# select_tut

## NAME



           select, pselect - synchronous I/O multiplexing
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           See select(2)
    


## DESCRIPTION



           The **select**() and **pselect**() system calls are used to efficiently
           monitor multiple file descriptors, to see if any of them is, or
           becomes, "ready"; that is, to see whether I/O becomes possible, or
           an "exceptional condition" has occurred on any of the file
           descriptors.
    
           This page provides background and tutorial information on the use
           of these system calls.  For details of the arguments and semantics
           of **select**() and **pselect**(), see select(2).
    
       **Combining signal and data events**
           **pselect**() is useful if you are waiting for a signal as well as for
           file descriptor(s) to become ready for I/O.  Programs that receive
           signals normally use the signal handler only to raise a global
           flag.  The global flag will indicate that the event must be
           processed in the main loop of the program.  A signal will cause
           the **select**() (or **pselect**()) call to return with _errno_ set to
           **EINTR**.  This behavior is essential so that signals can be
           processed in the main loop of the program, otherwise **select**()
           would block indefinitely.
    
           Now, somewhere in the main loop will be a conditional to check the
           global flag.  So we must ask: what if a signal arrives after the
           conditional, but before the **select**() call?  The answer is that
           **select**() would block indefinitely, even though an event is
           actually pending.  This race condition is solved by the **pselect**()
           call.  This call can be used to set the signal mask to a set of
           signals that are to be received only within the **pselect**() call.
           For instance, let us say that the event in question was the exit
           of a child process.  Before the start of the main loop, we would
           block **SIGCHLD** using sigprocmask(2).  Our **pselect**() call would
           enable **SIGCHLD** by using an empty signal mask.  Our program would
           look like:
    
           static volatile sig_atomic_t got_SIGCHLD = 0;
    
           static void
           child_sig_handler(int sig)
           {
               got_SIGCHLD = 1;
           }
    
           int
           main(int argc, char *argv[])
           {
               sigset_t sigmask, empty_mask;
               struct sigaction sa;
               fd_set readfds, writefds, exceptfds;
               int r;
    
               sigemptyset(&sigmask);
               sigaddset(&sigmask, SIGCHLD);
               if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == -1) {
                   perror("sigprocmask");
                   exit(EXIT_FAILURE);
               }
    
               sa.sa_flags = 0;
               sa.sa_handler = child_sig_handler;
               sigemptyset(&sa.sa_mask);
               if (sigaction(SIGCHLD, &sa, NULL) == -1) {
                   perror("sigaction");
                   exit(EXIT_FAILURE);
               }
    
               sigemptyset(&empty_mask);
    
               for (;;) {          /* main loop */
                   /* Initialize readfds, writefds, and exceptfds
                      before the pselect() call. (Code omitted.) */
    
                   r = pselect(nfds, &readfds, &writefds, &exceptfds,
                               NULL, &empty_mask);
                   if (r == -1 && errno != EINTR) {
                       /* Handle error */
                   }
    
                   if (got_SIGCHLD) {
                       got_SIGCHLD = 0;
    
                       /* Handle signalled event here; e.g., wait() for all
                          terminated children. (Code omitted.) */
                   }
    
                   /* main body of program */
               }
           }
    
       **Practical**
           So what is the point of **select**()?  Can't I just read and write to
           my file descriptors whenever I want?  The point of **select**() is
           that it watches multiple descriptors at the same time and properly
           puts the process to sleep if there is no activity.  UNIX
           programmers often find themselves in a position where they have to
           handle I/O from more than one file descriptor where the data flow
           may be intermittent.  If you were to merely create a sequence of
           read(2) and write(2) calls, you would find that one of your calls
           may block waiting for data from/to a file descriptor, while
           another file descriptor is unused though ready for I/O.  **select**()
           efficiently copes with this situation.
    
       **Select law**
           Many people who try to use **select**() come across behavior that is
           difficult to understand and produces nonportable or borderline
           results.  For instance, the above program is carefully written not
           to block at any point, even though it does not set its file
           descriptors to nonblocking mode.  It is easy to introduce subtle
           errors that will remove the advantage of using **select**(), so here
           is a list of essentials to watch for when using **select**().
    
