这里是C的新手。我正在使用sys/queue.h
进行简单的排队。我已经在SO和Google上进行了大量搜索,但找不到针对该特定问题的解决方案。
这很好:
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>
TAILQ_HEAD(, q_item) head;
typedef struct q_item {
int value;
TAILQ_ENTRY(q_item) entries;
} q_item;
void enqueue(int n) {
// enqueue the node with value n
q_item *item;
item = malloc(sizeof(q_item));
item->value = n;
printf("queued %d\n", item->value);
TAILQ_INSERT_TAIL(&head, item, entries);
}
void dequeue() {
q_item *returned_item;
returned_item = TAILQ_FIRST(&head);
printf("dequeued %d\n", returned_item->value);
TAILQ_REMOVE(&head, returned_item, entries);
free(returned_item);
}
int main() {
TAILQ_INIT(&head);
enqueue(1);
enqueue(2);
enqueue(3);
dequeue();
return 0;
}
我知道通常应该避免使用全局变量。尽管我也知道TAILQ_HEAD()是一个宏,但也许这改变了我对此的看法。无论如何,这不会编译:
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>
typedef struct q_item {
int value;
TAILQ_ENTRY(q_item) entries;
} q_item;
void enqueue(int n, TAILQ_HEAD(, q_item) * head) {
// enqueue the node with value n
q_item *item;
item = malloc(sizeof(q_item));
item->value = n;
printf("queued %d\n", item->value);
TAILQ_INSERT_TAIL(head, item, entries);
}
void dequeue(TAILQ_HEAD(, q_item) * head) {
q_item *returned_item;
returned_item = TAILQ_FIRST(head);
printf("dequeued %d\n", returned_item->value);
TAILQ_REMOVE(head, returned_item, entries);
free(returned_item);
}
int main() {
TAILQ_HEAD(, q_item) head; // <-- I've moved TAILQ_HEAD into main()
TAILQ_INIT(&head);
enqueue(1, &head);
enqueue(2, &head);
enqueue(3, &head);
dequeue(&head);
return 0;
}
当我尝试编译后者时,出现以下错误。他们无济于事,因为我无法分辨‘struct <anonymous> *’
和‘struct <anonymous> *’
之间的区别。
test_tailq_noglobal.c:32:13: warning: passing argument 2 of ‘enqueue’ from incompatible pointer type [-Wincompatible-pointer-types]
enqueue(1, &head);
^
test_tailq_noglobal.c:10:6: note: expected ‘struct <anonymous> *’ but argument is of type ‘struct <anonymous> *’
void enqueue(int n, TAILQ_HEAD(, q_item) * head) {
^~~~~~~
我知道TAILQ_HEAD的手册页显示您可以定义以下内容:
TAILQ_HEAD(tailhead, entry) head;
struct tailhead *headp; /* Tail queue head. */
但是我不确定该struct tailhead *headp
怎么办。我尝试如下将其作为指针传递来代替&head
,但这似乎也不起作用:
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>
typedef struct q_item {
int value;
TAILQ_ENTRY(q_item) entries;
} q_item;
void enqueue(int n, struct headname *headp) {
// enqueue the node with value n
q_item *item;
item = malloc(sizeof(q_item));
item->value = n;
printf("queued %d\n", item->value);
TAILQ_INSERT_TAIL(headp, item, entries);
}
void dequeue(struct headname *headp) {
q_item *returned_item;
returned_item = TAILQ_FIRST(headp);
printf("dequeued %d\n", returned_item->value);
TAILQ_REMOVE(headp, returned_item, entries);
free(returned_item);
}
int main() {
TAILQ_HEAD(headname, q_item) head;
struct headname *headp;
TAILQ_INIT(headp);
enqueue(1, headp);
enqueue(2, headp);
enqueue(3, headp);
dequeue(headp);
return 0;
}
错误:
test_tailq_headp.c: In function ‘dequeue’:
test_tailq_headp.c:21:18: error: dereferencing pointer to incomplete type ‘struct headname’
returned_item = TAILQ_FIRST(headp);
^
test_tailq_headp.c: In function ‘main’:
test_tailq_headp.c:33:13: warning: passing argument 2 of ‘enqueue’ from incompatible pointer type [-Wincompatible-pointer-types]
enqueue(1, headp);
^~~~~
test_tailq_headp.c:10:6: note: expected ‘struct headname *’ but argument is of type ‘struct headname *’
void enqueue(int n, struct headname *headp) {
^~~~~~~
有人可以告诉我我在做错什么,无论是在我的代码中还是在我思考这个问题的方式上?谢谢。
答案 0 :(得分:3)
我从不使用此队列,但是我发现的一种方法是在q_item自身内部添加TAILQ_HEAD()。它有助于避免全局使用head。
#include <stdio.h>
#include <stdlib.h>
#include <sys/queue.h>
typedef struct q_item {
int value;
TAILQ_ENTRY(q_item) entries;
TAILQ_HEAD(, q_item) head;
} q_item;
void enqueue(int n, q_item *q) {
// enqueue the node with value n
q_item *item;
item = malloc(sizeof(q_item));
item->value = n;
printf("queued %d\n", item->value);
TAILQ_INSERT_TAIL(&q->head, item, entries);
}
void dequeue(q_item *q) {
q_item *returned_item;
returned_item = TAILQ_FIRST(&q->head);
printf("dequeued %d\n", returned_item->value);
TAILQ_REMOVE(&q->head, returned_item, entries);
free(returned_item);
}
int main() {
q_item q;
TAILQ_INIT(&q.head);
enqueue(1, &q);
enqueue(2, &q);
enqueue(3, &q);
dequeue(&q);
dequeue(&q);
return 0;
}
另外,查看此宏的扩展方式可能很有用,只需使用gcc
选项使用-E
进行编译,您将看到例如TAILQ_HEAD(, qitem) head
扩展为
struct {
struct q_item *tqh_first;
struct q_item **tqh_last;
} head;
您还可以在<sys/queue.h>
header
/*
* Tail queue definitions.
*/
#define TAILQ_HEAD(name, type) \
struct name { \
struct type *tqh_first; /* first element */ \
struct type **tqh_last; /* addr of last next element */ \
}
这就是为什么您尝试使用void enqueue(int n, TAILQ_HEAD(, q_item) * head)
无效的原因,因为预处理程序可以简单地替换。