PThread创建不创建线程

Question

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

FILE * f;
sem_t * s1;
sem_t * s2;
int check;
int  v1;
int  v2;
int i;

static void * client (void *arg){


    sem_getvalue(s1, &v1); printf("Client pre wait(S1) in S1 => S1 = %d\n",v1);
    sem_wait(s1);
    printf("client works...\n");
    check = sem_getvalue(s1, &v1); printf("Client.wait(S1) in S1 => S1 = %d\n",v1);
    if(check != 0) printf("sem_getvalue error");


     return 0;
    }


int main(void){

    pthread_t tidc;
    pthread_t tids;
    int rc;
    int rs;

    //Semaforo 1
    s1 = (sem_t *) malloc(sizeof(sem_t));
    check = sem_init (s1, 0, 2);
    if (check != 0) perror("s1_init failed");




    sem_getvalue(s1, &v1);

    printf("Create the semaphores: S1 = %i\n",v1 );

    sem_wait(s1);
    printf("main waits\n");
    sem_getvalue(s1, &v1); printf("Main.wait(S1) in S1 => S1 = %d\n",v1);

    rc = pthread_create (&tidc, NULL, client, 0);
    printf(" thread created ==> rc= %i\n",rc);


    return 0;

   }

它返回这个输出：

Create the semaphores: S1 = 2
main waits
Main.wait(S1) in S1 => S1 = 1
 thread created ==> rc= 0

有时这是：

 Create the semaphores: S1 = 2
main waits
Main.wait(S1) in S1 => S1 = 1
 thread created ==> rc= 0
Client pre wait(S1) in S1 => S1 = 1
Client pre wait(S1) in S1 => S1 = 1
client works...
Client.wait(S1) in S1 => S1 = Client.wait(S1) in S1 => S1 = 0

似乎有时会产生两个线程，有时却没有人。我用gcc prog.c -lpthred编译，甚至用gcc -pthread prog.c编译

Answer 1

如果多线程程序在一次执行到另一次执行时不执行相同的操作，则可能是因为未初始化的变量(如非线程程序中的变量)，但也可能是因为https://stackoverflow.com/questions/34510/what-is-a-race-condition。

在这种情况下，争用条件在线程执行和程序退出之间。

由于在创建线程后立即退出主线程，线程将终止(main thread exit, does other exit too?)。有时，线程有时间做一些事情，这取决于操作系统的状态和加载。

如果在主程序中添加一些实际处理、长延迟或对pthread_join(tdic,NULL);的调用以等待线程终止，则将具有确定性行为。