c - 如何在 C 中用 pthreads 替换顺序操作？

Question

我有一个函数正在计算一个较大字符串的子字符串数量(使用较小的字符串进行匹配)，我试图用 pthreads 替换这个序列，从而在没有循环的情况下同时处理整个事情。

我正在寻找可以做到这一点的原则。到目前为止我所做的是创建一个 pthread_t 动态数组，这个数组的数量与字符的数量相同。对于更大的字符串，我正在将计算子字符串的函数分配给线程，我想我几乎把它搞砸了，我只需要向前推进就可以了。

pthreads.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>


#define MAX 1024

int total = 0;
int n1,n2;
int num_thr;
int *num_threads;
char *s1,*s2;
FILE *fp;

int readf(FILE *fp)
{
    if((fp=fopen("strings.txt", "r"))==NULL){
        printf("ERROR: can't open string.txt!\n");
        return 0;
    }
    s1=(char *)malloc(sizeof(char)*MAX);
    if(s1==NULL){
        printf("ERROR: Out of memory!\n");
        return -1;
    }
    s2=(char *)malloc(sizeof(char)*MAX);
    if(s1==NULL){
        printf("ERROR: Out of memory\n");
        return -1;
    }
    /*read s1 s2 from the file*/
    s1=fgets(s1, MAX, fp);
    s2=fgets(s2, MAX, fp);
    n1=strlen(s1)-1;  /*length of s1*/

    n2=strlen(s2)-1; /*length of s2*/
    num_thr=n1;
    printf("String 1 len = %d\n",n1);

    if(s1==NULL || s2==NULL || n1<n2)  /*when error exit*/
        return -1;
}

void *num_substring(void *vari);
void *num_substring(void *vari)
{
    int i,j,k;
    int count;

    for (i = 0; i <= (n1-n2); i++){   
        count=0;
        for(j = i,k = 0; k < n2; j++,k++){  /*search for the next string of size of n2*/  
            if (*(s1+j)!=*(s2+k)){
                break;
            }
            else
                count++;
            if(count==n2)    
                total++;        /*find a substring in this step*/                          
        }
    }
    printf("total= %d\n",total);
    //return total;
}








int main (int argc, char *argv[])
{

int count;

    readf(fp);

    printf("The number of substrings is: %d\n", count);



int i, ret=-1;
char *msg1= "a thread";
pthread_t * thread_arr = malloc(sizeof(pthread_t)*num_thr);
printf("num threads inside main = %d\n",num_thr);
for (i = 0; i < num_thr; i++) {

    ret = pthread_create(&thread_arr[i], NULL, num_substring, (void *) msg1);

    if(ret != 0) {
        printf ("Create pthread %d error!\n",i);
        exit (1);
    }
     printf("Main function thread %d created\n",i);
}

for (i=0;i < num_thr; i++){
pthread_join(thread_arr[i], NULL);
}




return 0;
} 

这个程序的正确输出必须是4，因为ab在另一个字符串中出现了4次，所以子串的个数必须等于4:

string.txt 内容:

abcdabsufsoababuosufba

一个

我的输出不是完全错误的，因为它最初显示 4 然后它开始递增(这不是我想要的，我想要的只是 4)但是，它仍然不完整，因为我没有用 pthreads 替换循环array (pthread_arr) 这是我猜测的最后一步，这是我的输出:

String 1 len = 22
The number of substrings is: 10219508
num threads inside main = 22
Main function thread 0 created
Main function thread 1 created
Main function thread 2 created
Main function thread 3 created
total= 4 // this is the correct number of substrings
total= 8
total= 12
Main function thread 4 created
total= 20
total= 16
Main function thread 5 created
Main function thread 6 created
total= 24
Main function thread 7 created
total= 28
Main function thread 8 created
total= 36
total= 32
Main function thread 9 created
total= 40
Main function thread 10 created
total= 44
Main function thread 11 created
Main function thread 12 created
total= 48
total= 52
Main function thread 13 created
total= 56
Main function thread 14 created
total= 60
Main function thread 15 created
Main function thread 16 created
Main function thread 17 created
Main function thread 18 created
Main function thread 19 created
Main function thread 20 created
Main function thread 21 created
total= 64
total= 68
total= 84
total= 76
total= 72
total= 80
total= 88

