我试图在两个部分实现双重链接列表：
第一个是创建列表的实际功能；
第二个是一个模拟器——包含几个线程、读写器和写入器（每个线程在while循环中弹出并推入到双链接列表中），还有一个垃圾收集器线程，如果列表太大，它会清理列表（根据argv）。
该列表使用互斥锁和条件变量以使其线程安全。
然而，每当我运行它时，就会出现一个双空闲/内存损坏（fasttop）错误，我不知道为什么。我很感激你的帮助。
#include <pthread.h>
#include <time.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#define USAGE_ERR "Usage: ./hw3 <WNUM> <RNUM> <MAX> <TIME>\n"
/* Argc parameter indexing consts */
#define INPUT_WNUM_IDX 1
#define INPUT_RNUM_IDX 2
#define OUTPUT_MAX_IDX 3
#define OUTPUT_TIME_IDX 4
typedef struct node {
int value;
struct node * previous;
struct node * next;
}
node;
typedef struct {
int length;
node * head;
node * tail;
pthread_mutex_t mutex;
}
list;
list * global_list;
int stop_threads = 0;
int MAX_LIST_SIZE;
pthread_cond_t gc_cond = PTHREAD_COND_INITIALIZER;
pthread_mutex_t gc_mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t read_cond = PTHREAD_COND_INITIALIZER;
pthread_mutex_t read_mutex = PTHREAD_MUTEX_INITIALIZER;
list * initlist_create();
void initlist_destroy(list * target_list);
list * initlist_push_head(list * target_list, int value);
int initlist_pop_tail(list * target_list);
void initlist_remove_last_k(list * target_list, int k);
int initlist_size(list * target_list);
pthread_mutex_t initlist_get_mutex(list * target_list);
void writer_thread();
void reader_thread();
void garbage_collect_thread();
list * initlist_create() {
size_t list_size = sizeof(list);
list * new_list = (list * ) malloc(list_size);
new_list->head = NULL;
new_list->tail = NULL;
new_list->length = 0;
if (0 != pthread_mutex_init( & (new_list->mutex), NULL)) {
exit(errno);
}
return new_list;
}
void initlist_destroy(list * target_list) {
node * current_node;
node * temp_node;
if (0 != pthread_mutex_lock( & (target_list->mutex))) {
exit(errno);
}
if (0 != target_list->length) {
current_node = target_list->head;
while (current_node != NULL) {
temp_node = current_node->next;
free(current_node);
current_node = temp_node;
}
}
if (0 != pthread_mutex_unlock( & (target_list->mutex))) {
exit(errno);
}
if (0 != pthread_mutex_destroy( & (target_list->mutex))) {
exit(errno);
}
free(target_list);
}
list * initlist_push_head(list * target_list, int value) {
node * new_node;
size_t node_size = sizeof(node);
if (0 != pthread_mutex_lock( & (target_list->mutex))) {
exit(errno);
}
new_node = (node * ) malloc(node_size);
if(NULL == new_node)
{
printf("Malloc failed\n" );
exit(errno);
}
new_node->value = value;
if (NULL != target_list->head) {
new_node->next = target_list->head;
new_node->previous = target_list->tail;
target_list->head->previous = new_node;
target_list->tail->previous = new_node;
}
else{
target_list->tail = new_node;
}
target_list->head = new_node;
target_list->length++;
if(0 < target_list->length){
pthread_cond_signal(&read_cond);
}
if (0 != pthread_mutex_unlock( & (target_list->mutex))) {
exit(errno);
}
return target_list;
}
int initlist_pop_tail(list * target_list) {
int deleted_node_value;
if (0 != pthread_mutex_lock( & (target_list->mutex))) {
exit(errno);
}
while(0 == target_list->length){
pthread_cond_wait(&read_cond, &target_list->mutex);
}
node * last_node = target_list->tail;
if (1 == target_list->length) {
last_node = target_list->head;
target_list->head = NULL;
target_list->tail = NULL;
} else {
target_list->tail = last_node->previous;
target_list->tail->next = target_list->head;
}
deleted_node_value = last_node->value;
// IMPORTANT
// If i uncomment this, segfault and memory corruption,no idea why
//free(last_node);
target_list->length--;
if (0 != pthread_mutex_unlock( & ( target_list->mutex) )) {
