如何使用C中的select()从客户端和FIFO读取()

I read()来自连接到服务器的客户端，同时，我让select()知道来自FIFO的数据。现在，当数据写入FIFO时，会选择将所有数据写入客户端，但会像"准备读取"一样不断返回。因此，下一次读取被设置为-1和errno == EAGAIN。它一直这样做，直到达到fdmax。不过它还可以。

但是，为什么我一直得到EAGAIN？有更好的方法来处理这个问题吗？还是这是正确的方式？

注意：我传递O_RDWR|O_NONBLOCK，这样它也可以保持客户端发送的read()数据，而不仅仅是FIFO。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <fcntl.h>
#include <errno.h>
#define PORT "9034"
int main(void) {
    fd_set master, read_fds;
    int fdmax, listener, newfd, sbytes, yes=1, i, j, rv; 
    struct sockaddr_storage remoteaddr; // client address
    socklen_t addrlen;
    char buf[256] = {0}, remoteIP[INET6_ADDRSTRLEN];
    struct addrinfo hints, *ai;
    FD_ZERO(&master);
    FD_ZERO(&read_fds);
    int fifo;
    if ((mkfifo("/tmp/fifo", 0666)) < 0)
        perror(strerror(errno));
    if ((fifo = open("/tmp/fifo", O_RDWR|O_NONBLOCK)) < 0)  
        perror(strerror(errno));
    // get us a socket and bind it
    memset(&hints, 0, sizeof hints);
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_flags = AI_PASSIVE;
    rv = getaddrinfo(NULL, PORT, &hints, &ai);
    listener = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);
    setsockopt(listener, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int));
    bind(listener, ai->ai_addr, ai->ai_addrlen);
    freeaddrinfo(ai);
    listen(listener, 10);
    FD_SET (fifo, &master);
    FD_SET(listener, &master);
    fdmax = listener;
    for (;;) {
        read_fds = master;
        if (select(fdmax + 1, &read_fds, NULL, NULL, NULL) == -1) exit(4);
        for (i = 0; i <= fdmax; i++) {
            if (FD_ISSET(i, &read_fds)) {
                if (i == listener) {
                    addrlen = sizeof remoteaddr;
                    newfd = accept(listener, (struct sockaddr * ) & remoteaddr, & addrlen);
                    FD_SET(newfd, & master);
                    if (newfd > fdmax)
                        fdmax = newfd;
                } else if (i != fifo) {
                    recv(i, buf, sizeof buf, 0); 
                }   
            }
            if (FD_ISSET(fifo, &read_fds)) {
                sbytes = read (fifo, buf, sizeof (buf));
                if (sbytes == -1 && errno == EAGAIN)
                    continue;
                for(j = 0; j <= fdmax; j++) {
                    // send to everyone!
                    if (FD_ISSET(j, &master)) {
                        if (j != listener && j != i && j != fifo) {
                            if (send(j, buf, sbytes, 0) == -1) {
                                perror("send");
                            }   
                        }   
                    }   
                }   
            }   
        }   
    }   
    return 0;
}

for (i = 0; i <= fdmax; i++) {

if (FD_ISSET(fifo, &read_fds)) {