C - 使用 dequeue 回绕时队列的分段错误
C - Queue's Segmentation Fault when wrapping around with dequeue
好吧，我想用 Queue 创建 3 个函数。它们是 create_queue()、enqueue() 和 dequeue()。我创建了所有这三个，并在 main 中测试它们，如果它已满，它应该抛出错误，无论头部还是尾部是否到达末端，它都应该环绕。在我的例子中，我得到了 enqueue() 环绕而不是 dequeue()。我被困在这里试图弄清楚我在哪里犯了错误。如果有人能发现它并让我知道如何修复它，我将不胜感激！ ^.^
queuetest.c
#include <stdlib.h>
#include <stdio.h>
#include "queue.h"
#include <errno.h>
typedef struct {
int x;
double y;
} Foo; // Just some arbitrary struct
int main() {
const int max_entries = 4; // size of stack
Foo* new_foo1;
Foo* new_foo2;
Foo* new_foo3;
Foo* new_foo4;
Foo* new_foo5;
Foo* returned_foo;
// First, create a stack
Queue *new_queue = create_queue(max_entries);
// Allocate a Foo and push it onto the queue.
new_foo1 = (Foo *) malloc(sizeof(Foo));
new_foo1->x = 100;
new_foo1->y = 1.11;
printf("Pushing: x = %5d, y = %10.3f\n", new_foo1->x, new_foo1->y);
enqueue(new_queue, (void *) new_foo1);
// Allocate another Foo and push it onto the queue.
new_foo2 = (Foo *) malloc(sizeof(Foo));
new_foo2->x = 200;
new_foo2->y = 2.22;
printf("Pushing: x = %5d, y = %10.3f\n", new_foo2->x, new_foo2->y);
enqueue(new_queue, (void *) new_foo2);
// Allocate another Foo and push it onto the queue.
new_foo3 = (Foo *) malloc(sizeof(Foo));
new_foo3->x = 300;
new_foo3->y = 3.33;
printf("Pushing: x = %5d, y = %10.3f\n", new_foo3->x, new_foo3->y);
enqueue(new_queue, (void *) new_foo3);
// Allocate another Foo and push it onto the queue.
new_foo4 = (Foo *) malloc(sizeof(Foo));
new_foo4->x = 400;
new_foo4->y = 4.44;
printf("Pushing: x = %5d, y = %10.3f\n", new_foo4->x, new_foo4->y);
enqueue(new_queue, (void *) new_foo4);
// Allocate another Foo and push it onto the queue.
new_foo5 = (Foo *) malloc(sizeof(Foo));
new_foo5->x = 500;
new_foo5->y = 5.55;
printf("Pushing: x = %5d, y = %10.3f\n", new_foo5->x, new_foo5->y);
enqueue(new_queue, (void *) new_foo5);
/////////////////////////////////////////////////////////////////
// Dequeue two Foos and print them.
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
/////////////////////////////////////////////////////////////////
// Add 3 Foos
printf("Pushing: x = %5d, y = %10.3f\n", new_foo5->x, new_foo5->y);
enqueue(new_queue, (void *) new_foo5);
printf("Pushing: x = %5d, y = %10.3f\n", new_foo1->x, new_foo1->y);
enqueue(new_queue, (void *) new_foo1);
printf("Pushing: x = %5d, y = %10.3f\n", new_foo2->x, new_foo2->y);
enqueue(new_queue, (void *) new_foo2);
/////////////////////////////////////////////////////////////////
// Dequeue 5 Foos and print them.
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
}
queue.c
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "queue.h"
/** Create a queue by allocating a Queue structure, initializing it,
* and allocating memory to hold the queue entries.
* @param max_cells Maximum entries in the queue
* @return Pointer to newly-allocated Queue structure, NULL if error.
*/
Queue *create_queue(int max_cells) {
Queue *new_queue; // Holds pointer to the newly-allocated queue structure.
new_queue = (Queue *) malloc(sizeof(Queue));
if (new_queue == NULL) return NULL; // Error--unable to allocate.
