SELECT(2)                  Linux Programmer's Manual                 SELECT(2)
       select,  pselect,  FD_CLR,  FD_ISSET, FD_SET, FD_ZERO - synchronous I/O

       /* 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);

       #define _XOPEN_SOURCE 600
       #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);

       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, with  three  dif-

       (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
       exceptfds will be watched for exceptions.  On exit, the sets are  modi-
       fied  in place to indicate which file descriptors actually changed sta-
       tus.  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

       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.  It may be zero, causing select() to return 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 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, n zero, and a non-
       NULL timeout as a fairly portable way to sleep  with  subsecond  preci-

       On  Linux,  select() modifies timeout to reflect the amount of time not
       slept; most other implementations do not do this.   (POSIX.1-2001  per-
       mits  either  behaviour.)   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 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.

       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.

       EINVAL nfds  is  negative  or  the  value  contained  within timeout is

       ENOMEM unable to allocate memory for internal tables.

       #include <stdio.h>
       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.h>

       main(void) {
           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");

           return 0;

       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.

       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  longs  (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 2.1-2.2.1 it gives pselect() when
       _GNU_SOURCE is defined,  under  glibc  2.2.2-2.2.4  it  gives  it  when
       _XOPEN_SOURCE  is  defined and has a value of 600 or larger.  No doubt,
       since POSIX.1-2001, it should give the prototype by default.

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

       The  Linux  pselect()  system call modifies its timeout argument.  How-
       ever, the glibc wrapper function hides this behaviour 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 behaviour required by POSIX.1-2001.

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

       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 mon-
       itored by select() in the main program.)

       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.

       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),    fea-

Linux 2.6.16                      2006-03-11                         SELECT(2)