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SELECT_TUT(2)		   Linux Programmers Manual		SELECT_TUT(2)

       select,	pselect,  FD_CLR,  FD_ISSET, FD_SET, FD_ZERO - synchronous I/O

       /* According to POSIX.1-2001 */

       /* According to earlier standards */

       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);


       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

       select() (or pselect()) is the pivot function of most C	programs  that
       handle more than one simultaneous file descriptor (or socket handle) in
       an efficient manner.  Its principal arguments are three arrays of  file
       descriptors:  readfds,  writefds, and exceptfds.  The way that select()
       is usually used is to block while waiting for a "change of  status"  on
       one or more of the file descriptors.  A "change of status" is when more
       characters become available from the file  descriptor,  or  when  space
       becomes	available  within the kernels internal buffers for more to be
       written to the file descriptor, or when a  file	descriptor  goes  into
       error  (in  the	case of a socket or pipe this is when the other end of
       the connection is closed).

       In summary, select() just watches multiple file descriptors, and is the
       standard Unix call to do so.

       The  arrays  of file descriptors are called file descriptor sets.  Each
       set is declared as type fd_set, and its contents can  be  altered  with
       the macros FD_CLR(), FD_ISSET(), FD_SET(), and FD_ZERO().  FD_ZERO() is
       usually the first function to be used on a newly declared set.	There
       after,  the  individual file descriptors that you are interested in can
       be added one by one with FD_SET().  select() modifies the  contents  of
       the sets according to the rules described below; after calling select()
       you can test if your file descriptor is still present in the  set  with
       the FD_ISSET() macro.  FD_ISSET() returns non-zero if the descriptor is
       present and zero if it is not.  FD_CLR() removes a file descriptor from
       the set.

	      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 with a  recv(2)  (for
	      sockets) or read(2) (for pipes, files, and sockets) call.

	      This  set  is  watched to see if there is space to write data to
	      any of its  file	descriptors.   After  select()	has  returned,
	      writefds	will  be  cleared  of  all file descriptors except for
	      those that are immediately available for writing with a  send(2)
	      (for  sockets) or write(2) (for pipes, files, and sockets) call.

	      This set is watched for exceptions or errors on any of the  file
	      descriptors.   However,  that is actually just a rumor.  How you
	      use exceptfds is to watch for out-of-band (OOB) data.  OOB  data
	      is  data	sent  on  a  socket  using the MSG_OOB flag, and hence
	      exceptfds only really  applies  to  sockets.   See  recv(2)  and
	      send(2) about this.  After select() has returned, exceptfds will
	      be cleared of all file descriptors except  for  those  that  are
	      available for reading OOB data.  You can only ever read one byte
	      of OOB data though (which is done with recv(2)), and writing OOB
	      data  (done  with  send(2)) can be done at any time and will not
	      block.  Hence there is no need for a fourth set to  check  if  a
	      socket is available for writing OOB data.

       nfds   This  is	an  integer  one  more	than  the  maximum of any file
	      descriptor in any of the sets.  In other words,  while  you  are
	      busy  adding  file  descriptors to your sets, you must calculate
	      the maximum integer value of all of them,  then  increment  this
	      value by one, and then pass this as nfds to select().

	      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 an event.
	      utimeout can be set to zero seconds, which  causes  select()  to
	      return immediately.  The structure struct timeval is defined as:

		  struct timeval {
		      time_t tv_sec;	/* seconds */
		      long tv_usec;	/* microseconds */

	      This argument has the same meaning as utimeout but struct  time
	      spec has nanosecond precision as follows:

		  struct timespec {
		      long tv_sec;    /* seconds */
		      long tv_nsec;   /* nanoseconds */

	      This argument holds a set of signals to allow while performing a
	      pselect() call (see sigaddset(3) and sigprocmask(2)).  It can be
	      passed  as  NULL,  in  which  case it does not modify the set of
	      allowed signals on entry and exit to the function.  It will then
	      behave just like select().

   Combining Signal and Data Events
       pselect()  must be used if you are waiting for a signal as well as data
       from a file descriptor.	Programs that receive signals as  events  nor
       mally  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 mask out signals that are
       not to be received except 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  sig
       procmask(2).  Our pselect() call would enable SIGCHLD by using the vir
       gin signal mask.  Our program would look like:

       int child_events = 0;

       child_sig_handler(int x)
	   signal(SIGCHLD, child_sig_handler);

       main(int argc, char **argv)
	   sigset_t sigmask, orig_sigmask;

	   sigaddset(&sigmask, SIGCHLD);
	   sigprocmask(SIG_BLOCK, &sigmask, &orig_sigmask);

	   signal(SIGCHLD, child_sig_handler);

	   for (;;) { /* main loop */
	       for (; child_events > 0; child_events--) {
		   /* do event work here */
	       r = pselect(nfds, &rd, &wr, &er, 0, &orig_sigmask);

	       /* main body of program */

       So what is the point of select()?  Cant I just read and	write  to  my
       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.  It does this while enabling you to han
       dle multiple simultaneous pipes and sockets.   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 avail
       able for data.  select() efficiently copes with this situation.

       A simple example of the use of select() can be found in	the  select(2)
       manual page.

