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SELECT_TUT(2)		   Linux Programmer's Manual		 SELECT_TUT(2)

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

SYNOPSIS
       #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);

       int   pselect(int  nfds,	 fd_set	 *readfds,  fd_set  *writefds,	fd_set
       *exceptfds, const struct timespec *ntimeout, sigset_t *sigmask);

       FD_CLR(int fd, fd_set *set);
       FD_ISSET(int fd, fd_set *set);
       FD_SET(int fd, fd_set *set);
       FD_ZERO(fd_set *set);

DESCRIPTION
       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 kernel's 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.

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
	      file descriptors that are immediately available for reading with
	      a recv() (for sockets) or read() (for pipes, files, and sockets)
	      call.

       writefds
	      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  file descriptors that are immediately available for writ‐
	      ing with a send() (for sockets) or write()  (for	pipes,	files,
	      and sockets) call.

       exceptfds
	      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 descriptors
	      that are available for reading OOB data. You can only ever  read
	      one  byte	 of  OOB  data though (which is done with recv()), and
	      writing OOB data (done with send()) 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().

       utimeout
	      This is the longest time select() must  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 */
	      };

       ntimeout
	      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 */
	      };

       sigmask
	      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 sig‐
       nals  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 sigprocmask(). Our
       pselect()  call	would  enable SIGCHLD by using the virgin signal mask.
       Our program would look like:

       int child_events = 0;

       void child_sig_handler (int x) {
	   child_events++;
	   signal (SIGCHLD, child_sig_handler);
       }

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

	   sigemptyset (&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 */
	   }
       }

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
       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() and write() 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.

PORT FORWARDING 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))

       static int listen_socket (int listen_port) {
	   struct sockaddr_in a;
	   int s;
	   int yes;
	   if ((s = socket (AF_INET, SOCK_STREAM, 0)) < 0) {
	       perror ("socket");
	       return -1;
	   }
	   yes = 1;
	   if (setsockopt
	       (s, SOL_SOCKET, SO_REUSEADDR,
		(char *) &yes, sizeof (yes)) < 0) {
	       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)) < 0) {
	       perror ("bind");
	       close (s);
	       return -1;
	   }
	   printf ("accepting connections on port %d\n",
		   (int) 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) {
	       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)) < 0) {
	       perror ("connect()");
	       shutdown (s, SHUT_RDWR);
	       close (s);
	       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

       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 (1);
	   }

	   signal (SIGPIPE, SIG_IGN);

	   forward_port = atoi (argv[2]);

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

	   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);
		   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)
		   continue;
	       if (r < 0) {
		   perror ("select()");
		   exit (1);
	       }
	       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) {
		       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 < 0) {
			   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;
		       errno = 0;
		       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;
		       errno = 0;
		       r = recv (fd2, &c, 1, MSG_OOB);
		       if (r < 1) {
			   SHUT_FD1;
		       } 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) {
			   SHUT_FD1;
		       } 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) {
			   SHUT_FD2;
		       } 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) {
			   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;
	       }
	   }
	   return 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 simultaneously.
       You might think it more efficient to use a fork()  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()  call.  This
       also  has  its  problems	 because you end up having to have 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.

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
	      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	appro‐
	      priately. 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(), recv(), write(), and send() 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 managing to send or receive a single byte.

       6.     Never  read/write only in single bytes at a time unless your 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(), recv(), write(), and send() 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 prop‐
	      erly 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 completeness.

       8.     Never call read(), recv(), write(),  or  send()  with  a	buffer
	      length of zero.

       9.     If  the  functions read(), recv(), write(), and send() fail with
	      errors other than those listed in 7., or one of the input	 func‐
	      tions  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 pre‐
	      vent 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.    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 descrip‐
	      tors  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() 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.

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  returned  sets  and  the  timeout
       struct  contents	 are undefined and should not be used.	pselect() how‐
       ever never modifies ntimeout.

NOTES
       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 that is more
       efficient  than	select(2) and poll(2) when monitoring large numbers of
       file descriptors.

SEE ALSO
       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)

Linux				  2006-05-13			 SELECT_TUT(2)
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