exit(errno);
}
// if (0 != pthread_mutex_unlock( & (read_mutex))) {
// exit(errno);
// }
return deleted_node_value;
}
void initlist_remove_last_k(list * target_list, int k) {
int remove_size = k;
if (0 != pthread_mutex_lock( & (target_list->mutex))) {
exit(errno);
}
if (k > target_list->length)
{
remove_size = target_list->length;
}
if (0 != pthread_mutex_unlock( & (target_list->mutex))) {
exit(errno);
}
for (int i = 0; i < remove_size; ++i) {
initlist_pop_tail(target_list);
}
}
int initlist_size(list * target_list) {
int list_size;
if (0 != pthread_mutex_lock( & (target_list->mutex))) {
exit(errno);
}
list_size = target_list->length;
if (0 != pthread_mutex_unlock( & (target_list->mutex))) {
exit(errno);
}
return list_size;
}
pthread_mutex_t initlist_get_mutex(list * target_list) {
pthread_mutex_t list_mutex;
if (0 != pthread_mutex_lock( & (target_list->mutex))) {
exit(errno);
}
list_mutex = target_list->mutex;
if (0 != pthread_mutex_unlock( & (target_list->mutex))) {
exit(errno);
}
return list_mutex;
}
void writer_thread()
{
while (1)
{
if(stop_threads){
return;
}
if (MAX_LIST_SIZE < initlist_size(global_list))
{
pthread_cond_signal(&gc_cond);
}
initlist_push_head(global_list, rand());
}
}
void reader_thread()
{
while (1)
{
if(stop_threads){
return;
}
if (MAX_LIST_SIZE < initlist_size(global_list))
{
pthread_cond_signal(&gc_cond);
}
initlist_pop_tail(global_list);
}
}
void garbage_collect_thread()
{
while(1){
if(stop_threads){
return;
}
pthread_cond_wait(&gc_cond, &gc_mutex);
int remove_count = (initlist_size(global_list) / 2);
initlist_remove_last_k(global_list, remove_count);
printf("GC – %d items removed from the list\r\n", remove_count);
if (0 != pthread_mutex_unlock( & (gc_mutex))) {
exit(errno);
}
}
}
int main(int argc, char * * argv) {
/* Validate arguments */
if (5 != argc) {
printf(USAGE_ERR);
exit(errno);
}
int writers_count = atoi(argv[INPUT_WNUM_IDX]);
int readers_count = atoi(argv[INPUT_RNUM_IDX]);
int max_run_time = atoi(argv[OUTPUT_TIME_IDX]);
MAX_LIST_SIZE = atoi(argv[OUTPUT_MAX_IDX]);
global_list = initlist_create();
pthread_t garbage_collector_thread;
pthread_t writer_threads[writers_count];
pthread_t reader_threads[readers_count];
if (0 != pthread_create(&garbage_collector_thread, NULL, garbage_collect_thread, NULL))
{ exit(errno);
}
for (int i = 0; i < writers_count; ++i)
{
if (0 != pthread_create(&writer_threads[i], NULL, writer_thread, NULL))
{
exit(errno);
}
}
for (int x = 0; x < readers_count; ++x)
{
if (0 != pthread_create(&reader_threads[x], NULL, reader_thread, NULL))
{
exit(errno);
}
}
sleep(max_run_time);
//IMPORTANT
//threads should die after this is set,but all threads wait for the mutex to free
//in gcc - see "info threads" command in gdb
stop_threads = 1;
int list_size = initlist_size(global_list);
printf("List size: %d", list_size);
for (int i = 0; i < list_size; ++i)
{
continue;
printf("num is %d", initlist_pop_tail(global_list));
}
//this gets stuck also when waiting for mutex obviously...
initlist_destroy(global_list);
}
最佳答案
我在调试器中运行了您的代码，有人试图将此代码中的同一指针从initlist_destroy中释放两次：
if (0 != target_list->length) {
current_node = target_list->head;
while (current_node != NULL) {
temp_node = current_node->next;
// Added the following line to view pointer values
printf("Freeing %p...\n", current_node);
free(current_node);
current_node = temp_node;
}
}
当我打印出指针值时，得到如下输出：
GC – 5 items removed from the list
List size: 4
Freeing 0x7fc1a1602d20...
Freeing 0x7fc1a15045e0...
Freeing 0x7fc1a3569c40...
Freeing 0x7fc1a3569c40...
