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

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

       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.h>

       int  select(int	n,  fd_set  *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval

       int pselect(int n, 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);

       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 avail-
       able from the file descriptor; or when space becomes available within the kernel's  inter-
       nal  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.
       Thereafter, 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 although I
       can't see the use for it in a clean program.

	      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.

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

	      This  set  is watched for exceptions or errors on any of the file descriptors. How-
	      ever, that is actually just a rumor. How you use exceptfds is to watch for  Out  of
	      Bounds  (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.

       n      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 n to select.

	      This is the longest time select must wait before returning, even if nothing  inter-
	      esting  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 timespec 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.

       pselect	must be used if you are waiting for a signal as well as data from a file descrip-
       tor. Programs that receive signals as events 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 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) {
	   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,

	   signal (SIGCHLD, child_sig_handler);

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

	       /* main body of program */

       Note that the above pselect call can be replaced with:

	       sigprocmask (SIG_BLOCK, &orig_sigmask, 0);
	       r = select (n, &rd, &wr, &er, 0);
	       sigprocmask (SIG_BLOCK, &sigmask, 0);

       but then there is still the possibility that a signal could arrive after  the  first  sig-
       procmask  and before the select. If you do do this, it is prudent to at least put a finite
       timeout so that the process does not block. At present glibc probably works this way.  The
       Linux  kernel  does  not  have a native pselect system call as yet so this is all probably
       much of a mute point.

       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	handle multiple simultaneous pipes and sockets. Unix programmers often find them-
       selves in a position where they have to handle IO 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 available for data. select
       efficiently copes with this situation.

       A classic example of select comes from the select man page:

       #include <stdio.h>
       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.h>

       main(void) {
	   fd_set rfds;
	   struct timeval tv;
	   int retval;

	   /* Watch stdin (fd 0) to see when it has input. */
	   FD_SET(0, &rfds);
	   /* Wait up to five seconds. */
	   tv.tv_sec = 5;
	   tv.tv_usec = 0;

	   retval = select(1, &rfds, NULL, NULL, &tv);
	   /* Don't rely on the value of tv now! */

	   if (retval)
	       printf("Data is available now.\n");
	       /* FD_ISSET(0, &rfds) will be true. */
	       printf("No data within five seconds.\n");


       Here is an example that better demonstrates the true utility of select. The listing  below
       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
		(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
		(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, n = 0;
	       fd_set rd, wr, er;
	       FD_ZERO (&rd);
	       FD_ZERO (&wr);
	       FD_ZERO (&er);
	       FD_SET (h, &rd);
	       n = max (n, h);
	       if (fd1 > 0 && buf1_avail < BUF_SIZE) {
		   FD_SET (fd1, &rd);
		   n = max (n, fd1);
	       if (fd2 > 0 && buf2_avail < BUF_SIZE) {
		   FD_SET (fd2, &rd);
		   n = max (n, fd2);
	       if (fd1 > 0
		   && buf2_avail - buf2_written > 0) {
		   FD_SET (fd1, &wr);
		   n = max (n, fd1);
	       if (fd2 > 0
		   && buf1_avail - buf1_written > 0) {
		   FD_SET (fd2, &wr);
		   n = max (n, fd2);
	       if (fd1 > 0) {
		   FD_SET (fd1, &er);
		   n = max (n, fd1);
	       if (fd2 > 0) {
		   FD_SET (fd2, &er);
		   n = max (n, fd2);

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

	       if (r == -1 && errno == EINTR)
	       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 {
		       buf1_avail = buf1_written = 0;
		       buf2_avail = buf2_written = 0;
		       fd1 = r;
		       fd2 =
			   connect_socket (forward_port,
		       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 -
		       if (r < 1) {
		       } else
			   buf2_written += r;
	       if (fd2 > 0)
		   if (FD_ISSET (fd2, &wr)) {
		       r =
			   write (fd2,
				  buf1 + buf1_written,
				  buf1_avail -
		       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) {
	   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 IO 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 connec-
       tion. At the moment, new connections cause the current connection to be dropped.

       Many people who try to use select come across behavior that is difficult 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 use select without a timeout. Your program should have noth-
	      ing to do if there is no data available. Code that depends on timeouts is not  usu-
	      ally portable and difficult to debug.

       2.     The value n 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 must be  checked.	Any  file
	      descriptor  that is available for writing must be written to, and any file descrip-
	      tor available for reading must be read, etc.

       5.     The functions read(), recv(), write(), and send() do not necessarily 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 as large as the maximum
	      possible packet size on your local network.

       7.     The functions read(), recv(), write(), and send() as well as the select() call  can
	      return  -1  with	an  errno  of EINTR or EAGAIN (EWOULDBLOCK) which are not errors.
	      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 IO, 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.     Except  as indicated in 7., the functions read(), recv(), write(), and send() never
	      have a return value less than 1 except if an error has occurred.	For  instance,	a
	      read()  on a pipe where the other end has died returns zero (so does an end-of-file
	      error), but only returns zero once (a followup  read  or	write  will  return  -1).
	      Should  any  of these functions return 0 or -1, you should not pass that descriptor
	      to select ever 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. pselect however does not modify its timeout

       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.  Hav-
	      ing  no  file  descriptors set is a useful way to sleep the process with sub-second
	      precision by using the timeout.  (See further on.)

       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 guarenteed 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 file descriptors sets should be all empty (but may not be on
       some systems). However the return value will definitely be zero.

       A  return  value of -1 indicates an error, with errno being set appropriately. In the case
       of an error, the returned sets and the timeout struct contents are  undefined  and  should
       not be used.  pselect however never modifies ntimeout.

       EBADF  A  set contained an invalid file descriptor. This error often occurs when you add a
	      file descriptor to a set that you have already issued a close on, or when that file
	      descriptor  has  experienced  some  kind of error. Hence you should cease adding to
	      sets any file descriptor that returns an error on reading or writing.

       EINTR  An interrupting signal was caught like SIGINT or SIGCHLD etc.   In  this	case  you
	      should rebuild your file descriptor sets and retry.

       EINVAL Occurs if n is negative.

       ENOMEM Internal memory allocation failure.

       Generally  speaking, all operating systems that support sockets, also support select. Some
       people consider select to be an esoteric and rarely used function. Indeed, many	types  of
       programs  become  extremely complicated without it. select can be used to solve many prob-
       lems in a portable and efficient way that naive programmers try	to  solve  with  threads,
       forking,  IPCs,	signals, memory sharing and other dirty methods. pselect is a newer func-
       tion that is less commonly used.

       The poll(2) system call has the same functionality as select, but with less subtle  behav-
       ior. It is less portable than select.

       4.4BSD (the select function first appeared in 4.2BSD).  Generally portable to/from non-BSD
       systems supporting clones of the BSD socket layer (including System V variants).  However,
       note  that  the	System V variant typically sets the timeout variable before exit, but the
       BSD variant does not.

       The pselect function is defined in IEEE Std  1003.1g-2000  (POSIX.1g).	It  is	found  in
       glibc2.1  and  later. Glibc2.0 has a function with this name, that however does not take a
       sigmask parameter.

       accept(2),  connect(2),	ioctl(2),  poll(2),   read(2),	 recv(2),   select(2),	 send(2),
       sigaddset(3), sigdelset(3), sigemptyset(3), sigfillset(3), sigismember(3), sigprocmask(2),

       This man page was written by Paul Sheer.

Linux 2.4				 October 21, 2001			    SELECT_TUT(2)
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