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


       Standard C library (libc, -lc)


       #include <sys/select.h>

       typedef /* ... */ fd_set;

       int select(int nfds, fd_set *_Nullable restrict readfds,
                  fd_set *_Nullable restrict writefds,
                  fd_set *_Nullable restrict exceptfds,
                  struct timeval *_Nullable restrict 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);

       int pselect(int nfds, fd_set *_Nullable restrict readfds,
                  fd_set *_Nullable restrict writefds,
                  fd_set *_Nullable restrict exceptfds,
                  const struct timespec *_Nullable restrict timeout,
                  const sigset_t *_Nullable restrict sigmask);

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

           _POSIX_C_SOURCE >= 200112L


       WARNING:  select() can monitor only file descriptors numbers that are less than FD_SETSIZE
       (1024)—an unreasonably low limit for many modern applications—and this limitation will not
       change.   All  modern  applications  should  instead use poll(2) or epoll(7), which do not
       suffer this limitation.

       select() allows 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),  or a sufficiently small write(2)) without

       A structure type that can represent a set of file descriptors.  According  to  POSIX,  the
       maximum  number  of  file  descriptors  in  an  fd_set structure is the value of the macro

   File descriptor sets
       The principal arguments of select() are three "sets" of file  descriptors  (declared  with
       the  type  fd_set),  which  allow  the  caller  to wait for three classes of events on the
       specified set of file descriptors.  Each of the fd_set arguments may be specified as  NULL
       if no file descriptors are to be watched for the corresponding class of events.

       Note  well: Upon return, each of the file descriptor sets is modified in place to indicate
       which file descriptors are currently "ready".  Thus, if using select() within a loop,  the
       sets must be reinitialized before each call.

       The contents of a file descriptor set can be manipulated using the following macros:

              This  macro  clears (removes all file descriptors from) set.  It should be employed
              as the first step in initializing a file descriptor set.

              This macro adds the file descriptor fd to set.  Adding a file  descriptor  that  is
              already present in the set is a no-op, and does not produce an error.

              This  macro  removes  the  file descriptor fd from set.  Removing a file descriptor
              that is not present in the set is a no-op, and does not produce an error.

              select() modifies the contents of the sets according to the rules described  below.
              After  calling  select(),  the  FD_ISSET()  macro  can  be  used  to test if a file
              descriptor is still present in a set.   FD_ISSET()  returns  nonzero  if  the  file
              descriptor fd is present in set, and zero if it is not.

       The arguments of select() are as follows:

              The  file descriptors in this set are watched to see if they are ready for reading.
              A file descriptor is ready for reading if a  read  operation  will  not  block;  in
              particular, a file descriptor is also ready on end-of-file.

              After select() has returned, readfds will be cleared of all file descriptors except
              for those that are ready for reading.

              The file descriptors in this set are watched to see if they are ready for  writing.
              A  file  descriptor  is  ready  for  writing  if  a write operation will not block.
              However, even if a file descriptor indicates as writable, a large write  may  still

              After  select()  has  returned,  writefds  will  be cleared of all file descriptors
              except for those that are ready for writing.

              The file descriptors in this set are watched  for  "exceptional  conditions".   For
              examples of some exceptional conditions, see the discussion of POLLPRI in poll(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 argument should be set to the highest-numbered file descriptor in any  of  the
              three  sets, plus 1.  The indicated file descriptors in each set are checked, up to
              this limit (but see BUGS).

              The timeout argument is a  timeval  structure  (shown  below)  that  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

              If  both  fields  of  the  timeval  structure  are  zero,  then  select()   returns
              immediately.  (This is useful for polling.)

              If  timeout  is  specified as NULL, select() blocks indefinitely waiting for a file
              descriptor to become ready.

       The pselect() system call allows an  application  to  safely  wait  until  either  a  file
       descriptor becomes ready or until a signal is caught.

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

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

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

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

       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.  (If sigmask is
       NULL, the signal mask is not modified during the pselect() call.)

       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 timeout argument for select() is a structure of the following type:

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

       The corresponding argument for pselect() is a timespec(3) structure.

       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).  The return value may be zero if the timeout expired before any file
       descriptors became ready.

       On error, -1 is returned, and errno is set to indicate the error; the file descriptor sets
       are unmodified, and timeout becomes undefined.


       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.)
              However, see BUGS.

       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 in Linux 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 returning, but the BSD variant does not.

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

       fd_set is defined in POSIX.1-2001 and later.


       The following header also provides the fd_set type: <sys/time.h>.

       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.

       The operation of select() and pselect() is not affected by the O_NONBLOCK flag.

       On some other UNIX systems, select() can fail with the error EAGAIN if the system fails to
       allocate kernel-internal resources, rather than ENOMEM as  Linux  does.   POSIX  specifies
       this  error  for  poll(2),  but not for select().  Portable programs may wish to check for
       EAGAIN and loop, just as with EINTR.

   The self-pipe trick
       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 pipe.)

   Emulating usleep(3)
       Before the advent of usleep(3), some code employed a call to select() with all three  sets
       empty,  nfds zero, and a non-NULL timeout as a fairly portable way to sleep with subsecond

   Correspondence between select() and poll() notifications
       Within the Linux  kernel  source,  we  find  the  following  definitions  which  show  the
       correspondence  between the readable, writable, and exceptional condition notifications of
       select() and the event notifications provided by poll(2) and epoll(7):

                                EPOLLHUP | EPOLLERR)
                              /* Ready for reading */
                              /* Ready for writing */
           #define POLLEX_SET  (EPOLLPRI)
                              /* Exceptional condition */

   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  process  reopens  the  file  descriptor between the time select()
       returned and the I/O operation is 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

   C library/kernel differences
       The  Linux kernel allows file descriptor sets of arbitrary size, determining the length of
       the sets to be checked from the value of nfds.  However, in the glibc implementation,  the
       fd_set type is fixed in size.  See also BUGS.

       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 kernel_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.   See  sigprocmask(2) for a discussion of the difference between the kernel and libc
       notion of the signal set.

   Historical glibc details
       glibc 2.0 provided an incorrect version of pselect() that did not take a sigmask argument.

       From glibc 2.1 to glibc 2.2.1,  one  must  define  _GNU_SOURCE  in  order  to  obtain  the
       declaration of pselect() from <sys/select.h>.


       POSIX  allows  an  implementation  to  define  an upper limit, advertised via the constant
       FD_SETSIZE, on the range of file descriptors that can be specified in  a  file  descriptor
       set.  The Linux kernel imposes no fixed limit, but the glibc implementation makes fd_set a
       fixed-size type, with  FD_SETSIZE  defined  as  1024,  and  the  FD_*()  macros  operating
       according  to  that  limit.  To monitor file descriptors greater than 1023, use poll(2) or
       epoll(7) instead.

       The implementation of the fd_set arguments as value-result arguments  is  a  design  error
       that is avoided in poll(2) and epoll(7).

       According to POSIX, select() should check all specified file descriptors in the three file
       descriptor sets, up to the limit nfds-1.  However, the current implementation ignores  any
       file descriptor in these sets that is greater than the maximum file descriptor number that
       the process currently has open.  According to POSIX, any  such  file  descriptor  that  is
       specified in one of the sets should result in the error EBADF.

       Starting  with  glibc  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 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 the 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/select.h>

           int             retval;
           fd_set          rfds;
           struct timeval  tv;

           /* 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), timespec(3), epoll(7), time(7)

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