C语言 Pthread 创建为分离

Pthread create as detached

提问by Manlio

I have a problem creating a thread as detached. Here's the code I wrote:

我在创建分离的线程时遇到问题。这是我写的代码：

void* testFunction() {

    pthread_attr_t attr;
    int chk,rc;

pthread_attr_init(&attr);
printf("thread_attr_init: %d\n",rc);

pthread_attr_getdetachstate(&attr, &chk);
printf("thread_attr_getdetachedstate: %d\n",rc);

if(chk == PTHREAD_CREATE_DETACHED ) 
    printf("Detached\n");
else if (chk == PTHREAD_CREATE_JOINABLE) 
    printf("Joinable\n");

return NULL;
}


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

pthread_t thread1;
pthread_attr_t attr;

int rc;

rc = pthread_attr_init(&attr);

printf("attr_init: %d\n",rc);
rc = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
printf("attr_setdetachedstate: %d\n",rc);

rc = pthread_create(&thread1, &attr, testFunction, NULL);
printf("attr_create: %d\n",rc);

sleep(4);
pthread_cancel(thread1);

return 0;
}

The problem is that testFunction()always print "Joinable". Can anyone tell me where I'm getting wrong?

问题是testFunction()总是打印“Joinable”。谁能告诉我哪里出错了？

回答by R.. GitHub STOP HELPING ICE

Your testFunctionis not examining anything about the current thread, rather just the initially-detached flag of a completely new attribute object you just created. Moreover, it is completely impossible, in the POSIX threads API, to recover the attributes a thread was created with or determine if a thread is detached or not. You simply have to trust that the implementation behaves as required, just like you have to trust that, if malloc(100)returns a non-null pointer, it points to a location at which you can store at least 100 bytes. This is the nature of C.

您testFunction不是在检查有关当前线程的任何内容，而只是检查您刚刚创建的全新属性对象的初始分离标志。此外，在 POSIX 线程 API 中，完全不可能恢复创建线程时使用的属性或确定线程是否已分离。您只需要相信实现的行为符合要求，就像您必须相信，如果malloc(100)返回一个非空指针，它会指向一个可以存储至少 100 个字节的位置。这就是 C 的本质。

回答by enthusiasticgeek

Try running the following code. That should clear the confusion.

尝试运行以下代码。这应该可以消除混淆。

/*------------------------------- join_01.c --------------------------------*
On Linux, compile with: 
cc -std=c99 -pthread join_01.c -o join_01  
    gcc join_01.c -o join_01 -std=c99 -lpthread (Ubuntu)

Check your system documentation how to enable C99 and POSIX threads on 
other Un*x systems.

Copyright Loic Domaigne. 
Licensed under the Apache License, Version 2.0.
*--------------------------------------------------------------------------*/

#include <unistd.h>  // sleep()
#include <pthread.h> 
#include <stdio.h>   
#include <stdlib.h>  // EXIT_SUCCESS
#include <string.h>  // strerror() 
#include <errno.h>

/***************************************************************************/
/* our macro for errors checking                                           */
/***************************************************************************/
#define COND_CHECK(func, cond, retv, errv) \
if ( (cond) ) \
{ \
   fprintf(stderr, "\n[CHECK FAILED at %s:%d]\n| %s(...)=%d (%s)\n\n",\
              __FILE__,__LINE__,func,retv,strerror(errv)); \
   exit(EXIT_FAILURE); \
}

#define ErrnoCheck(func,cond,retv)  COND_CHECK(func, cond, retv, errno)
#define PthreadCheck(func,rc) COND_CHECK(func,(rc!=0), rc, rc)

/*****************************************************************************/
/* thread- dummy thread                                                      */
/*****************************************************************************/
void*
thread(void* ignore)
{
   sleep(1);
   return NULL;
}

/*****************************************************************************/
/* detach_state. Print detachstate of a thread.                              */
/*****************************************************************************/
/* 
 * We find out indirectly if a thread is detached using pthread_join().  
 * If a thread is detached, then pthread_join() fails with EINVAL. 
 * Otherwise the thread is joined, and hence was joinable. 
 *
 */
void
detach_state(
   pthread_t   tid,  // thread to check detach status
   const char *tname // thread name
   )
{
   int rc; // return code

   rc = pthread_join(tid, NULL);
   if ( rc==EINVAL ) 
   {
      printf("%s is detached\n", tname);
   }
   else if ( rc==0 )  
   {  
      printf("%s was joinable\n", tname);
   }
   else 
   {
      printf("%s: pthread_join() = %d (%s)\n", 
             tname, rc, strerror(rc)
            );
   }
}

/*****************************************************************************/
/* main- main thread                                                         */
/*****************************************************************************/
int
main()
{
   pthread_t tid1, tid2, tid3; // thread 1,2 and 3.
   pthread_attr_t attr;        // thread's attribute
   int rc;  // return code

   /*--------------------------------------------------------*/
   /* 1st test: normal thread creation                       */
   /*--------------------------------------------------------*/
   rc = pthread_create(&tid1, NULL, thread, NULL);
   PthreadCheck("pthread_create", rc);
   detach_state(tid1, "thread1"); // expect: joinable 

   /*--------------------------------------------------------*/
   /* 2nd test: detach thread from main thread               */
   /*--------------------------------------------------------*/
   rc = pthread_create(&tid2, NULL, thread, NULL);
   PthreadCheck("pthread_create", rc);
   rc = pthread_detach(tid2);
   PthreadCheck("pthread_detach", rc);
   detach_state(tid2, "thread2"); // expect: detached

   /*--------------------------------------------------------*/
   /* 3rd test: create detached thread                       */
   /*--------------------------------------------------------*/

   // set detachstate attribute to DETACHED
   //
   rc=pthread_attr_init(&attr);
   PthreadCheck("pthread_attr_init", rc);
   rc=pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
   PthreadCheck("pthread_attr_setdetachstate", rc);

   // create thread now 
   //
   rc = pthread_create(&tid3, &attr, thread, NULL);
   PthreadCheck("pthread_create", rc);
   detach_state(tid3, "thread3");

   /*--------------------------------------------------------*/
   /* that's all folks!                                      */
   /*--------------------------------------------------------*/
   return EXIT_SUCCESS;
}

The output should be

输出应该是

$ ./join_01 
thread1 was joinable
thread2 is detached
thread3 is detached

回答by crazyscot

Your testFunction is not reading the current thread's attributes. Instead of calling pthread_attr_init(), pthread_getattr_np()might help you if it's present on your system, but you will need to pass the pthread_tfor the thread in question onto wherever it's being queried from.

您的 testFunction 没有读取当前线程的属性。如果它存在于您的系统上，而不是调用pthread_attr_init(),pthread_getattr_np()可能会帮助您，但是您需要将pthread_tfor 有问题的线程传递到它被查询的任何地方。