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


       /* 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 *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 the
       corresponding I/O operation (e.g., read(2)) without blocking.

       The operation of select() and pselect() is identical, with 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

       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 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 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

       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.

       timeout is an upper bound on the amount of time elapsed before select() returns.  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 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.

       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;

           sigprocmask(SIG_SETMASK, &sigmask, &origmask);
           ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
           sigprocmask(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.1-2001 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 precision.

       On Linux, select() modifies timeout to reflect the amount of time not  slept;  most  other
       implementations  do  not  do  this.   (POSIX.1-2001 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 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.   (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 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  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.


       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.1-2001 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.1-2001  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 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.

   Linux Notes
       The Linux pselect() 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.


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

       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 (where a signal handler writes a byte to a  pipe  whose
       other end is monitored by select() in the main program.)

       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.1-2001.   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


       #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");



       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), epoll(7), time(7)


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