlist (mylist); mylist = NULL;
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