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SELECT(2)                           Linux Programmer's Manual                           SELECT(2)

       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 a
       corresponding I/O operation (e.g., read(2)  without  blocking,  or  a  sufficiently  small

       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.   pse‐
              lect() does not change this argument.

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

       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 indefi‐

       sigmask is a pointer to a signal mask (see sigprocmask(2)); if it is not NULL,  then  pse‐
       lect()  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  pse‐
       lect() 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  con‐
       ditions.  (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 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  descrip‐
              tor 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).   Gener‐
       ally 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().

       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 require‐
       ments 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 ABI 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  wrap‐
       per  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

       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.  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.)

       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 cir‐
       cumstances 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  pse‐
       lect()  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");


       accept(2),  connect(2),  poll(2),  read(2),  recv(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 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2014-08-19                                  SELECT(2)

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