热门问题
时间线
聊天
视角
select (Unix)
来自维基百科,自由的百科全书
Remove ads
select是用於I/O多路轉接的一個系統呼叫函數。
![]() | 此條目包含過多行話或專業術語,可能需要簡化或提出進一步解釋。 (2012年11月29日) |
此條目沒有列出任何參考或來源。 (2010年5月5日) |
在C語言中,該系統呼叫在 sys/select.h 或 unistd.h 中聲明,語法如下:
int select(int nfds, fd_set* readfds, fd_set* writefds, fd_set* errorfds, struct timeval* timeout);
為了維護fd_set類型的參數,會使用下面四個宏:FD_SET(), FD_CLR(), FD_ZERO() 和 FD_ISSET()。
返回值:
这个函数将返回描述符集的个数, 如果超时返回为0,错误则返回-1。
參看:
- select(2)
- poll(2)
Remove ads
select與epoll的區別
說明:以上無論epoll_create, fd_set都受限於系統中單個行程能夠打開的檔案控制代碼數。
Remove ads
範例
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <sys/select.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#define PORT "9421"
/* function prototypes */
void die(const char*);
int main(int argc, char **argv)
{
int sockfd, new, maxfd, on = 1, nready, i;
struct addrinfo *res0, *res, hints;
char buffer[BUFSIZ];
fd_set master, readfds;
ssize_t nbytes;
(void)memset(&hints, '\0', sizeof(struct addrinfo));
hints.ai_family = AF_INET;
hints.ai_socktype = SOCK_STREAM;
hints.ai_protocol = IPPROTO_TCP;
hints.ai_flags = AI_PASSIVE;
if(-1 == (getaddrinfo(NULL, PORT, &hints, &res0)))
die("getaddrinfo()");
for(res = res0; res; res = res->ai_next)
{
if(-1 == (sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol)))
{
perror("socket()");
continue;
}
if(-1 == (setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, (char*)&on, sizeof(int))))
{
perror("setsockopt()");
continue;
}
if(-1 == (bind(sockfd, res->ai_addr, res->ai_addrlen)))
{
perror("bind");
continue;
}
break;
}
if(-1 == sockfd)
exit(EXIT_FAILURE);
freeaddrinfo(res0);
if(-1 == (listen(sockfd, 32)))
die("listen()");
if(-1 == (fcntl(sockfd, F_SETFD, O_NONBLOCK)))
die("fcntl()");
FD_ZERO(&master);
FD_ZERO(&readfds);
FD_SET(sockfd, &master);
maxfd = sockfd;
while(1)
{
memcpy(&readfds, &master, sizeof(master));
(void)printf("running select()\n");
if(-1 == (nready = select(maxfd+1, &readfds, NULL, NULL, NULL)))
die("select()");
(void)printf("Number of ready descriptor: %d\n", nready);
for(i=0; i<=maxfd && nready>0; i++)
{
if(FD_ISSET(i, &readfds))
{
nready--;
if(i == sockfd)
{
(void)printf("Trying to accept() new connection(s)\n");
if(-1 == (new = accept(sockfd, NULL, NULL)))
{
if(EWOULDBLOCK != errno)
die("accept()");
break;
}
else
{
if(-1 == (fcntl(new, F_SETFD, O_NONBLOCK)))
die("fcntl()");
FD_SET(new, &master);
if(maxfd < new)
maxfd = new;
}
}
else
{
(void)printf("recv() data from one of descriptors(s)\n");
nbytes = recv(i, buffer, sizeof(buffer), 0);
if(nbytes <= 0)
{
if(EWOULDBLOCK != errno)
die("recv()");
break;
}
buffer[nbytes] = '\0';
printf("%s", buffer);
(void)printf("%zi bytes received.\n", nbytes);
close(i);
FD_CLR(i, &master);
}
}
}
}
return 0;
}
void die(const char *msg)
{
perror(msg);
exit(EXIT_FAILURE);
}
Remove ads
參見
外部連結
Wikiwand - on
Seamless Wikipedia browsing. On steroids.
Remove ads