Socket multiplexing with select() does not work as expecting

Question

I'm trying to create a server that listens to two different ports, reads a message from the respective socket and prints it to stdout. When I connect for the first time to any of the available ports, the program works as expected. Then, when I try to connect to the second port, connect() will simply not unblock.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

int main(int argc, char** argv)
{
    struct sockaddr_in addr1;
    struct sockaddr_in addr2;
    int srv_fd1;
    int srv_fd2;
    int srv_fd;
    int accept_fd;
    fd_set srv_fds;
    char mem[1024];

    if ((srv_fd1 = socket(AF_INET, SOCK_STREAM, 0)) == -1 ||
        (srv_fd2 = socket(AF_INET, SOCK_STREAM, 0)) == -1)
    {
        fprintf(stderr, "socket(): %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }

    addr1.sin_family = AF_INET;
    addr1.sin_port   = htons(16000);
    addr1.sin_addr.s_addr = htonl(INADDR_ANY);

    addr2.sin_family = AF_INET;
    addr2.sin_port   = htons(16001);
    addr2.sin_addr.s_addr = htonl(INADDR_ANY);

    if (bind(srv_fd1, (struct sockaddr *)&addr1, (socklen_t)sizeof(addr1)) == -1 ||
        bind(srv_fd2, (struct sockaddr *)&addr2, (socklen_t)sizeof(addr2)) == -1)
    {
        fprintf(stderr, "bind(): %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }

    if (listen(srv_fd1, 128) == -1 || listen(srv_fd2, 128) == -1) {
        fprintf(stderr, "listen(): %s\n", strerror(errno));
        exit(EXIT_FAILURE);
    }

    FD_ZERO(&srv_fds);
    FD_SET(srv_fd1, &srv_fds);
    FD_SET(srv_fd2, &srv_fds);

    while (select(srv_fd2 +1, &srv_fds, NULL, NULL, NULL) != -1) {

        printf("Inside loop\n");
        fflush(stdout);
        memset(mem, 0, 1024);

        if (FD_ISSET(srv_fd1, &srv_fds))
            srv_fd = srv_fd1;
        else if (FD_ISSET(srv_fd2, &srv_fds))
            srv_fd = srv_fd2;

        accept_fd = accept(srv_fd, NULL, NULL);
        if (accept_fd == -1) {
            fprintf(stderr, "accept(): %s\n", strerror(errno));
            exit(EXIT_FAILURE);
        }

        read(accept_fd, mem, 1023);
        printf("%s", mem);

        close(accept_fd);
    }

    close(srv_fd1);
    close(srv_fd2);
    return 0;
}

Demonstration of the problem with telnet:

$ telnet 127.0.0.1 16000
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
FIRST ATTEMPT
Connection closed by foreign host.

$ telnet 127.0.0.1 16001
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
SECOND ATTEMPT
^]

telnet> quit
Connection closed.

Server response:

$ ./test
Inside loop
FIRST ATTEMPT
^C

Answer 1

you need to do

FD_ZERO(&srv_fds);
FD_SET(srv_fd1, &srv_fds);
FD_SET(srv_fd2, &srv_fds);

each time before you call select, because that one resets srv_fds to indicate which fd has something. In your case the first call of select cancel FD_SET(srv_fd2, &srv_fds); so the second call of select only manages srv_fd1

Note doing srv_fd2 +1 you also suppose srv_fd2 > srv_fd1 which can be false.

You also suppose you have only one ready doing

   if (FD_ISSET(srv_fd1, &srv_fds))
        srv_fd = srv_fd1;
    else if (FD_ISSET(srv_fd2, &srv_fds))
        srv_fd = srv_fd2;
   ...

but you can have them both ready.

---

A way to do can be :

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>

#define NSOCK 2
#define FIRST_PORT 16000

/* returns the max descriptor */
int do_fd_set(int * srv_fd, fd_set * srv_fds)
{
  int r = 0;
  int i;

  FD_ZERO(srv_fds);

  for (i = 0; i != NSOCK; ++i) {
    FD_SET(srv_fd[i], srv_fds);
    if (srv_fd[i] > r)
      r = srv_fd[i];
  }

  return r;
}

int main(int argc, char** argv)
{
    int srv_fd[NSOCK];
    fd_set srv_fds;
    int i;

    for (i = 0; i != NSOCK; ++i) {
      struct sockaddr_in addr;

      if ((srv_fd[i] = socket(AF_INET, SOCK_STREAM, 0)) == -1)
      {
        fprintf(stderr, "socket(): %s\n", strerror(errno));
        exit(EXIT_FAILURE);
      }

      addr.sin_family = AF_INET;
      addr.sin_port   = htons(FIRST_PORT + i);
      addr.sin_addr.s_addr = htonl(INADDR_ANY);

      if (bind(srv_fd[i], (struct sockaddr *)&addr, (socklen_t)sizeof(addr)) == -1)
      {
        fprintf(stderr, "bind(): %s\n", strerror(errno));
        exit(EXIT_FAILURE);
      }

      if (listen(srv_fd[i], 128) == -1) {
        fprintf(stderr, "listen(): %s\n", strerror(errno));
        exit(EXIT_FAILURE);
      }
    }

    while (select(do_fd_set(srv_fd, &srv_fds) + 1, &srv_fds, NULL, NULL, NULL) != -1) {
      printf("Inside loop\n");

      for (int i = 0; i != NSOCK; ++i) {
        if (FD_ISSET(srv_fd[i], &srv_fds)) {
          char mem[1024];
          int accept_fd;

          memset(mem, 0, 1024);
          accept_fd = accept(srv_fd[i], NULL, NULL);
          if (accept_fd == -1) {
            fprintf(stderr, "accept(): %s\n", strerror(errno));
            exit(EXIT_FAILURE);
          }
          read(accept_fd, mem, sizeof(mem) - 1);
          printf("from port %d : %s\n", FIRST_PORT + i, mem);
          close(accept_fd);
        }
      }
    }

    /* probably never reach that code */
    for (i = 0; i != NSOCK; ++i)
      close(srv_fd[i]);

    return 0;
}

Compilation and execution :

pi@raspberrypi:/tmp $ gcc -Wall so.c
pi@raspberrypi:/tmp $ ./a.out
Inside loop
from port 16000 : aze

Inside loop
from port 16000 : qsd

Inside loop
from port 16001 : wxc

^C
pi@raspberrypi:/tmp $ 

doing in an other terminal :

pi@raspberrypi:/tmp $ telnet localhost 16000
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
aze
Connection closed by foreign host.
pi@raspberrypi:/tmp $ telnet localhost 16000
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
qsd
Connection closed by foreign host.
pi@raspberrypi:/tmp $ telnet localhost 16001
Trying ::1...
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
wxc
Connection closed by foreign host.

Answer 2

bruno's answer diagnoses the problems correctly. As a supplement, here is a working version of the relevant code:

while(1) {
    FD_ZERO(&srv_fds);
    FD_SET(srv_fd1, &srv_fds);
    FD_SET(srv_fd2, &srv_fds);

    int maxFD = (srv_fd1 > srv_fd2) ? srv_fd1 : srv_fd2;
    if (select(maxFD+1, &srv_fds, NULL, NULL, NULL) == -1) break;

    [...]