以下代码应该使用多线程对数组进行归并排序。这个想法是将数组拆分为近似相同大小的子数组,并使用线程对它们进行并发排序。然后,全部排序后(pthread_join的for循环后),再合并到一起。但是,我的代码没有正确排序数组。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <pthread.h>
#include <unistd.h>
#include <assert.h>
typedef struct Arguments
{
long *nums;
int start;
int end;
long *result;
} SortThreadArgs;
/** The number of threads to be used for sorting. Default: 1 */
int thread_count = 2;
pthread_mutex_t merge_lock;
/**
* Print the given array of longs, an element per line.
*/
void print_long_array(const long *array, int count)
{
for (int i = 0; i < count; ++i)
{
printf("%ld\n", array[i]);
}
}
/**
* Merge two slices of nums into the corresponding portion of target.
*/
void merge(long nums[], int from, int mid, int to, long target[])
{
int left = from;
int right = mid;
// acquire lock before accessing shared data
pthread_mutex_lock(&merge_lock);
int i = from;
for (; i < to && left < mid && right < to; i++)
{
if (nums[left] <= nums[right])
{
target[i] = nums[left];
left++;
}
else
{
target[i] = nums[right];
right++;
}
}
if (left < mid)
{
memmove(&target[i], &nums[left], (mid - left) * sizeof(long));
}
else if (right < to)
{
memmove(&target[i], &nums[right], (to - right) * sizeof(long));
}
// release lock after modifying shared data
pthread_mutex_unlock(&merge_lock);
}
/**
* Sort the given slice of nums into target.
*
* Warning: nums gets overwritten.
*/
void merge_sort_aux(long nums[], int from, int to, long target[])
{
if (to - from <= 1)
{
return;
}
int mid = (from + to) / 2;
merge_sort_aux(target, from, mid, nums);
merge_sort_aux(target, mid, to, nums);
merge(nums, from, mid, to, target);
}
void *sort_thread(void *arg)
{
SortThreadArgs *args = (SortThreadArgs *)arg;
long *nums = args->nums;
int start = args->start;
int end = args->end;
long *result = args->result;
merge_sort_aux(nums, start, end, result);
pthread_exit(NULL);
}
/**
* Sort the given array and return the sorted version.
*
* The result is malloc'd so it is the caller's responsibility to free it.
*
* Warning: The source array gets overwritten.
*/
long *merge_sort(long nums[], int count)
{
// Check if thread_count is divisible by count
int mod = count % thread_count;
// Create a temp array that has the same elements as the given array
long *result = calloc(count, sizeof(long));
assert(result != NULL);
memmove(result, nums, count * sizeof(long));
pthread_mutex_init(&merge_lock, NULL);
pthread_t threads[thread_count];
for (int i = 0; i < thread_count; i++)
{
// Calculate the start and end index based on the thread_num
// i.e. Splitting the array into approximately same-size sub arrays
int start = i * (count / thread_count);
int end;
if (i == thread_count - 1 && mod != 0) {
end = count;
} else {
end = (i + 1) * (count / thread_count);
}
SortThreadArgs args = {nums, start, end, result};
pthread_create(&threads[i], NULL, sort_thread, &args);
}
for (int i = 0; i < thread_count; i++)
{
pthread_join(threads[i], NULL);
}
// perform final merge operation on all subarrays
for (int i = 0; i < thread_count; i++)
{
int start = i * (count / thread_count);
int mid = start;
int end;
if (i == thread_count - 1 && mod != 0) {
end = count;
} else {
end = (i + 1) * (count / thread_count);
}
merge(nums, 0, mid, end, result);
}
pthread_mutex_destroy(&merge_lock);
return result;
}
int main() {
int count = 10;
long array[10] = {4, 8, 3, 10, 6, 7, 5, 1, 9, 2};
long* result = merge_sort(array, count);
print_long_array(result, count);
}
我将数组拆分为子数组的方式是在 merge_sort 函数的第一个循环中。根据计数器 i,我计算开始和结束索引。例如,如果 thread_count = 2 且 count = 10,则对于 i = 0,start = 0,end = 5;对于 i = 1,开始 = 5,结束 = 10.
数组
{4, 8, 3, 10, 6, 7, 5, 1, 9, 2}
中,打印输出为:
1 2 2 4 5 8 3 9 10 6
这真的很奇怪,因为有一个重复的 2。假设我的想法应该正确工作,但我真的不知道导致这个的潜在错误是什么。任何帮助表示赞赏!
当我尝试在
pthread_join
之后的for循环中打印每个子数组时,它显示子数组甚至没有排序。
编辑:
我稍微清理了我的代码并按照建议将 memmove 更改为 memcpy。
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <pthread.h>
#include <unistd.h>
#include <assert.h>
/** The number of threads to be used for sorting. Default: 1 */
int thread_count = 1;
typedef struct Arguments
{
long *nums;
int from;
int to;
long *target;
} merge_sort_args;
/**
* Print the given array of longs, an element per line.
*/
void print_long_array(const long *array, int count)
{
for (int i = 0; i < count; ++i)
{
printf("%ld\n", array[i]);
}
}
/**
* Merge two slices of nums into the corresponding portion of target.
*/
void merge(long nums[], int from, int mid, int to, long target[])
{
printf("here\n");
int left = from;
int right = mid;
int i = from;
for (; i < to && left < mid && right < to; i++)
{
if (nums[left] <= nums[right])
{
target[i] = nums[left];
left++;
}
else
{
target[i] = nums[right];
right++;
}
}
if (left < mid)
{
memcpy(&target[i], &nums[left], (mid - left) * sizeof(long));
}
else if (right < to)
{
memcpy(&target[i], &nums[right], (to - right) * sizeof(long));
}
}
/**
* Sort the given slice of nums into target.
*
* Warning: nums gets overwritten.
*/
void *merge_sort_aux(void *arg)
{
merge_sort_args *msa = (merge_sort_args *)arg;
long *nums = msa->nums;
int from = msa->from;
int to = msa->to;
long *target = msa->target;
if (to - from <= 1)
{
pthread_exit(NULL);
}
int mid = (from + to) / 2;
merge_sort_args left_args = {target, from, mid, nums};
merge_sort_aux(&left_args);
merge_sort_args right_args = {target, mid, to, nums};
merge_sort_aux(&right_args);
merge(nums, from, mid, to, target);
pthread_exit(NULL);
}
/**
* Sort the given array and return the sorted version.
*
* The result is malloc'd so it is the caller's responsibility to free it.
*
* Warning: The source array gets overwritten.
*/
long *merge_sort(long nums[], int count)
{
long *result = calloc(count, sizeof(long));
assert(result != NULL);
memcpy(result, nums, count * sizeof(long));
pthread_t threads[thread_count];
// Split the array into sub arrays here
for (int i = 0; i < thread_count; i++)
{
int start = i * (count / thread_count);
int end = (i + 1) * (count / thread_count);
merge_sort_args args = {nums, start, end, result};
pthread_create(&threads[i], NULL, merge_sort_aux, &args);
}
for (int i = 0; i < thread_count; i++)
{
pthread_join(threads[i], NULL);
}
for (int i = 1; i < thread_count; i++)
{
int start = i * (count / thread_count);
int mid = start;
int end = (i + 1) * (count / thread_count);
merge(nums, 0, mid, end, result);
}
return result;
}
int main()
{
long array[2] = {2, 1};
int count = 2;
long *result = merge_sort(array, count);
print_long_array(result, count);
}
这次我在一个非常小的数组上尝试
2 1
,但我发现它甚至没有进入merge
函数,因为它没有打印出标志。可能是什么原因?