// Fill in the struct
new_queue->max_cells = max_cells;
new_queue->cells_used = 0; // Empty to start
new_queue->ePos = 1; //Starts at first position
new_queue->dPos = 1; //Starts at first position
// Now allocate space for the queue entries.
new_queue->head = (void **) calloc(sizeof(void *), max_cells);
if (new_queue->head == NULL) {
free(new_queue); // Unable to allocate queue entries, so free struct.
return NULL;
}
new_queue->tail = new_queue->head; // Start at head
return new_queue;
}
/** Deletes a queue, including the structure and the memory
* for holding the queue entries, but not the entries themselves.
* @param which_queue Pointer to Queue structure.
*/
void delete_queue(Queue *which_queue) {
free(which_queue->head); // Free memory block with queue entries.
free(which_queue); // Then free the struct.
}
/** Pushes a pointer onto a Queue.
* @param which_queue Pointer to queue you want to push onto.
* @param ptr Pointer to be pushed.
* @return 0 if successful, -1 if not.
*/
int enqueue(Queue *which_queue, void *ptr) {
// Check if queue is already full
if ((which_queue->cells_used) >= (which_queue->max_cells)) {
which_queue->cells_used = which_queue->max_cells;
printf("Error: Queue overflow\n");
return -1; // Queue overflow.
}
// Check if tail is at the end of the Queue
if ((which_queue->ePos) == (which_queue->max_cells)) {
//Sets tail to the beginning of queue
(which_queue->tail) = (which_queue->tail) - (which_queue->max_cells);
//Sets position back to beginning
(which_queue->ePos) = 1;
}
// Push onto queue.
*(which_queue->tail) = ptr; // Store the pointer on the stack
(which_queue->tail)++; // Point to next free cell
(which_queue->cells_used)++;
(which_queue->ePos)++;
return 0; // Success
}
/** Removes head of queue, and returns it.
* @param which_queue Pointer to Queue you want to dequeue from.
* @return Head entry of the queue, NULL if queue is empty.
*/
void* dequeue(Queue *which_queue) {
// Check if queue is empty
if ((which_queue->cells_used) <= 0) {
which_queue->cells_used = 0;
printf("Error: Queue underflow\n");
return NULL; // Queue empty
}
// Check if head is at the end of the Queue
if ((which_queue->dPos) == (which_queue->max_cells)) {
//Sets head to the beginning of queue
(which_queue->head) = (which_queue->head) - 3;
//Sets position back to beginning
(which_queue->dPos) = 1;
(which_queue->cells_used)--;
return (*((which_queue->head) + 3));
}
// Remove head from queue.
(which_queue->cells_used)--;
(which_queue->head)++;
(which_queue->dPos)++;
return (*((which_queue->head) - 1));
}
注意：我知道我使用了 - 3 和 + 3 而不是 max_cells - 1 等。我只是想看看这是否是一个问题，但事实并非如此。另一个注意事项是，如果我用 1 2 3 替换 -3 或 +3，它仍然会给我一个段错误。
queue.h
/** Struct to define a queue; each entry can hold a pointer to anything.
*/
struct queue {
void **head; // Pointer to head of queue
void **tail; // Pointer to next free cell (tail);
int max_cells; // Maximum number of entries in the queue
int cells_used; // Currently used number of cells
int ePos; //Position of the enqueue
int dPos; //Position of the dequeue
};
typedef struct queue Queue;
// Function prototypes
Queue *create_queue(int max_cells);
void delete_queue(Queue *which_queue);
int enqueue(Queue *which_queue, void *ptr);
void* dequeue(Queue *which_queue);
因此，在调用第二个 dequeue() 之后，打印行上出现了段错误。这意味着 returned_foo 有一个错误的值，这是由出列函数引起的，而不是 运行 在 if 语句部分正确环绕（至少我认为这是错误>.>）。任何帮助都会有所帮助！提前致谢。
您的 enqueue() 实现从未填充队列中的最后一个元素。
create_queue(); // ePos = 1, queue: x x x x
enqueue(A); // ePos = 2, queue: A x x x
enqueue(B); // ePos = 3, queue: A B x x
enqueue(C); // ePos = 4, queue: A B C x
enqueue(D); // This already wraps because ePos == max_cells!