           1.  You should always try to use **select**() without a timeout.  Your
               program should have nothing to do if there is no data
               available.  Code that depends on timeouts is not usually
               portable and is difficult to debug.
    
           2.  The value _nfds_ must be properly calculated for efficiency as
               explained above.
    
           3.  No file descriptor must be added to any set if you do not
               intend to check its result after the **select**() call, and
               respond appropriately.  See next rule.
    
           4.  After **select**() returns, all file descriptors in all sets
               should be checked to see if they are ready.
    
           5.  The functions read(2), recv(2), write(2), and send(2) do _not_
               necessarily read/write the full amount of data that you have
               requested.  If they do read/write the full amount, it's
               because you have a low traffic load and a fast stream.  This
               is not always going to be the case.  You should cope with the
               case of your functions managing to send or receive only a
               single byte.
    
           6.  Never read/write only in single bytes at a time unless you are
               really sure that you have a small amount of data to process.
               It is extremely inefficient not to read/write as much data as
               you can buffer each time.  The buffers in the example below
               are 1024 bytes although they could easily be made larger.
    
           7.  Calls to read(2), recv(2), write(2), send(2), and **select**() can
               fail with the error **EINTR** , and calls to read(2), recv(2),
               write(2), and send(2) can fail with _errno_ set to **EAGAIN**
               (**EWOULDBLOCK**).  These results must be properly managed (not
               done properly above).  If your program is not going to receive
               any signals, then it is unlikely you will get **EINTR**.  If your
               program does not set nonblocking I/O, you will not get **EAGAIN**.
    
           8.  Never call read(2), recv(2), write(2), or send(2) with a
               buffer length of zero.
    
           9.  If the functions read(2), recv(2), write(2), and send(2) fail
               with errors other than those listed in **7.** , or one of the input
               functions returns 0, indicating end of file, then you should
               _not_ pass that file descriptor to **select**() again.  In the
               example below, I close the file descriptor immediately, and
               then set it to -1 to prevent it being included in a set.
    
           10. The timeout value must be initialized with each new call to
               **select**(), since some operating systems modify the structure.
               **pselect**() however does not modify its timeout structure.
    
           11. Since **select**() modifies its file descriptor sets, if the call
               is being used in a loop, then the sets must be reinitialized
               before each call.
    


## RETURN VALUE



           See select(2).
    


## NOTES



           Generally speaking, all operating systems that support sockets
           also support **select**().  **select**() can be used to solve many
           problems in a portable and efficient way that naive programmers
           try to solve in a more complicated manner using threads, forking,
           IPCs, signals, memory sharing, and so on.
    
           The poll(2) system call has the same functionality as **select**(),
           and is somewhat more efficient when monitoring sparse file
           descriptor sets.  It is nowadays widely available, but
           historically was less portable than **select**().
    
           The Linux-specific epoll(7) API provides an interface that is more
           efficient than select(2) and poll(2) when monitoring large numbers
           of file descriptors.
    


## EXAMPLES



           Here is an example that better demonstrates the true utility of
           **select**().  The listing below is a TCP forwarding program that
           forwards from one TCP port to another.
    
           #include <arpa/inet.h>
           #include <errno.h>
           #include <netinet/in.h>
           #include <signal.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <string.h>
           #include <sys/select.h>
           #include <sys/socket.h>
           #include <sys/types.h>
           #include <unistd.h>
    
           static int forward_port;
    
