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


       /* According to POSIX 1003.1-2001 */
       #include <sys/select.h>

       /* According to earlier standards */
       #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 *timeout);

       int   pselect(int   n,   fd_set   *readfds,  fd_set  *writefds,  fd_set
       *exceptfds, const struct timespec *timeout, const 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);


       The functions  select()  and  pselect()  wait  for  a  number  of  file
       descriptors to change status.

       Their function is identical, with three differences:

       (i)    The  select()  function  uses a timeout that is a struct timeval
              (with seconds and microseconds), while pselect() uses  a  struct
              timespec (with seconds and nanoseconds).

       (ii)   The  select()  function  may  update  the  timeout  parameter to
              indicate how much time was left. The pselect() function does not
              change this parameter.

       (iii)  The  select()  function has no sigmask parameter, and behaves as
              pselect() called with NULL sigmask.

       Three independent sets of descriptors are  watched.   Those  listed  in
       readfds  will  be  watched  to  see  if characters become available for
       reading  (more  precisely,  to  see  if  a  read  will  not  block;  in
       particular,  a  file descriptor is also ready on end-of-file), those in
       writefds will be watched to see if a write will not block, and those in
       exceptfds  will  be  watched  for  exceptions.   On  exit, the sets are
       modified in  place  to  indicate  which  descriptors  actually  changed

       Four  macros are provided to manipulate the sets.  FD_ZERO() will clear
       a set.  FD_SET() and FD_CLR() add or remove a given descriptor  from  a
       set.   FD_ISSET() tests to see if a descriptor is part of the set; this
       is useful after select() returns.

       n is the highest-numbered descriptor in any of the three sets, plus  1.

       timeout is an upper bound on the amount of time elapsed before select()
       returns. It may be zero, causing select() to return immediately.  (This
       is  useful  for polling.) If timeout is NULL (no timeout), select() can
       block indefinitely.

       sigmask is a pointer to a signal mask (see sigprocmask(2));  if  it  is
       not  NULL, then pselect() first replaces the current signal mask by the
       one pointed to by sigmask, then does the ‘select’  function,  and  then
       restores the original signal mask again.

       The idea of pselect() is that if one wants to wait for an event, either
       a signal or something on a file descriptor, an atomic test is needed to
       prevent race conditions. (Suppose the signal handler sets a global flag
       and returns. Then a test of this global flag  followed  by  a  call  of
       select()  could  hang indefinitely if the signal arrived just after the
       test but just before the call. On the other hand, pselect() allows  one
       to first block signals, handle the signals that have come in, then call
       pselect() with the desired sigmask, avoiding the  race.)   Since  Linux
       today does not have a pselect() system call, the current glibc2 routine
       still contains this race.

   The timeout
       The time structures involved are defined in <sys/time.h> and look like

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


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

       (However, see below on the POSIX 1003.1-2001 versions.)

       Some code calls select() with all three sets empty, n zero, and a  non-
       null  timeout  as  a  fairly  portable  way  to  sleep  with  subsecond

       On Linux, the function select() modifies timeout to reflect the  amount
       of  time  not  slept;  most other implementations do not do this.  This
       causes problems both when Linux code which reads timeout is  ported  to
       other operating systems, and when code is ported to Linux that reuses a
       struct timeval for multiple select()s in a loop without  reinitializing
       it.  Consider timeout to be undefined after select() returns.


       On  success,  select()  and  pselect() return the number of descriptors
       contained in the three returned descriptor sets  (that  is,  the  total
       number  of  one bits in readfds, writefds, exceptfds) which may be zero
       if the timeout expires before anything interesting happens.  On  error,
       -1  is  returned,  and errno is set appropriately; the sets and timeout
       become undefined, so do not rely on their contents after an error.


       EBADF  An invalid file descriptor was given in one of the sets.

       EINTR  A non blocked signal was caught.

       EINVAL n is negative or the value contained within timeout is  invalid.

       ENOMEM select() was unable to allocate memory for internal tables.


       #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 == -1)
           else if (retval)
               printf("Data is available now.\n");
               /* FD_ISSET(0, &rfds) will be true. */
               printf("No data within five seconds.\n");

           return 0;


       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),
       and part of POSIX 1003.1-2001.  It is  found  in  glibc2.1  and  later.
       Glibc2.0  has  a  function with this name, that however does not take a
       sigmask parameter.


       An fd_set is a fixed size buffer. Executing FD_CLR  or  FD_SET  with  a
       value  of  fd that is negative or is equal to or larger than FD_SETSIZE
       will result in undefined behavior. Moreover, POSIX requires fd to be  a
       valid file descriptor.

       Concerning  the types involved, the classical situation is that the two
       fields of a struct timeval are longs (as shown above), and  the  struct
       is defined in <sys/time.h>.  The POSIX 1003.1-2001 situation is

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

       where the struct is defined in <sys/select.h> and the data types time_t
       and suseconds_t are defined in <sys/types.h>.

       Concerning prototypes, the  classical  situation  is  that  one  should
       include <time.h> for select().  The POSIX 1003.1-2001 situation is that
       one should include <sys/select.h> for select()  and  pselect().   Libc4
       and  libc5  do  not  have  a <sys/select.h> header; under glibc 2.0 and
       later this header exists.  Under glibc 2.0 it unconditionally gives the
       wrong prototype for pselect(), under glibc 2.1-2.2.1 it gives pselect()
       when _GNU_SOURCE is defined, under glibc 2.2.2-2.2.4 it gives  it  when
       _XOPEN_SOURCE  is  defined and has a value of 600 or larger.  No doubt,
       since POSIX 1003.1-2001, it should give the prototype by default.


       pselect() is currently emulated with a user-space wrapper  that  has  a
       race  condition.  For reliable (and more portable) signal trapping, use
       the self-pipe trick.  (Where a signal handler writes to  a  pipe  whose
       other end is read by the main loop.)

       Under Linux, select() may report a socket file descriptor as "ready for
       reading", while nevertheless a subsequent read blocks. This  could  for
       example  happen  when  data  has arrived but upon examination has wrong
       checksum and is discarded. There may be other circumstances.   Thus  it
       may be safer to use O_NONBLOCK on sockets that should not block.


       For a tutorial with discussion and examples, see select_tut(2).

       For vaguely related stuff, see accept(2), connect(2), poll(2), read(2),
       recv(2), send(2), sigprocmask(2), write(2)