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man:select_tut

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

     /* According to POSIX.1-2001, POSIX.1-2008 */
     #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

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 file 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.
     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 that are immediately available for writing.
     exceptfds
            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
            ioctl_tty(2).)  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.
     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()
     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 to be received only 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; }
     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
     file  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.  Calls to read(2), recv(2), write(2), send(2), and select() can fail
         with the error EINTR, and calls to read(2), recv(2)  write(2),  and
         send(2)  can  fail  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.
     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
         file  descriptor  to select() again.  In the example below, I close
         the file 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 sec-
         onds */ select(0, NULL, NULL, NULL, &tv);
     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  <sig-
     nal.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 addr;
         int lfd;
         int yes;
         lfd = socket(AF_INET, SOCK_STREAM, 0);
         if (lfd == -1) {
             perror("socket");
             return -1;
         }
         yes = 1;
         if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR,
                 &yes, sizeof(yes)) == -1) {
             perror("setsockopt");
             close(lfd);
             return -1;
         }
         memset(&addr, 0, sizeof(addr));
         addr.sin_port = htons(listen_port);
         addr.sin_family = AF_INET;
         if (bind(lfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
             perror("bind");
             close(lfd);
             return -1;
         }
         printf("accepting connections on port %d\n", listen_port);
         listen(lfd, 10);
         return lfd; }
     static int connect_socket(int connect_port, char *address) {
         struct sockaddr_in addr;
         int cfd;
         cfd = socket(AF_INET, SOCK_STREAM, 0);
         if (cfd == -1) {
             perror("socket");
             return -1;
         }
         memset(&addr, 0, sizeof(addr));
         addr.sin_port = htons(connect_port);
         addr.sin_family = AF_INET;
         if (!inet_aton(address, (struct in_addr *)  &addr.sin_addr.s_addr))
     {
             perror("bad IP address format");
             close(cfd);
             return -1;
         }
         if (connect(cfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
             perror("connect()");
             shutdown(cfd, SHUT_RDWR);
             close(cfd);
             return -1;
         }
         return cfd; }
     #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
     int main(int argc, char *argv[]) {
         int h;
         int fd1 = -1, fd2 = -1;
         char buf1[BUF_SIZE], buf2[BUF_SIZE];
         int buf1_avail = 0, buf1_written = 0;
         int buf2_avail = 0, buf2_written = 0;
         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 ready, nfds = 0;
             ssize_t nbytes;
             fd_set readfds, writefds, exceptfds;
             FD_ZERO(&readfds);
             FD_ZERO(&writefds);
             FD_ZERO(&exceptfds);
             FD_SET(h, &readfds);
             nfds = max(nfds, h);
             if (fd1 > 0 && buf1_avail < BUF_SIZE)
                 FD_SET(fd1, &readfds);
                 /* Note: nfds is updated below, when fd1 is added to
                    exceptfds. */
             if (fd2 > 0 && buf2_avail < BUF_SIZE)
                 FD_SET(fd2, &readfds);
             if (fd1 > 0 && buf2_avail - buf2_written > 0)
                 FD_SET(fd1, &writefds);
             if (fd2 > 0 && buf1_avail - buf1_written > 0)
                 FD_SET(fd2, &writefds);
             if (fd1 > 0) {
                 FD_SET(fd1, &exceptfds);
                 nfds = max(nfds, fd1);
             }
             if (fd2 > 0) {
                 FD_SET(fd2, &exceptfds);
                 nfds = max(nfds, fd2);
             }
             ready  =  select(nfds  +  1,  &readfds,  &writefds, &exceptfds,
     NULL);
             if (ready == -1 && errno == EINTR)
                 continue;
             if (ready == -1) {
                 perror("select()");
                 exit(EXIT_FAILURE);
             }
             if (FD_ISSET(h, &readfds)) {
                 socklen_t addrlen;
                 struct sockaddr_in client_addr;
                 int fd;
                 addrlen = sizeof(client_addr);
                 memset(&client_addr, 0, addrlen);
                 fd = accept(h, (struct sockaddr *) &client_addr, &addrlen);
                 if (fd == -1) {
                     perror("accept()");
                 } else {
                     SHUT_FD1;
                     SHUT_FD2;
                     buf1_avail = buf1_written = 0;
                     buf2_avail = buf2_written = 0;
                     fd1 = fd;
                     fd2 = connect_socket(forward_port, argv[3]);
                     if (fd2 == -1)
                         SHUT_FD1;
                     else
                         printf("connect from %s\n",
                                 inet_ntoa(client_addr.sin_addr));
                     /* Skip any events on the old, closed file descriptors.
     */
                     continue;
                 }
             }
             /* NB: read OOB data before normal reads */
             if (fd1 > 0 && FD_ISSET(fd1, &exceptfds)) {
                 char c;
                 nbytes = recv(fd1, &c, 1, MSG_OOB);
                 if (nbytes < 1)
                     SHUT_FD1;
                 else
                     send(fd2, &c, 1, MSG_OOB);
             }
             if (fd2 > 0 && FD_ISSET(fd2, &exceptfds)) {
                 char c;
                 nbytes = recv(fd2, &c, 1, MSG_OOB);
                 if (nbytes < 1)
                     SHUT_FD2;
                 else
                     send(fd1, &c, 1, MSG_OOB);
             }
             if (fd1 > 0 && FD_ISSET(fd1, &readfds)) {
                 nbytes = read(fd1, buf1 + buf1_avail,
                           BUF_SIZE - buf1_avail);
                 if (nbytes < 1)
                     SHUT_FD1;
                 else
                     buf1_avail += nbytes;
             }
             if (fd2 > 0 && FD_ISSET(fd2, &readfds)) {
                 nbytes = read(fd2, buf2 + buf2_avail,
                           BUF_SIZE - buf2_avail);
                 if (nbytes < 1)
                     SHUT_FD2;
                 else
                     buf2_avail += nbytes;
             }
             if (fd1 > 0 && FD_ISSET(fd1, &writefds) && buf2_avail > 0) {
                 nbytes = write(fd1, buf2 + buf2_written,
                            buf2_avail - buf2_written);
                 if (nbytes < 1)
                     SHUT_FD1;
                 else
                     buf2_written += nbytes;
             }
             if (fd2 > 0 && FD_ISSET(fd2, &writefds) && buf1_avail > 0) {
                 nbytes = write(fd2, buf1 + buf1_written,
                            buf1_avail - buf1_written);
                 if (nbytes < 1)
                     SHUT_FD2;
                 else
                     buf1_written += nbytes;
             }
             /* 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

     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)

COLOPHON

     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at
     https://www.kernel.org/doc/man-pages/.

Linux 2017-09-15 SELECT_TUT(2)

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