           #undef max
           #define max(x, y) ((x) > (y) ? (x) : (y))
    
           static int
           listen_socket(int listen_port)
           {
               int                 lfd;
               int                 yes;
               struct sockaddr_in  addr;
    
               lfd = socket(AF_INET, SOCK_STREAM, 0);
               if (lfd == -1) {
                   perror("socket");
                   return -1;
               }
    
               yes = 1;
               if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR,
                              &yes, sizeof(yes)) == -1)
               {
                   perror("setsockopt");
                   close(lfd);
                   return -1;
               }
    
               memset(&addr, 0, sizeof(addr));
               addr.sin_port = htons(listen_port);
               addr.sin_family = AF_INET;
               if (bind(lfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
                   perror("bind");
                   close(lfd);
                   return -1;
               }
    
               printf("accepting connections on port %d\n", listen_port);
               listen(lfd, 10);
               return lfd;
           }
    
           static int
           connect_socket(int connect_port, char *address)
           {
               int                 cfd;
               struct sockaddr_in  addr;
    
               cfd = socket(AF_INET, SOCK_STREAM, 0);
               if (cfd == -1) {
                   perror("socket");
                   return -1;
               }
    
               memset(&addr, 0, sizeof(addr));
               addr.sin_port = htons(connect_port);
               addr.sin_family = AF_INET;
    
               if (!inet_aton(address, (struct in_addr *) &addr.sin_addr.s_addr)) {
                   fprintf(stderr, "inet_aton(): bad IP address format\n");
                   close(cfd);
                   return -1;
               }
    
               if (connect(cfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
                   perror("connect()");
                   shutdown(cfd, SHUT_RDWR);
                   close(cfd);
                   return -1;
               }
               return cfd;
           }
    
           #define SHUT_FD1 do {                                \
                                if (fd1 >= 0) {                 \
                                    shutdown(fd1, SHUT_RDWR);   \
                                    close(fd1);                 \
                                    fd1 = -1;                   \
                                }                               \
                            } while (0)
    
           #define SHUT_FD2 do {                                \
                                if (fd2 >= 0) {                 \
                                    shutdown(fd2, SHUT_RDWR);   \
                                    close(fd2);                 \
                                    fd2 = -1;                   \
                                }                               \
                            } while (0)
    
           #define BUF_SIZE 1024
    
           int
           main(int argc, char *argv[])
           {
               int      h;
               int      ready, nfds;
               int      fd1 = -1, fd2 = -1;
               int      buf1_avail = 0, buf1_written = 0;
               int      buf2_avail = 0, buf2_written = 0;
               char     buf1[BUF_SIZE], buf2[BUF_SIZE];
               fd_set   readfds, writefds, exceptfds;
               ssize_t  nbytes;
    
               if (argc != 4) {
                   fprintf(stderr, "Usage\n\tfwd <listen-port> "
                           "<forward-to-port> <forward-to-ip-address>\n");
                   exit(EXIT_FAILURE);
               }
    
               signal(SIGPIPE, SIG_IGN);
    
               forward_port = atoi(argv[2]);
    
               h = listen_socket(atoi(argv[1]));
               if (h == -1)
                   exit(EXIT_FAILURE);
    
               for (;;) {
                   nfds = 0;
    
                   FD_ZERO(&readfds);
                   FD_ZERO(&writefds);
                   FD_ZERO(&exceptfds);
                   FD_SET(h, &readfds);
                   nfds = max(nfds, h);
    
                   if (fd1 > 0 && buf1_avail < BUF_SIZE)
                       FD_SET(fd1, &readfds);
                       /* Note: nfds is updated below, when fd1 is added to
                          exceptfds. */
                   if (fd2 > 0 && buf2_avail < BUF_SIZE)
                       FD_SET(fd2, &readfds);
    
                   if (fd1 > 0 && buf2_avail - buf2_written > 0)
                       FD_SET(fd1, &writefds);
                   if (fd2 > 0 && buf1_avail - buf1_written > 0)
                       FD_SET(fd2, &writefds);
    
                   if (fd1 > 0) {
                       FD_SET(fd1, &exceptfds);
                       nfds = max(nfds, fd1);
                   }
                   if (fd2 > 0) {
                       FD_SET(fd2, &exceptfds);
                       nfds = max(nfds, fd2);
                   }
    
                   ready = select(nfds + 1, &readfds, &writefds, &exceptfds, NULL);
    
                   if (ready == -1 && errno == EINTR)
                       continue;
    
                   if (ready == -1) {
                       perror("select()");
                       exit(EXIT_FAILURE);
                   }
    
                   if (FD_ISSET(h, &readfds)) {
                       socklen_t addrlen;
                       struct sockaddr_in client_addr;
                       int fd;
    
