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


       /* 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 *timeout);

       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 *timeout,
                   const sigset_t *sigmask);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       pselect(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600


       select() and  pselect()  allow  a  program  to  monitor  multiple  file
       descriptors,  waiting  until one or more of the file descriptors become
       "ready" for some class of I/O operation (e.g., input possible).  A file
       descriptor  is  considered  ready  if  it  is  possible  to  perform  a
       corresponding I/O operation  (e.g.,  read(2)  without  blocking,  or  a
       sufficiently small write(2)).

       The  operation of select() and pselect() is identical, other than these
       three differences:

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

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

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

       Three independent sets of file 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 space is available for write (though
       a  large write may still block), and those in exceptfds will be watched
       for exceptions.  On exit, the sets are modified in  place  to  indicate
       which file descriptors actually changed status.  Each of the three file
       descriptor sets may be specified as NULL if no file descriptors are  to
       be watched for the corresponding class of events.

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

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

       The  timeout argument specifies the interval that select() should block
       waiting for a file descriptor to become ready.   The  call  will  block
       until either:

       *  a file descriptor becomes ready;

       *  the call is interrupted by a signal handler; or

       *  the timeout expires.

       Note  that  the timeout interval will be rounded up to the system clock
       granularity, and  kernel  scheduling  delays  mean  that  the  blocking
       interval  may overrun by a small amount.  If both fields of the timeval
       structure are zero, then select() returns immediately.  (This is useful
       for  polling.)   If  timeout  is  NULL (no timeout), select() can block

       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.

       Other than the difference in the precision of the timeout argument, the
       following pselect() call:

           ready = pselect(nfds, &readfds, &writefds, &exceptfds,
                           timeout, &sigmask);

       is equivalent to atomically executing the following calls:

           sigset_t origmask;

           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
           ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
           pthread_sigmask(SIG_SETMASK, &origmask, NULL);

       The reason that pselect() is needed is that if one wants  to  wait  for
       either  a  signal  or  for  a  file descriptor to become ready, then 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.  By
       contrast, pselect() allows one  to  first  block  signals,  handle  the
       signals  that  have  come  in,  then  call  pselect()  with the desired
       sigmask, avoiding the 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.1 versions.)

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

       On Linux, select() modifies timeout to reflect the amount of  time  not
       slept;  most  other  implementations  do not do this.  (POSIX.1 permits
       either behavior.)  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   file
       descriptors  contained  in the three returned descriptor sets (that is,
       the total number of bits that are set 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  to  indicate  the
       error;  the  file  descriptor  sets are unmodified, and timeout becomes


       EBADF  An invalid file  descriptor  was  given  in  one  of  the  sets.
              (Perhaps  a  file  descriptor that was already closed, or one on
              which an error has occurred.)

       EINTR  A signal was caught; see signal(7).

       EINVAL nfds is negative or exceeds  the  RLIMIT_NOFILE  resource  limit
              (see getrlimit(2)).

       EINVAL the value contained within timeout is invalid.

       ENOMEM unable to allocate memory for internal tables.


       pselect()  was  added  to  Linux  in  kernel  2.6.16.   Prior  to this,
       pselect() was emulated in glibc (but see BUGS).


       select() conforms to POSIX.1-2001, POSIX.1-2008, and  4.4BSD  (select()
       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.

       pselect() is defined in POSIX.1g, and in POSIX.1-2001 and POSIX.1-2008.


       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 timeval structure are typed as long (as shown  above),  and
       the structure is defined in <sys/time.h>.  The POSIX.1 situation is

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

       where  the  structure  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.1  situation is that one
       should include <sys/select.h> for select() and pselect().

       Under  glibc  2.0,  <sys/select.h>  gives  the  wrong   prototype   for
       pselect().    Under  glibc  2.1  to  2.2.1,  it  gives  pselect()  when
       _GNU_SOURCE is defined.  Since glibc 2.2.2,  the  requirements  are  as
       shown in the SYNOPSIS.

   Multithreaded applications
       If  a  file descriptor being monitored by select() is closed in another
       thread, the result is unspecified.   On  some  UNIX  systems,  select()
       unblocks  and  returns,  with an indication that the file descriptor is
       ready (a subsequent I/O operation  will  likely  fail  with  an  error,
       unless  another  the file descriptor reopened between the time select()
       returned and the I/O operations was performed).   On  Linux  (and  some
       other  systems),  closing  the file descriptor in another thread has no
       effect on select().  In summary,  any  application  that  relies  on  a
       particular behavior in this scenario must be considered buggy.

   C library/kernel differences
       The pselect() interface described in this page is implemented by glibc.
       The underlying Linux system call is named pselect6().  This system call
       has somewhat different behavior from the glibc wrapper function.

       The  Linux  pselect6()  system  call  modifies  its  timeout  argument.
       However, the glibc wrapper function hides  this  behavior  by  using  a
       local  variable  for  the timeout argument that is passed to the system
       call.  Thus, the glibc pselect() function does not modify  its  timeout
       argument; this is the behavior required by POSIX.1-2001.

       The  final  argument  of the pselect6() system call is not a sigset_t *
       pointer, but is instead a structure of the form:

           struct {
               const sigset_t *ss;     /* Pointer to signal set */
               size_t          ss_len; /* Size (in bytes) of object pointed
                                          to by 'ss' */

       This allows the system call to obtain both a pointer to the signal  set
       and  its  size,  while  allowing  for  the fact that most architectures
       support a maximum of 6 arguments to a system call.


       Glibc 2.0 provided a version of pselect() that did not take  a  sigmask

       Starting  with  version  2.1,  glibc provided an emulation of pselect()
       that  was  implemented  using  sigprocmask(2)   and   select().    This
       implementation  remained  vulnerable  to  the  very race condition that
       pselect() was designed to prevent.  Modern versions of  glibc  use  the
       (race-free) pselect() system call on kernels where it is provided.

       On  systems  that  lack  pselect(), reliable (and more portable) signal
       trapping can be achieved using the self-pipe trick.  In this technique,
       a  signal  handler writes a byte to a pipe whose other end is monitored
       by select() in the main program.   (To  avoid  possibly  blocking  when
       writing  to  a pipe that may be full or reading from a pipe that may be
       empty, nonblocking I/O is used when reading from  and  writing  to  the

       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 in which a
       file  descriptor is spuriously reported as ready.  Thus it may be safer
       to use O_NONBLOCK on sockets that should not block.

       On Linux, select() also modifies timeout if the call is interrupted  by
       a signal handler (i.e., the EINTR error return).  This is not permitted
       by POSIX.1.  The Linux pselect() system call has the same behavior, but
       the glibc wrapper hides this behavior by internally copying the timeout
       to a local variable and passing that variable to the system call.


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

           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");



       accept(2), connect(2), poll(2), read(2),  recv(2),  restart_syscall(2),
       send(2), sigprocmask(2), write(2), epoll(7), time(7)

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


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