   Select Law
       Many people who try to use select() come across behavior that is diffi
       cult to understand and produces	non-portable  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  non-
       blocking  mode  at  all (see ioctl(2)).	It is easy to introduce subtle
       errors that will remove the advantage of using select(), hence  I  will
       present a list of essentials to watch for when using the select() call.

       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, its 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 only manag
	   ing to send or receive 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 above 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  non-blocking	I/O,  you will not get
	   EAGAIN.  Nonetheless you should still cope with  these  errors  for

       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 above example, 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.

       11. I  have  heard that the Windows socket layer does not cope with OOB
	   data properly.  It also does not cope with select() calls  when  no
	   file descriptors are set at all.  Having no file descriptors set is
	   a useful way to sleep the  process  with  sub-second  precision  by
	   using the timeout.  (See further on.)

   Usleep Emulation
       On systems that do not have a usleep(3) function, you can call select()
       with a finite timeout and no file descriptors as follows:

	   struct timeval tv;
	   tv.tv_sec = 0;
	   tv.tv_usec = 200000;  /* 0.2 seconds */
	   select(0, NULL, NULL, NULL, &tv);

       This is only guaranteed to work on Unix systems, however.

       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.

       Generally  speaking,  all  operating systems that support sockets, also
       support select().  Many types of programs become extremely  complicated
       without	the use of select().  select() can be used to solve many prob
       lems in a portable and efficient way  that  naive  programmers  try  to
       solve  in  a more complicated manner using threads, forking, IPCs, sig
       nals, 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.

       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.


       static int forward_port;

       #undef max
       #define max(x,y) ((x) > (y) ? (x) : (y))

       static int
       listen_socket(int listen_port)
	   struct sockaddr_in a;
	   int s;
	   int yes;

	   if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
	       return -1;
	   yes = 1;
	   if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR,
		   (char *) &yes, sizeof(yes)) < 0) {
	       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)) < 0) {
	       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;

	   if ((s = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
	       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");
	       return -1;

	   if (connect(s, (struct sockaddr *) &a, sizeof(a)) < 0) {
	       shutdown(s, SHUT_RDWR);
	       return -1;
	   return s;

       #define SHUT_FD1 {		       \
	       if (fd1 >= 0) {		       \
		   shutdown(fd1, SHUT_RDWR);   \
		   close(fd1);		       \
		   fd1 = -1;		       \
	       }			       \

       #define SHUT_FD2 {		       \
	       if (fd2 >= 0) {		       \
		   shutdown(fd2, SHUT_RDWR);   \
		   close(fd2);		       \
		   fd2 = -1;		       \
	       }			       \

       #define BUF_SIZE 1024

       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) {
			"Usage\n\tfwd  "
			" \n");

	   signal(SIGPIPE, SIG_IGN);

	   forward_port = atoi(argv[2]);

	   h = listen_socket(atoi(argv[1]));
	   if (h < 0)

	   for (;;) {
	       int r, nfds = 0;
	       fd_set rd, wr, 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);
		   nfds = max(nfds, fd1);
	       if (fd2 > 0) {
		   FD_SET(fd2, &er);
		   nfds = max(nfds, fd2);

	       r = select(nfds + 1, &rd, &wr, &er, NULL);

	       if (r == -1 && errno == EINTR)
	       if (r < 0) {
	       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 < 0) {
		   } else {
		       buf1_avail = buf1_written = 0;
		       buf2_avail = buf2_written = 0;
		       fd1 = r;
		       fd2 =
			   connect_socket(forward_port, argv[3]);
		       if (fd2 < 0) {
		       } else
			   printf("connect from %s\n",
       /* NB: read oob data before normal reads */
	       if (fd1 > 0)
		   if (FD_ISSET(fd1, &er)) {
		       char c;
		       errno = 0;
		       r = recv(fd1, &c, 1, MSG_OOB);
		       if (r < 1) {
		       } else
			   send(fd2, &c, 1, MSG_OOB);
	       if (fd2 > 0)
		   if (FD_ISSET(fd2, &er)) {
		       char c;
		       errno = 0;
		       r = recv(fd2, &c, 1, MSG_OOB);
		       if (r < 1) {
		       } 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 (r < 1) {
		       } else
			   buf1_avail += r;
	       if (fd2 > 0)
		   if (FD_ISSET(fd2, &rd)) {
		       r =
			   read(fd2, buf2 + buf2_avail,
				 BUF_SIZE - buf2_avail);
		       if (r < 1) {
		       } else
			   buf2_avail += r;
	       if (fd1 > 0)
		   if (FD_ISSET(fd1, &wr)) {
		       r =
			   write(fd1, buf2 + buf2_written,
				  buf2_avail - buf2_written);
		       if (r < 1) {
		       } 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) {
		       } 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) {
	       if (fd2 < 0 && buf2_avail - buf2_written == 0) {

       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  non-blocking	I/O  using  an
       ioctl(2)  call.	 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  connec
       tions cause the current connection to be dropped.

       accept(2),  connect(2), ioctl(2), poll(2), read(2), recv(2), select(2),
       send(2), sigprocmask(2), write(2), sigaddset(3), sigdelset(3),  sigemp
       tyset(3), sigfillset(3), sigismember(3), epoll(7)

       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2007-12-18			 SELECT_TUT(2)

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