Input/Output Multiplexing

Requests can be multiplexed among multiple sockets or files. Use select(3C) to do this:

#include <sys/time.h>
#include <sys/types.h>
#include <sys/select.h>
 ...
fd_set readmask, writemask, exceptmask;
struct timeval timeout;
 ...
select(nfds, &readmask, &writemask, &exceptmask, &timeout);

The first argument of select(3C) is the number of file descriptors in the lists pointed to by the next three arguments.

The second, third, and fourth arguments of select(3C) point to three sets of file descriptors: a set of descriptors to read on, a set to write on, and a set on which exception conditions are accepted. Out-of-band data is the only exceptional condition. Any of these pointers can be a properly cast null. Each set is a structure containing an array of long integer bit masks. The size of the array is set by FD_SETSIZE (defined in select.h). The array is long enough to hold one bit for each FD_SETSIZE file descriptor.

The macros FD_SET(fd, &mask) and FD_CLR(fd, &mask) add and delete, respectively, the file descriptor fd in the set mask. The set should be zeroed before use, and the macro FD_ZERO(&mask) clears the set mask.

The fifth argument of select(3C) allows a time-out value to be specified. If the timeout pointer is NULL, select(3C) blocks until a descriptor is selectable, or until a signal is received. If the fields in timeout are set to 0, select(3C) polls and returns immediately.

select(3C) normally returns the number of file descriptors selected. select(3C) returns a 0 if the time-out has expired. select(3C) returns -1 for an error or interrupt with the error number in errno and the file descriptor masks unchanged. For a successful return, the three sets indicate which file descriptors are ready to be read from, written to, or have exceptional conditions pending.

You should test the status of a file descriptor in a select mask with the FD_ISSET(fd, &mask) macro. It returns a nonzero value if fd is in the set mask, and 0 if it is not. Use select(3C) followed by a FD_ISSET(fd, &mask) macro on the read set to check for queued connect requests on a socket.

Example 2-5 shows how to select on a "listening" socket for readability to determine when a new connection can be picked up with a call to accept(3SOCKET). The program accepts connection requests, reads data, and disconnects on a single socket.

Example 2-5 Using select(3C) to Check for Pending Connections

#include <sys/types.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
 
#define TRUE 1
 
/*
 * This program uses select to check that someone is
 * trying to connect before calling accept.
 */
 
main()
{
   int sock, length;
   struct sockaddr_in6 server;
   int msgsock;
   char buf[1024];
   int rval;
   fd_set ready;
   struct timeval to;
 
   /* Open a socket and bind it as in previous examples. */
 
   /* Start accepting connections. */
   listen(sock, 5);
   do {
      FD_ZERO(&ready);
      FD_SET(sock, &ready);
      to.tv_sec = 5;
      to.tv_usec = 0;
      if (select(sock + 1, &ready, (fd_set *)0, (fd_set *)0, &to) == -1) {
         perror("select");
         continue;
      }
      if (FD_ISSET(sock, &ready)) {
         msgsock = accept(sock, (struct sockaddr *)0,
            (int *)0);
         if (msgsock == -1)
            perror("accept");
         else do {
            memset(buf, 0, sizeof buf);
            if ((rval = read(msgsock, buf, 1024)) == -1)
               perror("reading stream message");
            else if (rval == 0)
               printf("Ending connection\n");
            else
               printf("-->%s\n", buf);
         } while (rval > 0);
         close(msgsock);
      } else
         printf("Do something else\n");
   } while (TRUE);
	   exit(0);
}

In previous versions of the select(3C) routine, its arguments were pointers to integers instead of pointers to fd_sets. This style of call still works if the number of file descriptors is smaller than the number of bits in an integer.

select(3C) provides a synchronous multiplexing scheme. The SIGIO and SIGURG signals (described in "Advanced Topics") provide asynchronous notification of output completion, input availability, and exceptional conditions.