POSIX Poll


Example

In this example we create a pair of connected sockets and send 4 strings from one to another and print received strings to console. Note, that the number of times we will call send may not be equal to number of times we call recv

#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;
}