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    Command:

    select_tut

    
    
    
    

    SYNOPSIS

           /* According to POSIX.1-2001 */
           #include <sys/select.h>
    
           /* According to earlier standards */
           #include <sys/time.h>
           #include <sys/types.h>
           #include <unistd.h>
    
           int select(int nfds, fd_set *readfds, fd_set *writefds,
                      fd_set *exceptfds, struct timeval *utimeout);
    
           void FD_CLR(int fd, fd_set *set);
           int  FD_ISSET(int fd, fd_set *set);
           void FD_SET(int fd, fd_set *set);
           void FD_ZERO(fd_set *set);
    
           #include <sys/select.h>
    
           int pselect(int nfds, fd_set *readfds, fd_set *writefds,
                       fd_set *exceptfds, const struct timespec *ntimeout,
                       const sigset_t *sigmask);
    
       Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
    
           pselect(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600
    
    
    

    DESCRIPTION

           select() (or pselect()) is 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 descriptors.
    
           Its  principal arguments are three "sets" of file descriptors: readfds,
           writefds, and exceptfds.  Each set is declared as type fd_set, and  its
           contents  can  be  manipulated  with  the  macros FD_CLR(), FD_ISSET(),
           FD_SET(), and FD_ZERO().  A newly declared set should first be  cleared
           using  FD_ZERO().  select() modifies the contents of the sets according
           to the rules described below; after calling select() you can test if  a
           file  descriptor  is  still present in a set with the FD_ISSET() macro.
           FD_ISSET() returns nonzero if a specified file descriptor is present in
           a set and zero if it is not.  FD_CLR() removes a file descriptor from a
           set.
    
       Arguments
           readfds
                  This set is watched to see if data is available for reading from
                  any  of  its  file  descriptors.   After  select() has returned,
                  readfds will be cleared of all file descriptors except for those
                  that are immediately available for reading.
    
                  cleared  of  all  file descriptors except for those for which an
                  exceptional condition has occurred.
    
           nfds   This is an integer  one  more  than  the  maximum  of  any  file
                  descriptor  in  any  of  the sets.  In other words, while adding
                  file descriptors to each of the sets,  you  must  calculate  the
                  maximum  integer value of all of them, then increment this value
                  by one, and then pass this as nfds.
    
           utimeout
                  This is the longest time select()  may  wait  before  returning,
                  even  if  nothing interesting happened.  If this value is passed
                  as NULL, then select() blocks indefinitely waiting  for  a  file
                  descriptor  to  become  ready.  utimeout can be set to zero sec-
                  onds, which causes select() to return immediately, with informa-
                  tion  about the readiness of file descriptors at the time of the
                  call.  The structure struct timeval is defined as:
    
                      struct timeval {
                          time_t tv_sec;    /* seconds */
                          long tv_usec;     /* microseconds */
                      };
    
           ntimeout
                  This argument for pselect() has the same  meaning  as  utimeout,
                  but struct timespec has nanosecond precision as follows:
    
                      struct timespec {
                          long tv_sec;    /* seconds */
                          long tv_nsec;   /* nanoseconds */
                      };
    
           sigmask
                  This  argument  holds  a  set  of signals that the kernel should
                  unblock (i.e., remove  from  the  signal  mask  of  the  calling
                  thread),  while  the caller is blocked inside the pselect() call
                  (see sigaddset(3) and sigprocmask(2)).  It may be NULL, in which
                  case  the call does not modify the signal mask on entry and exit
                  to the function.  In this case, pselect() will then behave  just
                  like select().
    
       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, other-
           wise 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()
               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
           descriptors  whenever I want?  The point of select() is that it watches
           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 pro-
               gram 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  appropri-
               ately.  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 neces-
               sarily  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.  The  functions  read(2),  recv(2), write(2), and send(2) as well as
               the select() call can return -1 with errno set to  EINTR,  or  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
               descriptor  to  select()  again.  In the example below, I close the
               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.   pse-
               lect() however does not modify its timeout structure.
    
           This is guaranteed to work only on UNIX systems, however.
    
    
    

    RETURN VALUE

           On success, select() returns the total number of file descriptors still
           present in the file descriptor sets.
    
           If  select()  timed  out, then the return value will be zero.  The file
           descriptors set should be all empty (but may not be on some systems).
    
           A return value of -1 indicates an error, with errno being set appropri-
           ately.   In the case of an error, the contents of the returned sets and
           the struct timeout contents are undefined and should not be used.  pse-
           lect() however never modifies ntimeout.
    
    
    

    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.
    
