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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

       /* 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

       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

	      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.

	      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.

	      This set is watched for "exceptional conditions".	 In  practice,
	      only  one such exceptional condition is common: the availability
	      of out-of-band (OOB) data for reading from a  TCP	 socket.   See
	      recv(2),	send(2),  and  tcp(7) for more details about OOB data.
	      (One other less common case where select(2) indicates an	excep‐
	      tional condition occurs with pseudoterminals in packet mode; see
	      tty_ioctl(4).)  After select() has returned, exceptfds  will  be
	      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.

	      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 */

	      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 */

	      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()
       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 only to be received 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;

       main(int argc, char *argv[])
	   sigset_t sigmask, empty_mask;
	   struct sigaction sa;
	   fd_set readfds, writefds, exceptfds;
	   int r;

	   sigaddset(&sigmask, SIGCHLD);
	   if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == -1) {

	   sa.sa_flags = 0;
	   sa.sa_handler = child_sig_handler;
	   if (sigaction(SIGCHLD, &sa, NULL) == -1) {


	   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 */

       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.	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 diffi‐
       cult 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 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.

       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.

   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 guaranteed to work only 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().   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.

       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)) == -1) {
	       return -1;
	   yes = 1;
	   if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR,
		   &yes, sizeof(yes)) == -1) {
	       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) {
	       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)) == -1) {
	       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)) == -1) {
	       shutdown(s, SHUT_RDWR);
	       return -1;
	   return s;

       #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

       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");

	   signal(SIGPIPE, SIG_IGN);

	   forward_port = atoi(argv[2]);

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

	   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 == -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 == -1) {
		   } else {
		       buf1_avail = buf1_written = 0;
		       buf2_avail = buf2_written = 0;
		       fd1 = r;
		       fd2 = connect_socket(forward_port, argv[3]);
		       if (fd2 == -1)
			   printf("connect from %s\n",

	       /* 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)
			   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)
			   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)
			   buf1_avail += r;
	       if (fd2 > 0)
		   if (FD_ISSET(fd2, &rd)) {
		       r = read(fd2, buf2 + buf2_avail,
				 BUF_SIZE - buf2_avail);
		       if (r < 1)
			   buf2_avail += r;
	       if (fd1 > 0)
		   if (FD_ISSET(fd1, &wr)) {
		       r = write(fd1, buf2 + buf2_written,
				  buf2_avail - buf2_written);
		       if (r < 1)
			   buf2_written += r;
	       if (fd2 > 0)
		   if (FD_ISSET(fd2, &wr)) {
		       r = write(fd2, buf1 + buf1_written,
				  buf1_avail - buf1_written);
		       if (r < 1)
			   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 nonblocking I/O using  fcntl(2).
       This  also  has its problems because you end up using inefficient time‐

       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 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2012-08-03			 SELECT_TUT(2)

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