pselect6

       multiplexing

SYNOPSIS
       /* 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

DESCRIPTION
       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 corre-
       sponding 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.  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 particu-
       lar, 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
       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  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 indefinitely.

       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 sig-
       nal 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 */
           };

       and

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

       after select() returns.

RETURN VALUE
       On success, select() and pselect() return the number of  file  descrip-
       tors  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.

ERRORS
       EBADF  An invalid file descriptor was given in one of the sets.   (Per-
              haps  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  the  value  contained  within  timeout  is
              invalid.

       ENOMEM unable to allocate memory for internal tables.

VERSIONS
       pselect()  was  added  to  Linux in kernel 2.6.16.  Prior to this, pse-
       lect() was emulated in glibc (but see BUGS).

CONFORMING TO
       select() conforms to POSIX.1-2001 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.

NOTES
       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>.
       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  par-
       ticular 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.  How-
       ever, 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  argu-
       ment; 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 sup-
       port a maximum of 6 arguments to a system call.

BUGS
       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 implemen-
       tation  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
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.h>

       int
       main(void)
       {
           fd_set rfds;
           struct timeval tv;
           int retval;

           /* Watch stdin (fd 0) to see when it has input. */
           FD_ZERO(&rfds);
           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)
               perror("select()");
           else if (retval)
               printf("Data is available now.\n");
               /* FD_ISSET(0, &rfds) will be true. */
           else
               printf("No data within five seconds.\n");

           exit(EXIT_SUCCESS);
       }

SEE ALSO
       accept(2),  connect(2),  poll(2),  read(2),  recv(2), send(2), sigproc-
       mask(2), write(2), epoll(7), time(7)

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

COLOPHON
       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 http://www.kernel.org/doc/man-pages/.



Linux                             2013-09-04                         SELECT(2)
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