1 基本原理
注：select 原理图，摘自 IBM iSeries 信息中心。
1 数据结构与函数原型
1.1 select
int select( int nfds, fd_set *readset, fd_set *writeset, fd_set* exceptset, struct timeval *timeout );
select
- 位于：
#include <sys/select.h>
struct timeval
- 位于：
#include <sys/time.h>
返回对应位仍然为1的fd的总数。
- nfds：第一个参数是：最大的文件描述符值+1；
- readset：可读描述符集合；
- writeset：可写描述符集合；
- exceptset：异常描述符；
- timeout：select 的监听时长，如果这短时间内所监听的 socket 没有事件发生。
1.2 fd_set
1.2.1 清空描述符集合
FD_ZERO(fd_set *)
1.2.2 向描述符集合添加指定描述符
FD_SET(int, fd_set *)
1.2.3 从描述符集合删除指定描述符
FD_CLR(int, fd_set *)
1.2.4 检测指定描述符是否在描述符集合中
FD_ISSET(int, fd_set *)
1.2.5 描述符最大数量
#define FD_SETSIZE 1024
1.3 描述符集合
可读描述符集合中可读的描述符，为1，其他为0；可写也类似。异常描述符集合中有异常等待处理的描述符的值为1，其他为0。
1.4 ioctl
- 函数原型：
int ioctl(int handle, int cmd,[int *argdx, int argcx]);
- 头文件：
#include <sys/ioctl.h>
- 返回值：
- 0 - 成功
- 1 - 失败
2 示例
程序各部分的解释在注释中。
#include <sys/socket.h>
#include <string.h>
#include <sys/time.h>
#include <netinet/in.h>
#include <sys/ioctl.h>
#include <stdlib.h>
#include <errno.h>
#include <stdio.h>
#include <unistd.h>
#define TRUE 1
#define FALSE 0
int main(int argc, char *argv[])
{
int i, len, rc, on = TRUE;
int listen_sd, new_sd = 0, max_sd;
int desc_ready;
char buffer[80];
int close_conn, end_server = FALSE;
struct sockaddr_in server_addr;
struct timeval timeout;
struct fd_set master_set, working_set;
// Listen
listen_sd = socket(AF_INET, SOCK_STREAM, 0);
if (listen_sd < 0)
{
perror("socket() failed");
exit(-1);
}
// Set socket options
rc = setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, (char *) &on, sizeof(on));
if (rc < 0)
{
perror("setsockopt() failed");
close(listen_sd);
exit(-1);
}
// Set IO control
rc = ioctl(listen_sd, FIONBIO, (char *) &on);
if (rc < 0)
{
perror("ioctl() failed");
close(listen_sd);
exit(-1);
}
// Bind
memset(&server_addr, 0, sizeof(server_addr));
server_addr.sin_family = AF_INET;
server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
server_addr.sin_port = htons(atoi(argv[1]));
rc = bind(listen_sd, (struct sockaddr *) &server_addr, sizeof(server_addr));
if (rc < 0)
{
perror("bind() failed\n");
close(listen_sd);
exit(-1);
}
// Listen
rc = listen(listen_sd, 32);
if (rc < 0)
{
perror("listen() failed\n");
close(listen_sd);
exit(-1);
}
// Intialize sd set
FD_ZERO(&master_set);
max_sd = listen_sd;
FD_SET(listen_sd, &master_set);
timeout.tv_sec = 3 * 60;
timeout.tv_usec = 0;
// Start
do
{
// Copy master_set into working_set
memcpy(&working_set, &master_set, sizeof(master_set));
printf("Waiting on select()...\n");
rc = select(max_sd + 1, &working_set, NULL, NULL, &timeout);
if (rc < 0)
{
perror(" select() failed\n");
break;
}
if (rc == 0)
{
printf(" select() timed out. End program.\n");
break;
}
desc_ready = rc; // number of sds ready in working_set
// Check each sd in working_set
for (i = 0; i <= max_sd && desc_ready > 0; ++i)
{
// Check to see if this sd is ready
if (FD_ISSET(i, &working_set))
{
--desc_ready;
// Check to see if this is the listening sd
if (i == listen_sd)
{
printf(" Listeing socket is readable\n");
do
{
// Accept
new_sd = accept(listen_sd, NULL, NULL);
// Nothing to be accepted
if (new_sd < 0)
{
// All have been accepted
if (errno != EWOULDBLOCK)
{
perror(" accept() failed\n");
end_server = TRUE;
}
break;
}
// Insert new_sd into master_set
printf(" New incoming connection - %d\n", new_sd);
FD_SET(new_sd, &master_set);
if (new_sd > max_sd)
{
max_sd = new_sd;
}
}
while (new_sd != -1);
}
// This is not the listening sd
else
{
close_conn = FALSE;
printf(" Descriptor %d is avaliable\n", i);
do
{
rc = recv(i, buffer, sizeof(buffer), 0);
// Receive data on sd "i", until failure occurs
if (rc < 0)
{
// Normal failure
if (errno != EWOULDBLOCK)
{
perror(" recv() failed\n");
close_conn = TRUE;
}
break;
}
// The connection has been closed by the client
if (rc == 0)
{
printf(" Connection closed\n");
close_conn = TRUE;
break;
}
/* Receiving data succeeded and echo it back
the to client */
len = rc;
printf(" %d bytes received\n", len);
rc = send(i, buffer, len, 0);
if (rc < 0)
{
perror(" send() failed");
close_conn = TRUE;
break;
}
}
while (TRUE);
// If unknown failure occured
if (close_conn)
{
// Close the sd and remove it from master_set
close(i);
FD_CLR(i, &master_set);
// If this is the max sd
if (i == max_sd)
{
// Find the max sd in master_set now
while (FD_ISSET(max_sd, &master_set) == FALSE)
{
--max_sd;
}
} // End of if (i == max_sd)
} // End of if (close_conn)
}
}
}
}
while (end_server == FALSE);
/* Close each sd in master_set */
for (i = 0; i < max_sd; ++i)
{
if (FD_ISSET(i, &master_set))
{
close(i);
}
}
return 0;
}
