The Single UNIX ® Specification, Version 2
Copyright © 1997 The Open Group


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


#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 be either circular or linear. An application using insque() or remque() must define a structure in which 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 are not subject to any special restriction.

The insque() function inserts the element pointed to by element into a queue immediately after the element pointed to by pred.

The remque() function removes the element pointed to by element from a 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, will initialise the forward and backward pointers of element to null pointers.

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


The insque() and remque() functions do not return a value.


No errors are defined.




The historical implementations of these functions described the arguments as being of type struct qelem * rather than as being of type void * as defined here. 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.

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

If this method had been carried forward with an ISO C compiler and the historical function prototype, most applications would have to be modified to cast pointers to the structures 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 (unless they specifically referenced struct qelem and depended on it being defined in <search.h>).





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Copyright © 1997 The Open Group
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