select (Unix)

<h2 id="example">Example</h2>
#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 <err.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;

int error;

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 (0 != (error = getaddrinfo(NULL, PORT, &hints, &res0)))
        errx(EXIT_FAILURE, "%s", gai_strerror(error));

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);
}

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Berkeley_sockets/edRVDP8C">Berkeley sockets</a></li>
<li><a href="/facts/Polling_(computer_science)/WVa88JIg">Polling</a></li>
<li><a href="/facts/Epoll/1Ve5Mqds">epoll</a></li>
<li><a href="/facts/Kqueue/AbszxLuA">kqueue</a></li>
<li><a href="/facts/Input/output_completion_port/N5CIu3Mh">Input/output completion port</a> (IOCP)</li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="https://pubs.opengroup.org/onlinepubs/9799919799/functions/select.html">select</a> – System Interfaces Reference, <a href="/facts/Single_Unix_Specification/w1ba5bDt">The Single UNIX Specification</a>, Version 4 from <a href="/facts/The_Open_Group/uRoDdEae">The Open Group</a></li>
<li><a href="http://mdoc.su/-/select.2">man-pages for select(2) in FreeBSD, NetBSD, OpenBSD and DragonFly BSD</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Computer Systems Research Group (1994). "select, pselect — synchronous I/O multiplexing". BSD Cross Reference. NetBSD. <a href="/wiki/Computer_Systems_Research_Group" target="_blank">/wiki/Computer_Systems_Research_Group</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"Connection processing methods". nginx.org. <a href="http://nginx.org/en/docs/events.html" target="_blank">http://nginx.org/en/docs/events.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
</ol>

select (Unix) open-in-new

select (Unix)