Answer 1

你非常接近，但有一些错误。

1. 每个线程都在扫描整个 s1 而不仅仅是一个子字符串。
2. main 应传递一个参数，指示 s1
中子字符串的起始偏移量
3. 线程函数中只需要一个循环[或期望]。
4. 每个线程递增一个全局总数 [没有线程锁定]。最好将值作为线程的返回值传回
5. 因为 s1 中的每个字符都有一个线程，所以线程将 [仅] 将 0 或 1 作为计数。

我已经修复了你的代码[请原谅不必要的样式清理]:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>

#define MAX 1024

int n1;
int n2;
int num_thr;
int *num_threads;
char *s1;
char *s2;
FILE *fp;

int
readf(FILE * fp)
{
    char *cp;

    if ((fp = fopen("strings.txt", "r")) == NULL) {
        printf("ERROR: can't open string.txt!\n");
        return 0;
    }

    if (s1 == NULL)
        s1 = (char *) malloc(sizeof(char) * MAX);
    if (s1 == NULL) {
        printf("ERROR: Out of memory!\n");
        return -1;
    }

    if (s2 == NULL)
        s2 = (char *) malloc(sizeof(char) * MAX);
    if (s1 == NULL) {
        printf("ERROR: Out of memory\n");
        return -1;
    }

    /* read s1 s2 from the file */
    cp = fgets(s1, MAX, fp);
    if (cp == NULL)
        return -1;

    cp = fgets(s2, MAX, fp);
    if (cp == NULL)
        return -1;

    n1 = strlen(s1) - 1;                /* length of s1 */
    n2 = strlen(s2) - 1;                /* length of s2 */

    num_thr = n1;
    printf("String 1 len = %d\n", n1);

    if (n1 < n2)
        return -1;

    return 0;
}

void *
num_substring(void *vari)
{
    int i;
    long subtotal;

    subtotal = 0;

#if 0
    int count;
    int j;
    int k;
    for (int i = 0; i <= (n1 - n2); i++) {
        count = 0;
        /* search for the next string of size of n2 */
        for (int j = i, k = 0; k < n2; j++, k++) {
            if (*(s1 + j) != *(s2 + k)) {
                break;
            }
            else
                count++;
            if (count == n2)
                subtotal++;             /* find a substring in this step */
        }
    }
#else
    int t = (long) vari;
    char *cp = &s1[t];
    subtotal = 1;
    for (i = 0; i < n2; i++) {
        //printf("t%d: TRY: %c %c\n",t,cp[i],s2[i]);
        if (cp[i] != s2[i]) {
            subtotal = 0;
            break;
        }
    }
#endif

    //printf("t%d: subtotal= %ld\n", subtotal);

    return (void *) subtotal;
}

int
main(int argc, char *argv[])
{
    int total = 0;
    void *ptr;

    //int count;

    readf(fp);

    //printf("The number of substrings is: %d\n", count);

    long i;
    int ret = -1;
    //char *msg1 = "a thread";
    pthread_t *thread_arr = malloc(sizeof(pthread_t) * num_thr);

    printf("num threads inside main = %d\n", num_thr);
    for (i = 0; i < num_thr; i++) {
#if 0
        ret = pthread_create(&thread_arr[i], NULL, num_substring,(void *) msg1);
#else
        ret = pthread_create(&thread_arr[i], NULL, num_substring,(void *) i);
#endif

        if (ret != 0) {
            printf("Create pthread %ld error!\n", i);
            exit(1);
        }
        printf("Main function thread %ld created\n", i);
    }

    total = 0;
    for (i = 0; i < num_thr; i++) {
        pthread_join(thread_arr[i], &ptr);
        total += (long) ptr;
    }

    printf("TOTAL: %d\n",total);

    return 0;
}