轮询

在这个例子中，我们创建了一对连接的套接字，并从一个到另一个发送 4 个字符串，并将接收的字符串打印到控制台。请注意，我们调用 send 的次数可能不等于我们调用 recv 的次数

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <poll.h>

#define BUFSIZE 512

int main()
{    
    #define CKERR(msg) {if(ret < 0) { perror(msg); \
        close(sockp[0]); close(sockp[1]); exit(EXIT_FAILURE); } }
    const char* strs_to_write[] = {"hello ", "from ", "other ", "side "};
    
    int sockp[2] = {-1, -1};
    ssize_t ret = socketpair (AF_UNIX, SOCK_STREAM, 0, sockp);
    CKERR("Socket pair creation error")
    
    struct pollfd pfds[2];
    for(int i=0; i<2; ++i) {
        pfds[i] = (struct pollfd){sockp[i], POLLIN|POLLOUT, 0};
        fcntl(sockp[i], F_SETFL|O_NONBLOCK); // nonblocking fds are
                // literally mandatory for IO multiplexing; non-portable
    }
    char buf[BUFSIZE];
    
    size_t snt = 0, msgs = sizeof(strs_to_write)/sizeof(char*);
    while(1) {
        int ret = poll(pfds,
            2 /*length of pollfd array*/,
            5 /*milliseconds to wait*/);
        CKERR("Poll error")

        if (pfds[0].revents & POLLOUT && snt < msgs) {
            // Checking POLLOUT before writing to ensure there is space
            // available in socket's kernel buffer to write, otherwise we
            // may face EWOULDBLOCK / EAGAIN error
            ssize_t ret = send(sockp[0], strs_to_write[snt], strlen(strs_to_write[snt]), 0);
            if(++snt >= msgs)
                close(sockp[0]);
            CKERR("send error")
            if (ret == 0) {
                puts("Connection closed");
                break;
            }
            if (ret > 0) {
                // assuming that all bytes were written
                // if ret != %sent bytes number%, send other bytes later
            }
        }
        if (pfds[1].revents & POLLIN) {
            // There is something to read
            ssize_t ret = recv(sockp[1], buf, BUFSIZE, 0);
            CKERR("receive error")
            if (ret == 0) {
                puts("Connection closed");
                break;
            }
            if (ret > 0) {
                printf("received str: %.*s\n", (int)ret, buf);
            }
        }

    }
    close(sockp[1]);
    return EXIT_SUCCESS;
}