insque(S)

insque, remque -- insert or remove an element of a queue

Syntax

cc ...-lc

include <search.h>

void insque(void *elem, void *pred);
void remque(void *elem);

Description

insque- insert an entry into a queue

remque- remove an entry from a queue

insque(S) and remque(S) manipulate queues built from doubly linked lists. The queues can be either linear or circular.

An application that uses insque( ) or remque( ) must define a structure in which the first two members are pointers to the same type of structure, and any other members are application-specific. The first member 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, it terminates with null pointers.

The names of the structure and members are not restricted in any special way.

Each element in the queue must be in the following form:

   struct qelem {
   	struct	qelem *q_forw;
   	struct	qelem *q_back;
   	char	q_data[];
   };
   insque(elem, pred)
   struct qelem *elem, *pred;
   
   remque(elem)
   struct qelem *elem;

insque( ) inserts the element pointed to by elem into a queue immediately after the preceding element, pred. remque( ) removes the entry pointed to by elem from a queue.

For a linear list, you can initialize the forward and backward pointers of elem to null pointers by invoking:

insque (&element,NULL)

For a circular list, the application must initialize the forward and backward pointers of the initial element to their own addresses.

History

Older implementations of these functions described them as type struct qelem * rather than type void *. In those implementations, struct qelem * was usually defined in search.h as:

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

This structure provided no room for actual data in the elements, so applications using insque( ) or remque( ) could not use the structure directly. Therefore, most applications had to define structures that contained the two pointers as initial elements and also provided space for, or pointers to, the object's data. Applications that updated more than one type of table also had the problem of specifying two or more different structures with the same name, if they used struct qelem * literally as specified.

As described here, the implementations actually expected a structure type where the first two members were forward and backward pointers to structures. With C compilers that did not provide function prototypes, applications used structures as specified in ``Description'', above, and the compiler did what the application expected.

If this method had been used with an ANSI compiler and the historical function prototype, to avoid compiler warnings you would have to modify most applications to cast pointers to the structures actually used, to cast pointers to struct qelem *. However, if applications specify an argument type of void *, you do not need to change them unless they refer to struct qelem * and depend on its being defined in search.h.

Return values

insque( ) and remque( ) do not return any values.

Diagnostics

This function does not set errno.

Standards conformance

insque(S) and remque(S) are conformant with:

X/Open CAE Specification, System Interfaces and Headers, Issue 4, Version 2. .