                       addrlen = sizeof(client_addr);
                       memset(&client_addr, 0, addrlen);
                       fd = accept(h, (struct sockaddr *) &client_addr, &addrlen);
                       if (fd == -1) {
                           perror("accept()");
                       } else {
                           SHUT_FD1;
                           SHUT_FD2;
                           buf1_avail = buf1_written = 0;
                           buf2_avail = buf2_written = 0;
                           fd1 = fd;
                           fd2 = connect_socket(forward_port, argv[3]);
                           if (fd2 == -1)
                               SHUT_FD1;
                           else
                               printf("connect from %s\n",
                                      inet_ntoa(client_addr.sin_addr));
    
                           /* Skip any events on the old, closed file
                              descriptors. */
    
                           continue;
                       }
                   }
    
                   /* NB: read OOB data before normal reads. */
    
                   if (fd1 > 0 && FD_ISSET(fd1, &exceptfds)) {
                       char c;
    
                       nbytes = recv(fd1, &c, 1, MSG_OOB);
                       if (nbytes < 1)
                           SHUT_FD1;
                       else
                           send(fd2, &c, 1, MSG_OOB);
                   }
                   if (fd2 > 0 && FD_ISSET(fd2, &exceptfds)) {
                       char c;
    
                       nbytes = recv(fd2, &c, 1, MSG_OOB);
                       if (nbytes < 1)
                           SHUT_FD2;
                       else
                           send(fd1, &c, 1, MSG_OOB);
                   }
                   if (fd1 > 0 && FD_ISSET(fd1, &readfds)) {
                       nbytes = read(fd1, buf1 + buf1_avail,
                                     BUF_SIZE - buf1_avail);
                       if (nbytes < 1)
                           SHUT_FD1;
                       else
                           buf1_avail += nbytes;
                   }
                   if (fd2 > 0 && FD_ISSET(fd2, &readfds)) {
                       nbytes = read(fd2, buf2 + buf2_avail,
                                     BUF_SIZE - buf2_avail);
                       if (nbytes < 1)
                           SHUT_FD2;
                       else
                           buf2_avail += nbytes;
                   }
                   if (fd1 > 0 && FD_ISSET(fd1, &writefds) && buf2_avail > 0) {
                       nbytes = write(fd1, buf2 + buf2_written,
                                      buf2_avail - buf2_written);
                       if (nbytes < 1)
                           SHUT_FD1;
                       else
                           buf2_written += nbytes;
                   }
                   if (fd2 > 0 && FD_ISSET(fd2, &writefds) && buf1_avail > 0) {
                       nbytes = write(fd2, buf1 + buf1_written,
                                      buf1_avail - buf1_written);
                       if (nbytes < 1)
                           SHUT_FD2;
                       else
                           buf1_written += nbytes;
                   }
    
                   /* Check if write data has caught read data. */
    
                   if (buf1_written == buf1_avail)
                       buf1_written = buf1_avail = 0;
                   if (buf2_written == buf2_avail)
                       buf2_written = buf2_avail = 0;
    
                   /* One side has closed the connection, keep
                      writing to the other side until empty. */
    
                   if (fd1 < 0 && buf1_avail - buf1_written == 0)
                       SHUT_FD2;
                   if (fd2 < 0 && buf2_avail - buf2_written == 0)
                       SHUT_FD1;
               }
               exit(EXIT_SUCCESS);
           }
    
           The above program properly forwards most kinds of TCP connections
           including OOB signal data transmitted by **telnet** servers.  It
           handles the tricky problem of having data flow in both directions
           simultaneously.  You might think it more efficient to use a
           fork(2) call and devote a thread to each stream.  This becomes
           more tricky than you might suspect.  Another idea is to set
           nonblocking I/O using fcntl(2).  This also has its problems
           because you end up using inefficient timeouts.
    
           The program does not handle more than one simultaneous connection
           at a time, although it could easily be extended to do this with a
           linked list of buffers—one for each connection.  At the moment,
           new connections cause the current connection to be dropped.
    


* * *