整体情况是这样的:开启一个线程持续向一个文件中写入数据，而主线程则使用select监听文件描述符是否可读，

写入操作都是成功的，但是select的结果却一直是超时。难道是普通文件不能select？但是man手册并未看到明确说明。 PO下代码:

#include <stdio.h>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/time.h>
#include <errno.h>
#include <pthread.h>
#include <string.h>
#include <sys/socket.h>

#define LOCAL_CONST_IOBUF_LEN	(64)
#define LOCAL_CONST_LOOP_AMOUNT	(500)

void* write_thread(void* pv)
{
	const pthread_t tid = pthread_self();
	if (NULL == pv)
	{
		printf("thread:%u report: arg is null, exit.\n", tid);
		return pv;
	}
	
	int fd = *((int*)pv);
	size_t nWrote;
	char szWRBuf[LOCAL_CONST_IOBUF_LEN];
	sprintf(szWRBuf, "writer thread %u.\n", tid);
	size_t nLoopCtr = 0;
	while (nLoopCtr++ < LOCAL_CONST_LOOP_AMOUNT)
	{
		nWrote = write(fd, szWRBuf, strlen(szWRBuf));
		printf("write to fd(%d) %u bytes.\n", fd, nWrote);
		sleep(2);
	}
	printf("thread %u exit.\n", tid);
	return pv;
}

int main(int argc, char** argv)
{
	const char* fpath = "/tmp/cond";
	const int fd = open(fpath, O_TRUNC | O_RDWR);
	if (fd <= 0)
	{
		printf("open %s failed.\n", fpath);
		return -1;
	}
	printf("test beg, fd(%d).\n", fd);
	
	pthread_t nWriteTid = 0;
	if (0 == pthread_create(&nWriteTid, NULL, write_thread, (void*)&fd))
	{
		size_t nLoopCtr = 0;
		while (nLoopCtr++ < LOCAL_CONST_LOOP_AMOUNT)
		{
			fd_set rdfs;
			FD_ZERO(&rdfs);
			FD_SET(fd, &rdfs);
			
			struct timeval tv;
			tv.tv_sec = 5;
	        	tv.tv_usec = 0;
			
			const int nSelect = select(2, &rdfs, NULL, NULL, &tv);
			if (nSelect < 0)
			{
				printf("select:error,ret %d, errno %d.\n", nSelect, errno);
				break;
			}
			else if (0 == nSelect)
			{
				printf("select:no data for reading.\n");
				continue;
			}
			
			char szRDBuf[LOCAL_CONST_IOBUF_LEN];
			memset(szRDBuf, '\0', sizeof(szRDBuf));
			const size_t nRead = read(fd, szRDBuf, sizeof(szRDBuf)-1);
			if (-1 == nRead)
			{
				printf("select:read failed(-1).errno %d", errno);
				break;
			}
			else if (0 == nRead)
			{
				printf("select:read nothing, EOF reached.errno %d.\n", errno);
				continue;
			}
			szRDBuf[sizeof(szRDBuf)-1] = '\0';
			printf("select:szRDBuf:%s\n", szRDBuf);
		}
		pthread_cancel(nWriteTid);
		void* pvRet = NULL;
		pthread_join(nWriteTid, &pvRet);
	}

	close(fd);
	printf("main exit.\n");
	return 0;
}