// ePos = 2, queue: D B C x
当dequeue()到达队列的最后一个元素时，它读取NULL（这是最后一个元素包含的内容，因为calloc用零填充内存块），然后程序崩溃了。
好吧，我想用 Queue 创建 3 个函数。它们是 create_queue()、enqueue() 和 dequeue()。我创建了所有这三个，并在 main 中测试它们，如果它已满，它应该抛出错误，无论头部还是尾部是否到达末端，它都应该环绕。在我的例子中，我得到了 enqueue() 环绕而不是 dequeue()。我被困在这里试图弄清楚我在哪里犯了错误。如果有人能发现它并让我知道如何修复它，我将不胜感激！ ^.^
queuetest.c
#include <stdlib.h>
#include <stdio.h>
#include "queue.h"
#include <errno.h>
typedef struct {
int x;
double y;
} Foo; // Just some arbitrary struct
int main() {
const int max_entries = 4; // size of stack
Foo* new_foo1;
Foo* new_foo2;
Foo* new_foo3;
Foo* new_foo4;
Foo* new_foo5;
Foo* returned_foo;
// First, create a stack
Queue *new_queue = create_queue(max_entries);
// Allocate a Foo and push it onto the queue.
new_foo1 = (Foo *) malloc(sizeof(Foo));
new_foo1->x = 100;
new_foo1->y = 1.11;
printf("Pushing: x = %5d, y = %10.3f\n", new_foo1->x, new_foo1->y);
enqueue(new_queue, (void *) new_foo1);
// Allocate another Foo and push it onto the queue.
new_foo2 = (Foo *) malloc(sizeof(Foo));
new_foo2->x = 200;
new_foo2->y = 2.22;
printf("Pushing: x = %5d, y = %10.3f\n", new_foo2->x, new_foo2->y);
enqueue(new_queue, (void *) new_foo2);
// Allocate another Foo and push it onto the queue.
new_foo3 = (Foo *) malloc(sizeof(Foo));
new_foo3->x = 300;
new_foo3->y = 3.33;
printf("Pushing: x = %5d, y = %10.3f\n", new_foo3->x, new_foo3->y);
enqueue(new_queue, (void *) new_foo3);
// Allocate another Foo and push it onto the queue.
new_foo4 = (Foo *) malloc(sizeof(Foo));
new_foo4->x = 400;
new_foo4->y = 4.44;
printf("Pushing: x = %5d, y = %10.3f\n", new_foo4->x, new_foo4->y);
enqueue(new_queue, (void *) new_foo4);
// Allocate another Foo and push it onto the queue.
new_foo5 = (Foo *) malloc(sizeof(Foo));
new_foo5->x = 500;
new_foo5->y = 5.55;
printf("Pushing: x = %5d, y = %10.3f\n", new_foo5->x, new_foo5->y);
enqueue(new_queue, (void *) new_foo5);
/////////////////////////////////////////////////////////////////
// Dequeue two Foos and print them.
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
/////////////////////////////////////////////////////////////////
// Add 3 Foos
printf("Pushing: x = %5d, y = %10.3f\n", new_foo5->x, new_foo5->y);
enqueue(new_queue, (void *) new_foo5);
printf("Pushing: x = %5d, y = %10.3f\n", new_foo1->x, new_foo1->y);
enqueue(new_queue, (void *) new_foo1);
printf("Pushing: x = %5d, y = %10.3f\n", new_foo2->x, new_foo2->y);
enqueue(new_queue, (void *) new_foo2);
/////////////////////////////////////////////////////////////////
// Dequeue 5 Foos and print them.
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
returned_foo = (Foo *) dequeue(new_queue);
printf("Removed: x = %5d, y = %10.3f\n", returned_foo->x, returned_foo->y);
}
queue.c
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "queue.h"
/** Create a queue by allocating a Queue structure, initializing it,
* and allocating memory to hold the queue entries.
* @param max_cells Maximum entries in the queue
* @return Pointer to newly-allocated Queue structure, NULL if error.
*/
Queue *create_queue(int max_cells) {
Queue *new_queue; // Holds pointer to the newly-allocated queue structure.
new_queue = (Queue *) malloc(sizeof(Queue));
if (new_queue == NULL) return NULL; // Error--unable to allocate.
