C-多线程

多线程的坑

有如下代码

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

volatile int th_num = 10;
volatile int sum = 0;
pthread_mutex_t lock;
void *add_th(void *arg){
    int in_arg = *(int *)arg;
    pthread_t tid;
    tid = pthread_self();
    for(int i=0;i<2000;i++){
        // 使用锁
        pthread_mutex_lock(&lock);
        sum += 1;
        pthread_mutex_unlock(&lock);
    }
    printf("thread arg = %d\n",in_arg);
    printf("thread id = %lu\n",tid);
    th_num --;
    return NULL;
}
int main() {
    pthread_t handle;
    int re = -1;
    for (int i = th_num;i>0;i--){
//        使用原有的i
//        int *arg = &i;

//        新建变量
        int *arg = malloc(sizeof(int));
        *arg = i;
//
//        int d = i;
//        int *arg = &d;
        re = pthread_create(&handle,NULL,add_th,arg);
        if(re == 0){
            printf("创建线程成功\n");
        }else {
            printf("创建线程失败，code %d\n",re);
        }
    }
    while (th_num > 0){
        printf("查询线程是否已经退出,th_num : %d\n",th_num);
        sleep(1);
    }
    printf("sum = %d\n",sum);
    return 0;
}
分析
  1. 将for循环中的i,直接作为线程的入参,有什么问题吗？
    直接将i作为入参,后面的几个线程都是使用同一个i变量。i后面的值会影响前面的值。需要使用新的变量作为入参才能满足使用要求。
  2. 多个线程对同一个变量进行操作需要加锁，使用volatile是无法避免的。必须使用锁才能保证数据不会出现冲突。
    注意:这里的多线程是基于linux系统的,windows使用线程开发方式不一样。

windows 多线程开发

windows创建一个线程

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#include <process.h>

void th_testThread(void *arg) {
    // 线程处理
    int *data = (int *)arg;
}

int main(int argc, char *argv[]) {
    int data = 2;
    _beginthread(th_testThread, 0, &data);

    // 主线程处理
}

Pthread内存泄露情况

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    pthread_t pthread;
#ifdef PTHREAD_LIMIT
    pthread_attr_t attr;
    pthread_attr_init(&attr);
    size_t    stacksize = 512 * 1024;
    pthread_attr_setstacksize(&attr, stacksize);
    if (0 != pthread_create(&pthread, &attr, th_handler, NULL)) {
        pthread_attr_destroy(&attr);
#else
    if (0 != pthread_create(&pthread, NULL, th_handler, NULL)) {
#endif
        log_error("create th_handler thread failed.\n");
        return;
    }
#ifdef PTHREAD_LIMIT
    pthread_attr_destroy(&attr);
#endif

valgrind检查出现泄露:

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 272 bytes in 1 blocks are possibly lost in loss record 34 of 44
==558==    at 0x4C33B25: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==558==    by 0x4013646: allocate_dtv (dl-tls.c:286)
==558==    by 0x4013646: _dl_allocate_tls (dl-tls.c:530)
==558==    by 0x4E46227: allocate_stack (allocatestack.c:627)
==558==    by 0x4E46227: pthread_create@@GLIBC_2.2.5 (pthread_create.c:644)

原因分析:创建线程之后,没有释放pthread,需要使用pthread_detach或者pthread_join进行释放。

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    pthread_t pthread;
#ifdef PTHREAD_LIMIT
    pthread_attr_t attr;
    pthread_attr_init(&attr);
    size_t    stacksize = 512 * 1024;
    pthread_attr_setstacksize(&attr, stacksize);
    if (0 != pthread_create(&pthread, &attr, th_handler, NULL)) {
        pthread_attr_destroy(&attr);
#else
    if (0 != pthread_create(&pthread, NULL, th_handler, NULL)) {
#endif
        log_error("create th_handler thread failed.\n");
        return;
    }
#ifdef PTHREAD_LIMIT
    pthread_attr_destroy(&attr);
#endif
    pthread_detach(pthread);

Pthread内存泄露情况

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pthread_t pthread;
pthread_attr_t attr;
pthread_attr_init(&attr);
size_t    stacksize = 512 * 1024;
pthread_attr_setstacksize(&attr, stacksize);
if (0 != pthread_create(&pthread, &attr, th_handler, NULL))     {
    pthread_attr_destroy(&attr);
    log_error("create th_handler thread failed.\n");
    return;
}
pthread_attr_destroy(&attr);

valgrind检查出现泄露:

