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



NAME
       select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O 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 corresponding I/O operation (e.g., read(2)) without blocking.

       The operation of select() and pselect() is identical, with 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 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  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.

       timeout is an upper bound on the amount of time elapsed before select() returns.  If both fields of the timeval
       stucture 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;

           sigprocmask(SIG_SETMASK, &sigmask, &origmask);
           ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
           sigprocmask(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 indefi-
       nitely 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 */
           };

       and

           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  time-
       out  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.

RETURN VALUE
       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.

ERRORS
       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 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, pselect() 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>.

       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.

   Linux Notes
       The  Linux pselect() 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.

BUGS
       Glibc 2.0 provided a version of pselect() that did not take a sigmask argument.

       Since  version  2.1,  glibc has provided an emulation of pselect() that is implemented using sigprocmask(2) and
       select().  This implementation remains vulnerable to the very race condition that  pselect()  was  designed  to
       prevent.   On  systems  that lack pselect(), reliable (and more portable) signal trapping can be achieved using
       the self-pipe trick (where a signal handler writes a byte to a pipe whose other end is monitored by select() in
       the main program.)

       Under  Linux,  select() may report a socket file descriptor as "ready for reading", while nevertheless a subse-
       quent 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.1-2001.  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.

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

       For  vaguely  related  stuff,  see  accept(2),  connect(2), poll(2), read(2), recv(2), send(2), sigprocmask(2),
       write(2), epoll(7), time(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-12-05                         SELECT(2)