// Fill in the struct
new_queue->max_cells = max_cells;
new_queue->cells_used = 0; // Empty to start
new_queue->ePos = 1; //Starts at first position
new_queue->dPos = 1; //Starts at first position
// Now allocate space for the queue entries.
new_queue->head = (void **) calloc(sizeof(void *), max_cells);
if (new_queue->head == NULL) {
free(new_queue); // Unable to allocate queue entries, so free struct.
return NULL;
}
new_queue->tail = new_queue->head; // Start at head
return new_queue;
}
/** Deletes a queue, including the structure and the memory
* for holding the queue entries, but not the entries themselves.
* @param which_queue Pointer to Queue structure.
*/
void delete_queue(Queue *which_queue) {
free(which_queue->head); // Free memory block with queue entries.
free(which_queue); // Then free the struct.
}
/** Pushes a pointer onto a Queue.
* @param which_queue Pointer to queue you want to push onto.
* @param ptr Pointer to be pushed.
* @return 0 if successful, -1 if not.
*/
int enqueue(Queue *which_queue, void *ptr) {
// Check if queue is already full
if ((which_queue->cells_used) >= (which_queue->max_cells)) {
which_queue->cells_used = which_queue->max_cells;
printf("Error: Queue overflow\n");
return -1; // Queue overflow.
}
// Check if tail is at the end of the Queue
if ((which_queue->ePos) == (which_queue->max_cells)) {
//Sets tail to the beginning of queue
(which_queue->tail) = (which_queue->tail) - (which_queue->max_cells);
//Sets position back to beginning
(which_queue->ePos) = 1;
}
// Push onto queue.
*(which_queue->tail) = ptr; // Store the pointer on the stack
(which_queue->tail)++; // Point to next free cell
(which_queue->cells_used)++;
(which_queue->ePos)++;
return 0; // Success
}
/** Removes head of queue, and returns it.
* @param which_queue Pointer to Queue you want to dequeue from.
* @return Head entry of the queue, NULL if queue is empty.
*/
void* dequeue(Queue *which_queue) {
// Check if queue is empty
if ((which_queue->cells_used) <= 0) {
which_queue->cells_used = 0;
printf("Error: Queue underflow\n");
return NULL; // Queue empty
}
// Check if head is at the end of the Queue
if ((which_queue->dPos) == (which_queue->max_cells)) {
//Sets head to the beginning of queue
(which_queue->head) = (which_queue->head) - 3;
//Sets position back to beginning
(which_queue->dPos) = 1;
(which_queue->cells_used)--;
return (*((which_queue->head) + 3));
}
// Remove head from queue.
(which_queue->cells_used)--;
(which_queue->head)++;
(which_queue->dPos)++;
return (*((which_queue->head) - 1));
}
注意：我知道我使用了 - 3 和 + 3 而不是 max_cells - 1 等。我只是想看看这是否是一个问题，但事实并非如此。另一个注意事项是，如果我用 1 2 3 替换 -3 或 +3，它仍然会给我一个段错误。
queue.h
/** Struct to define a queue; each entry can hold a pointer to anything.
*/
struct queue {
void **head; // Pointer to head of queue
void **tail; // Pointer to next free cell (tail);
int max_cells; // Maximum number of entries in the queue
int cells_used; // Currently used number of cells
int ePos; //Position of the enqueue
int dPos; //Position of the dequeue
};
typedef struct queue Queue;
// Function prototypes
Queue *create_queue(int max_cells);
void delete_queue(Queue *which_queue);
int enqueue(Queue *which_queue, void *ptr);
void* dequeue(Queue *which_queue);
因此，在调用第二个 dequeue() 之后，打印行上出现了段错误。这意味着 returned_foo 有一个错误的值，这是由出列函数引起的，而不是 运行 在 if 语句部分正确环绕（至少我认为这是错误>.>）。任何帮助都会有所帮助！提前致谢。
您的 enqueue() 实现从未填充队列中的最后一个元素。
create_queue(); // ePos = 1, queue: x x x x
enqueue(A); // ePos = 2, queue: A x x x
enqueue(B); // ePos = 3, queue: A B x x
enqueue(C); // ePos = 4, queue: A B C x
enqueue(D); // This already wraps because ePos == max_cells!
// ePos = 2, queue: D B C x
当dequeue()到达队列的最后一个元素时，它读取NULL（这是最后一个元素包含的内容，因为calloc用零填充内存块），然后程序崩溃了。