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


       #include <sys/select.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);

       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


       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

       select()  can monitor only file descriptors numbers that are less than FD_SETSIZE; poll(2)
       and epoll(7) do not have this limitation.  See BUGS.

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

       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() has the following type:

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

       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 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 returning, 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.

       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 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 buggy.

   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.

       In glibc versions 2.1 to 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.

       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 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  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>

           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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