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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 <&lt;sys/select.h>&gt;

       /* According to earlier standards */
       #include <&lt;sys/time.h>&gt;
       #include <&lt;sys/types.h>&gt;
       #include <&lt;unistd.h>&gt;

       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 <&lt;sys/select.h>&gt;

       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, with  three  dif-
       ferences:

       (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
       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  time-
       out 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 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 &lt;sys/time.h&gt; 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  pre-
       cision.

       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 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.

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 &lt;sys/time.h&gt;.  The  POSIX.1-2001  situation
       is

           struct timeval {
               time_t         tv_sec;     /* seconds */
               suseconds_t    tv_usec;    /* microseconds */
           };

       where  the  structure  is  defined in &lt;sys/select.h&gt; and the data types
       time_t and suseconds_t are defined in &lt;sys/types.h&gt;.

       Concerning prototypes, the  classical  situation  is  that  one  should
       include  &lt;time.h&gt; for select().  The POSIX.1-2001 situation is that one
       should include &lt;sys/select.h&gt; for select() and  pselect().   Libc4  and
       libc5  do  not  have a &lt;sys/select.h&gt; 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.

   Linux Notes
       The Linux pselect() 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.

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 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.

       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  behav-
       ior,  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 3.05 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                             2008-07-14                         SELECT(2)