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[thirdparty/man-pages.git] / man2 / select.2

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.\" This manpage is copyright (C) 1992 Drew Eckhardt,
.\"                 copyright (C) 1995 Michael Shields.
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.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1995-05-18 by Jim Van Zandt <jrv@vanzandt.mv.com>
.\" Sun Feb 11 14:07:00 MET 1996  Martin Schulze  <joey@linux.de>
.\"	* layout slightly modified
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.\" Modified Mon Oct 21 23:05:29 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Thu Feb 24 01:41:09 CET 2000 by aeb
.\" Modified Thu Feb  9 22:32:09 CET 2001 by bert hubert <ahu@ds9a.nl>, aeb
.\" Modified Mon Nov 11 14:35:00 PST 2002 by Ben Woodard <ben@zork.net>
.\"
.TH SELECT 2 2001-02-09 "Linux 2.4" "Linux Programmer's Manual"
.SH NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO \- synchronous I/O multiplexing
.SH SYNOPSIS
.nf
/* According to POSIX 1003.1-2001 */
.br
.B #include <sys/select.h>
.sp
/* According to earlier standards */
.br
.B #include <sys/time.h>
.br
.B #include <sys/types.h>
.br
.B #include <unistd.h>
.sp
\fBint select(int \fIn\fB, fd_set *\fIreadfds\fB, fd_set *\fIwritefds\fB, 
           fd_set *\fIexceptfds\fB, struct timeval *\fItimeout\fB);
.sp
\fBint pselect(int \fIn\fB, fd_set *\fIreadfds\fB, fd_set *\fIwritefds\fB, 
           fd_set *\fIexceptfds\fB, const struct timespec *\fItimeout\fB, 
           const sigset_t *\fIsigmask\fB);
.sp
.BI "void FD_CLR(int " fd ", fd_set *" set );
.br
.BI "int FD_ISSET(int " fd ", fd_set *" set );
.br
.BI "void FD_SET(int " fd ", fd_set *" set );
.br
.BI "void FD_ZERO(fd_set *" set );
.fi
.SH DESCRIPTION
.BR select ()
and
.BR pselect ()
wait for one or more file descriptors to change status.
.PP
Their operation is identical, with three differences:
.TP
(i)
.BR select ()
uses a timeout that is a
.I struct timeval
(with seconds and microseconds), while
.BR pselect ()
uses a
.I struct timespec
(with seconds and nanoseconds).
.TP
(ii)
.BR select ()
may update the
.I timeout
argument to indicate how much time was left.
.BR pselect ()
does not change this argument.
.TP
(iii)
.BR select ()
has no
.I sigmask
argument, and behaves as
.BR pselect ()
called with NULL
.IR sigmask .
.PP
Three independent sets of descriptors are watched.  Those listed in
.I 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
.I writefds
will be watched to see if a write will not block, and
those in
.I exceptfds
will be watched for exceptions.  On exit, the sets are modified in place
to indicate which descriptors actually changed status.
.PP
Four macros are provided to manipulate the sets.
.BR FD_ZERO ()
clears a set.
.BR FD_SET ()
and
.BR FD_CLR ()
respectively add and remove a given descriptor from a set.
.BR FD_ISSET ()
tests to see if a descriptor is part of the set; this is useful after
.BR select ()
returns.
.PP
.I n
is the highest-numbered descriptor in any of the three sets, plus 1.
.PP
.I timeout
is an upper bound on the amount of time elapsed before
.BR select ()
returns. It may be zero, causing
.BR select ()
to return immediately. (This is useful for polling.) If
.I timeout
is NULL (no timeout),
.BR select ()
can block indefinitely.
.PP
.I sigmask
is a pointer to a signal mask (see
.BR sigprocmask (2));
if it is not NULL, then
.BR pselect ()
first replaces the current signal mask by the one pointed to by
.IR sigmask ,
then does the `select' function, and then restores the original
signal mask.
.PP
The idea of
.BR pselect ()
is that if one wants to wait for an event, either a signal
or something on a file descriptor, 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
.BR select ()
could hang indefinitely if the signal arrived just after the test
but just before the call. On the other hand,
.BR pselect ()
allows one to first block signals, handle the signals that have come in,
then call
.BR pselect ()
with the desired
.IR sigmask ,
avoiding the race.)
.SS "The timeout"
The time structures involved are defined in
.I <sys/time.h>
and look like

.RS
.nf
struct timeval { 
    long    tv_sec;         /* seconds */
    long    tv_usec;        /* microseconds */
};
.fi
.RE

and

.RS
.nf
struct timespec {
    long    tv_sec;         /* seconds */
    long    tv_nsec;        /* nanoseconds */
};
.fi
.RE

