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


   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)).

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

   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.

   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.

   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 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 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.


   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) instead.

   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).


   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at

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