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 272 bytes in 1 blocks are possibly lost in loss record 34 of 44
==558==    at 0x4C33B25: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==558==    by 0x4013646: allocate_dtv (dl-tls.c:286)
==558==    by 0x4013646: _dl_allocate_tls (dl-tls.c:530)
==558==    by 0x4E46227: allocate_stack (allocatestack.c:627)
==558==    by 0x4E46227: pthread_create@@GLIBC_2.2.5 (pthread_create.c:644)

原因分析:创建线程之后,没有释放pthread,需要使用pthread_detach或者pthread_join进行释放。

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pthread_t pthread;
pthread_attr_t attr;
pthread_attr_init(&attr);
size_t    stacksize = 512 * 1024;
pthread_attr_setstacksize(&attr, stacksize);
if (0 != pthread_create(&pthread, &attr, th_handler, NULL))     {
    pthread_attr_destroy(&attr);
    log_error("create th_handler thread failed.\n");
    return;
}
pthread_attr_destroy(&attr);
pthread_detach(pthread);

使用detach会剥离线程,只能等待线程释放自己的资源
使用pthread_join会等待子线程退出,子线程阻塞也会等待。
使用pthread_cancel会给子线程发送一个信号,这个信号会中断子线程,使其退出。

使用例子

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#include <zconf.h>
#include "stdint.h"
#include "stdio.h"
#include "pthread.h"

void *th_handler(void *arg) {
    pthread_t id = pthread_self();
    int32_t count = *((int32_t *)(arg));
    while (--(*((int32_t *)(arg))) > 0){
        printf("%llu count %d\n",id, (*((int32_t *)(arg))));
        sleep(1);
    }
    printf("exit Thread[%llu]\n", id);
    *((int32_t *)(arg)) = 33;
    pthread_exit(arg);
    return NULL;
}

int32_t main(int argc, char *argv[]) {
    printf("Hello World\n");

    // 测试pthread_detach
    pthread_t handle;
    int32_t count = 12;
    if (0 != pthread_create(&handle,NULL,th_handler ,&count)){
        printf("create thread1 failed[%s]", strerror(errno));
    }else{
        // 剥离线程
        pthread_detach(handle);
    }

    // 测试pthread_join
    pthread_t handle2;
    int32_t count2 = 12;
    if (0 != pthread_create(&handle2, NULL, th_handler, &count2)) {
        printf("create thread2 failed[%s]", strerror(errno));
    }else{
        void *th_ret = 0;
        pthread_join(handle2, &th_ret);
        // 查看返回值
        printf("pthread_join ret %d\n", *((int32_t *)th_ret));
    }

    // 测试pthread_cancel
    pthread_t handle3;
    int32_t count3 = 300;
    if (0 != pthread_create(&handle3, NULL, th_handler, &count3)) {
        printf("create thread3 failed[%s]\n", strerror(errno));
    }else{
        sleep(2);
        printf("cancel thread 3....\n");
        pthread_cancel(handle3);
    }


    return 0;
}

C线程创建和退出

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#include <zconf.h>
#include <errno.h>
#include "pthread.h"
#include "stdio.h"
#include "string.h"

void *th(void *argv) {
    size_t count = 0;
    char name[20] = {0};
    strcpy(name, argv);
    while (1) {
        printf("%s count[%ld]\n",name, ++count);
        usleep(300 * 1000);
    }
    pthread_exit(NULL);
}

int main(int argc, char *argv[]) {
    
    pthread_t handle;
    if (0 != pthread_create(&handle, NULL, th, "first")) {
        printf("create thread failed[%s]\n", strerror(errno));
    }else{
        printf("create thread success\n");
    }
    pthread_detach(handle);
    
    pthread_t handle2;
    if (0 != pthread_create(&handle2, NULL, th, "second")) {
        printf("create thread failed[%s]\n", strerror(errno));
    }else{
        printf("create thread success\n");
    }
    pthread_detach(handle2);
    
    sleep(10);
    printf("shutdown handle1[%lu] \n", handle);
    pthread_cancel(handle);
    
    printf("shutdown [%lu] finish\n", handle);
    sleep(10);
    
    printf("shutdown handle2[%lu]\n", handle2);
    pthread_cancel(handle2);
    printf("shutdown [%lu] finish\n", handle2);
    printf("finish");
    
    return 0;
}

pthread_cancel之后也可以使用pthread_join,甚至也可以使用pthread_detach之后再pthread_join。反正没啥用。

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