如您所见，您尝试释放同一指针两次。你的列表处理代码有缺陷。这并不奇怪。链表代码很难写。
我的建议是编写一个单线程程序
创建链接列表
向列表中添加少量项（例如3）
从列表中删除每个项
验证列表是否为空
我将编写一个函数，以易于阅读的方式打印列表指针（例如十六进制值表）。每次修改列表后调用该函数。很容易看出你做错了什么。
更新
我写了一个程序来实现上面的建议：
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <pthread.h>
#include <inttypes.h>
#define PTR_FMT "0x%016" PRIXPTR
void perror_location(const char *file, int line_number) {
char location[80];
snprintf(location, sizeof(location), "%s:%d", file, line_number);
perror(location);
exit(errno);
}
#define PERROR() perror_location(__FILE__, __LINE__)
void *malloc_safe(size_t size) {
void *result = malloc(size);
if (NULL == result) {
PERROR();
}
return result;
}
#define MALLOC(type) \
(type *) malloc_safe(sizeof(type))
void lock(pthread_mutex_t *mutex) {
if (0 != pthread_mutex_lock(mutex)) {
PERROR();
}
}
void unlock(pthread_mutex_t *mutex) {
if (0 != pthread_mutex_unlock(mutex)) {
PERROR();
}
}
typedef struct _node {
int value;
struct _node *next;
struct _node *prev;
} node;
typedef struct _list {
node *head;
node *tail;
size_t length;
pthread_mutex_t mutex;
} list;
list *list_init(void) {
list *result = MALLOC(list);
result->head = NULL;
result->tail = NULL;
result->length = 0;
if (0 != pthread_mutex_init(&result->mutex, NULL)) {
PERROR();
}
return result;
}
node *list_add(list *list_ptr, int value) {
node *result = MALLOC(node);
result->value = value;
result->next = NULL;
result->prev = NULL;
lock(&list_ptr->mutex);
if (list_ptr->head == NULL) {
list_ptr->head = result;
list_ptr->tail = result;
}
else {
result->prev = list_ptr->tail;
list_ptr->tail->next = result;
list_ptr->tail = result;
}
list_ptr->length++;
unlock(&list_ptr->mutex);
return result;
}
node *list_remove(list *list_ptr) {
node *result = NULL;
lock(&list_ptr->mutex);
if (list_ptr->head != NULL) {
result = list_ptr->tail;
list_ptr->tail = result->prev;
if (result->prev != NULL) {
result->prev->next = NULL;
}
else {
list_ptr->head = NULL;
}
list_ptr->length--;
}
unlock(&list_ptr->mutex);
if (result != NULL) {
result->next = NULL;
result->prev = NULL;
}
return result;
}
void list_print(list *list_ptr) {
lock(&list_ptr->mutex);
printf("----------\n");
printf("length = %ld\n", list_ptr->length);
printf("%-18s %-18s %-18s\n", "list", "list->head", "list->tail");
printf(PTR_FMT " " PTR_FMT " " PTR_FMT "\n",
(uintptr_t) list_ptr,
(uintptr_t) list_ptr->head,
(uintptr_t) list_ptr->tail);
if (list_ptr->head != NULL) {
printf("%-18s %-18s %-18s\n", "node", "node->next", "node->prev");
node *current = list_ptr->head;
do {
printf(PTR_FMT " " PTR_FMT " " PTR_FMT "\n",
(uintptr_t) current,
(uintptr_t) current->next,
(uintptr_t) current->prev);
current = current->next;
} while (current != NULL);
}
unlock(&list_ptr->mutex);
}
int main(void) {
list *linked_list = list_init();
printf("Add items...\n");
for (int value = 1; value <= 3; value++) {
list_add(linked_list, value);
list_print(linked_list);
}
printf("Remove items...\n");
while (1) {
node *result = list_remove(linked_list);
if (NULL == result) {
break;
}
free(result);
list_print(linked_list);
}
if ((linked_list->head != NULL) || (linked_list->tail != NULL)) {
printf("ERROR: List is not empty\n");
return EFAULT;
}
free(linked_list);
return 0;
}
输出
Add items...
----------
length = 1
list list->head list->tail
0x00007F8180C031B0 0x00007F8180C03210 0x00007F8180C03210
node node->next node->prev
0x00007F8180C03210 0x0000000000000000 0x0000000000000000
----------
length = 2
list list->head list->tail
0x00007F8180C031B0 0x00007F8180C03210 0x00007F8180C03230
node node->next node->prev
0x00007F8180C03210 0x00007F8180C03230 0x0000000000000000
0x00007F8180C03230 0x0000000000000000 0x00007F8180C03210
----------
length = 3
list list->head list->tail
0x00007F8180C031B0 0x00007F8180C03210 0x00007F8180C03250
node node->next node->prev
0x00007F8180C03210 0x00007F8180C03230 0x0000000000000000
0x00007F8180C03230 0x00007F8180C03250 0x00007F8180C03210
0x00007F8180C03250 0x0000000000000000 0x00007F8180C03230
Remove items...
----------
length = 2
list list->head list->tail
0x00007F8180C031B0 0x00007F8180C03210 0x00007F8180C03230
node node->next node->prev
0x00007F8180C03210 0x00007F8180C03230 0x0000000000000000
0x00007F8180C03230 0x0000000000000000 0x00007F8180C03210
----------
length = 1
list list->head list->tail
0x00007F8180C031B0 0x00007F8180C03210 0x00007F8180C03210
node node->next node->prev
0x00007F8180C03210 0x0000000000000000 0x0000000000000000
----------
length = 0
list list->head list->tail
0x00007F8180C031B0 0x0000000000000000 0x0000000000000000
我使用了一个调试器来修复我自己的列表处理。我建议你用它来修好你的。