insque(), remque()

manipulate queues 



#include <search.h>

void insque(void *element, void *pred);

void remque(void *element);


The insque() and remque() functions manipulate queues built from doubly-linked lists. The queue can either be circular or linear. An application using insque() or remque() must define a structure where the first two members of the structure are pointers to the same type of structure, and any further members are application-specific. The first member of the structure is a forward pointer to the next entry in the queue. The second member is a backward pointer to the previous entry in the queue. If the queue is linear, the queue is terminated with null pointers. The names of the structure and of the pointer members is not subject to any special restriction.

The insque() function inserts the specified element into a queue immediately after the specified predecessor element.

The remque() function removes the specified element from the queue.

If the queue is to be used as a linear list, invoking

insque(&element, NULL);

where element is the initial element of the queue, initializes the forward and backward pointers of element to null pointers.

If the queue is to be used as a circular list, the application must initialize the forward pointer and the backward pointer of the initial element of the queue to the element's own address.



Is the element to be inserted into, or removed from, the queue.


Is the predecessor element that the specified element is to be inserted after.








The historical implementation of these functions described the arguments as being of type struct qelem * rather than as being of type void * as defined by UNIX 98. In those implementations, struct qelem was commonly defined in <search.h> as

struct qelem {
	struct qelem *q_forw;
	struct qelem *q_back;

Applications using these functions, however, were never able to use this structure directly, since it provided no room for the actual data contained in the elements. Most applications defined structures that contained the two pointers as the initial elements and also provided space for, or pointers to, the object's data. Applications that used these functions to update more than one type of table also had the problem of specifying two or more different structures with the same name, if they literally used struct qelem as specified.

The implementations of these functions, however, were actually expecting a structure as defined in the description above. With C compilers that didn't provide function prototypes, applications used structures as specified and the compiler did what the applications expected.

If this method had been carried forward with an ANSI C compiler and the historical function prototype, most applications would have to be modified to cast pointers to the structure actually used to be pointers to struct qelem to avoid compilation warnings. By specifying void * as the argument type, applications won't need to change.


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