对于函数原型 int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine) (void *), void *arg) 里的参数 arg，之前一直有疑问，就是把 &thread 传给 arg时，新创建的线程里是否能取到这个值呢？看例子：

#include <pthread.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <ctype.h>

#define handle_error_en(en, msg) \
	   do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)

#define handle_error(msg) \
	   do { perror(msg); exit(EXIT_FAILURE); } while (0)

static void *thread_start(void *arg)
{	
	size_t stack_size = 0;
	pthread_attr_t attr;
	printf("in child thread id: %lu\n", *((pthread_t*)arg));
	int ret = pthread_getattr_np(pthread_self(), &attr);
	if(ret != 0)
	{
		handle_error_en(ret, "pthread_attr_init");
	}
	ret = pthread_attr_getstacksize(&attr, &stack_size);
	if (ret != 0)
	{
	   handle_error_en(ret, "pthread_attr_setstacksize");
	}
	printf("current thread stack_size = %lu\n", stack_size);

	sleep(200);
	return 0;
}

void (*cancel_hook) (void *);
int main(int argc, char *argv[])
{
	int s, tnum, opt, num_threads;
	pthread_t thread_id;
	pthread_attr_t attr;
	int stack_size = 0x80000;
	void *res;
	printf("pid of a.out %lu\n", getpid());
	printf("sizeof(pthread_mutex_t) = %lu\n", sizeof(pthread_mutex_t));
	
	printf("pointer of function = %lu\n", sizeof(cancel_hook));
	/* Initialize thread creation attributes */
	s = pthread_attr_init(&attr);
	if (s != 0)
	{
	   handle_error_en(s, "pthread_attr_init");
	} 

	if (stack_size > 0) 
	{
	   s = pthread_attr_setstacksize(&attr, stack_size);
	   if (s != 0)
	   {
		   handle_error_en(s, "pthread_attr_setstacksize");
	   }
	}


	printf("set stack size %lu\n", stack_size);
	s = pthread_create(&thread_id, &attr, &thread_start, &thread_id);

	if (s != 0)
	{
	   handle_error_en(s, "pthread_create");
	}
	printf("the child threadId %lu\n", thread_id);

	/* Destroy the thread attributes object, since it is no
	  longer needed */
	s = pthread_attr_destroy(&attr);
	if (s != 0)
	{
	   handle_error_en(s, "pthread_attr_destroy");
	}

	s = pthread_join(thread_id, &res);
	if (s != 0)
	{
	   handle_error_en(s, "pthread_join");
	}

	exit(EXIT_SUCCESS);
}

新线程里打印了入参 arg 的值：printf("in child thread id: %lu\n", *((pthread_t*)arg));  因为 arg 是 void* 类型，所以需要强转一下再解引用。

可以看到在 main 线程和子线程里打印出来的线程 id 是一样的，即在调用 pthread_create(&thread_id, &attr, &thread_start, &thread_id); 时，在新创建的线程里已经能取到 thread_id 的值。看源码片段：

在GDB 跟踪里可以看到，在调用 create_thread 前，已经把入参 arg 赋值给了 pd->arg，经过*newthread = (pthread_t)pd，此时 arg 已经有值了，即 pd->arg 也是有值 了。