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

       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 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 speci-
       fied  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.

       The timeout argument specifies the interval that select() should block waiting for a  file
       descriptor  to  become ready.  This interval will be rounded up to the system clock granu-
       larity, 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 indefinitely.

       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 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 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().

       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.

   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.

   Linux notes
       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 3.54 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at

Linux                                       2013-09-04                                  SELECT(2)

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