    
    

    EXAMPLE

           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 <stdlib.h>
           #include <stdio.h>
           #include <unistd.h>
           #include <sys/time.h>
           #include <sys/types.h>
           #include <string.h>
           #include <signal.h>
           #include <sys/socket.h>
           #include <netinet/in.h>
           #include <arpa/inet.h>
           #include <errno.h>
    
           static int forward_port;
    
           #undef max
           #define max(x,y) ((x) > (y) ? (x) : (y))
    
                       &yes, sizeof(yes)) == -1) {
                   perror("setsockopt");
                   close(s);
                   return -1;
               }
               memset(&a, 0, sizeof(a));
               a.sin_port = htons(listen_port);
               a.sin_family = AF_INET;
               if (bind(s, (struct sockaddr *) &a, sizeof(a)) == -1) {
                   perror("bind");
                   close(s);
                   return -1;
               }
               printf("accepting connections on port %d\n", listen_port);
               listen(s, 10);
               return s;
           }
    
           static int
           connect_socket(int connect_port, char *address)
           {
               struct sockaddr_in a;
               int s;
    
               s = socket(AF_INET, SOCK_STREAM, 0);
               if (s == -1) {
                   perror("socket");
                   close(s);
                   return -1;
               }
    
               memset(&a, 0, sizeof(a));
               a.sin_port = htons(connect_port);
               a.sin_family = AF_INET;
    
               if (!inet_aton(address, (struct in_addr *) &a.sin_addr.s_addr)) {
                   perror("bad IP address format");
                   close(s);
                   return -1;
               }
    
               if (connect(s, (struct sockaddr *) &a, sizeof(a)) == -1) {
                   perror("connect()");
                   shutdown(s, SHUT_RDWR);
                   close(s);
                   return -1;
               }
               return s;
           }
    
           #define SHUT_FD1 do {                                \
                                if (fd1 >= 0) {                 \
    
           #define BUF_SIZE 1024
    
           int
           main(int argc, char *argv[])
           {
               int h;
               int fd1 = -1, fd2 = -1;
               char buf1[BUF_SIZE], buf2[BUF_SIZE];
               int buf1_avail, buf1_written;
               int buf2_avail, buf2_written;
    
               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 (;;) {
                   int r, nfds = 0;
                   fd_set rd, wr, er;
    
                   FD_ZERO(&rd);
                   FD_ZERO(&wr);
                   FD_ZERO(&er);
                   FD_SET(h, &rd);
                   nfds = max(nfds, h);
                   if (fd1 > 0 && buf1_avail < BUF_SIZE) {
                       FD_SET(fd1, &rd);
                       nfds = max(nfds, fd1);
                   }
                   if (fd2 > 0 && buf2_avail < BUF_SIZE) {
                       FD_SET(fd2, &rd);
                       nfds = max(nfds, fd2);
                   }
                   if (fd1 > 0 && buf2_avail - buf2_written > 0) {
                       FD_SET(fd1, &wr);
                       nfds = max(nfds, fd1);
                   }
                   if (fd2 > 0 && buf1_avail - buf1_written > 0) {
                       FD_SET(fd2, &wr);
                       nfds = max(nfds, fd2);
                   }
                   if (fd1 > 0) {
                       FD_SET(fd1, &er);
                       exit(EXIT_FAILURE);
                   }
    
                   if (FD_ISSET(h, &rd)) {
                       unsigned int l;
                       struct sockaddr_in client_address;
    
                       memset(&client_address, 0, l = sizeof(client_address));
                       r = accept(h, (struct sockaddr *) &client_address, &l);
                       if (r == -1) {
                           perror("accept()");
                       } else {
                           SHUT_FD1;
                           SHUT_FD2;
                           buf1_avail = buf1_written = 0;
                           buf2_avail = buf2_written = 0;
                           fd1 = r;
                           fd2 = connect_socket(forward_port, argv[3]);
                           if (fd2 == -1)
                               SHUT_FD1;
                           else
                               printf("connect from %s\n",
                                       inet_ntoa(client_address.sin_addr));
                       }
                   }
    
                   /* NB: read oob data before normal reads */
    
                   if (fd1 > 0)
                       if (FD_ISSET(fd1, &er)) {
                           char c;
    
                           r = recv(fd1, &c, 1, MSG_OOB);
                           if (r < 1)
                               SHUT_FD1;
                           else
                               send(fd2, &c, 1, MSG_OOB);
                       }
                   if (fd2 > 0)
                       if (FD_ISSET(fd2, &er)) {
                           char c;
    
                           r = recv(fd2, &c, 1, MSG_OOB);
                           if (r < 1)
                               SHUT_FD2;
                           else
                               send(fd1, &c, 1, MSG_OOB);
                       }
                   if (fd1 > 0)
                       if (FD_ISSET(fd1, &rd)) {
                           r = read(fd1, buf1 + buf1_avail,
                                     BUF_SIZE - buf1_avail);
    
                   if (fd1 > 0)
                       if (FD_ISSET(fd1, &wr)) {
                           r = write(fd1, buf2 + buf2_written,
                                      buf2_avail - buf2_written);
                           if (r < 1)
                               SHUT_FD1;
                           else
                               buf2_written += r;
                       }
                   if (fd2 > 0)
                       if (FD_ISSET(fd2, &wr)) {
                           r = write(fd2, buf1 + buf1_written,
                                      buf1_avail - buf1_written);
                           if (r < 1)
                               SHUT_FD2;
                           else
                               buf1_written += r;
                       }
    
                   /* 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  simultane-
           ously.   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  time-
           outs.
    
           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 connec-
           tions cause the current connection to be dropped.
    
    
    

    SEE ALSO

    
    
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