(However, see below on the POSIX 1003.1-2001 versions.)
.PP
Some code calls
.BR select ()
with all three sets empty,
.I n
zero, and a non-null
.I timeout
as a fairly portable way to sleep with subsecond precision.
.PP
On Linux, 
.BR select ()
modifies
.I timeout
to reflect the amount of time not slept; most other implementations
do not do this.
(POSIX.1-2001 permits either behaviour.)
This causes problems both when Linux code which reads
.I timeout
is ported to other operating systems, and when code is ported to Linux
that reuses a struct timeval for multiple
.BR select ()s
in a loop without reinitializing it.  Consider
.I timeout
to be undefined after
.BR select ()
returns.
.\" .PP - it is rumoured that:
.\" On BSD, when a timeout occurs, the file descriptor bits are not changed.
.\" - it is certainly true that:
.\" Linux follows SUSv2 and sets the bit masks to zero upon a timeout.
.SH "RETURN VALUE"
On success,
.BR select ()
and
.BR pselect ()
return the number of descriptors contained in the three returned
descriptor sets (that is, the total number of one bits in
.IR readfds ,
.IR writefds ,
.IR exceptfds )
which may be zero if the timeout expires before anything interesting happens.
On error, \-1 is returned, and
.I errno
is set appropriately; the sets and
.I timeout
become undefined, so do not
rely on their contents after an error.
.SH ERRORS
.TP
.B EBADF
An invalid file descriptor was given in one of the sets.
.TP
.B EINTR
A non blocked signal was caught.
.TP
.B EINVAL
.I n
is negative or the value contained within
.I timeout
is invalid.
.TP
.B ENOMEM
.BR select ()
was unable to allocate memory for internal tables.
.SH EXAMPLE
.nf
#include <stdio.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");

    return 0;
}
.fi
.SH "CONFORMING TO"
4.4BSD 
.RB ( 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.
.PP
.BR pselect ()
is defined in IEEE Std 1003.1g-2000 (POSIX.1g), and in
POSIX 1003.1-2001.
.SH NOTES
An fd_set is a fixed size buffer. Executing FD_CLR or FD_SET with a value of
.I fd
that is negative or is equal to or larger than FD_SETSIZE will result
in undefined behavior. Moreover, POSIX requires
.I fd
to be a valid file descriptor.

Concerning the types involved, the classical situation is that
the two fields of a struct timeval are longs (as shown above),
and the struct is defined in
.IR <sys/time.h> .
The POSIX 1003.1-2001 situation is

.RS
.nf
struct timeval {
    time_t         tv_sec;     /* seconds */
    suseconds_t    tv_usec;    /* microseconds */
};
.fi
.RE

where the struct is defined in
.I <sys/select.h>
and the data types time_t and suseconds_t are defined in
.IR <sys/types.h> .
.LP
Concerning prototypes, the classical situation is that one should
include
.I <time.h>
for
.BR select ().
The POSIX 1003.1-2001 situation is that one should include
.I <sys/select.h>
for
.BR select ()
and
.BR pselect ().
Libc4 and libc5 do not have a
.I <sys/select.h>
header; under glibc 2.0 and later this header exists.
Under glibc 2.0 it unconditionally gives the wrong prototype for
.BR pselect (),
under glibc 2.1-2.2.1 it gives
.BR pselect ()
when
.B _GNU_SOURCE
is defined, under glibc 2.2.2-2.2.4 it gives it when
.B _XOPEN_SOURCE
is defined and has a value of 600 or larger.
No doubt, since POSIX 1003.1-2001, it should give the prototype by default.
.SH VERSIONS
.BR pselect ()
was added to Linux in kernel 2.6.16.

Prior to this,
.BR pselect ()
was emulated in glibc (but see BUGS).
.SH "LINUX NOTES"
The Linux
.BR pselect ()
system call modifies its timeout argument.
However, the glibc wrapper functions hides this behaviour
by using a local variable for the timeout argument that
is passed to the system call.
Thus, the glibc 
.BR pselect ()
function does not modify its timeout argument;
this is the behaviour required by POSIX.1-2001.
.SH BUGS
Glibc 2.0 provided a version of
.BR pselect ()
that did not take a 
.I sigmask 
argument.

Since version 2.1, glibc has provided an emulation of
.BR pselect () 
that is implemented using 
.BR sigprocmask (2)
and
.BR select ().
This implementation remains vulnerable to the very race condition that
.BR pselect ()
was designed to prevent.
On systems that lack
.BR 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 
.BR select ()
in the main program.)

Under Linux,
.BR 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.
.\" Stevens discusses a case where accept can block after select
.\" returns successfully because of an intervening RST from the client.
Thus it may be safer to use O_NONBLOCK on sockets that should not block.
.\" Maybe the kernel should have returned EIO in such a situation?
.SH "SEE ALSO"
For a tutorial with discussion and examples, see
.BR select_tut (2).
.LP
For vaguely related stuff, see
.BR accept (2),
.BR connect (2),
.BR poll (2),
.BR read (2),
.BR recv (2),
.BR send (2),
.BR sigprocmask (2),
.BR write (2)