man-pages/man2/select.2

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.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
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.\" Sun Feb 11 14:07:00 MET 1996  Martin Schulze  <joey@linux.de>
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.\"
.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
/* 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 "FD_CLR(int " fd ", fd_set *" set );
.br
.BI "FD_ISSET(int " fd ", fd_set *" set );
.br
.BI "FD_SET(int " fd ", fd_set *" set );
.br
.BI "FD_ZERO(fd_set *" set );
.fi
.SH DESCRIPTION
The functions
.B select
and
.B pselect
wait for a number of file descriptors to change status.
.PP
Their function is identical, with three differences:
.TP
(i)
The
.B select
function uses a timeout that is a
.I struct timeval
(with seconds and microseconds), while
.B pselect
uses a
.I struct timespec
(with seconds and nanoseconds).
.TP
(ii)
The
.B select
function may update the
.I timeout
parameter to indicate how much time was left. The
.B pselect
function does not change this parameter.
.TP
(iii)
The
.B select
function has no
.I sigmask
parameter, and behaves as
.B 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.
.B FD_ZERO
will clear a set.
.B FD_SET
and
.B FD_CLR
add or remove a given descriptor from a set.
.B FD_ISSET
tests to see if a descriptor is part of the set; this is useful after
.B 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
.B select
returns. It may be zero, causing
.B select
to return immediately. (This is useful for polling.) If
.I timeout
is NULL (no timeout),
.B select
can block indefinitely.
.PP
.I sigmask
is a pointer to a signal mask (see
.BR sigprocmask (2));
if it is not NULL, then
.B 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 again.
.PP
The idea of
.B 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,
.B 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.)
Since Linux today does not have a
.BR pselect ()
system call, the current glibc2 routine still contains this 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
.B 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, the function
.B select
modifies
.I timeout
to reflect the amount of time not slept; most other implementations
do not do this.  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
.B 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,
.B select
and
.B 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
.B 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 (the
.B select
function 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
The
.B pselect
function is defined in IEEE Std 1003.1g-2000 (POSIX.1g), and part of
POSIX 1003.1-2001.
It is found in glibc2.1 and later.
Glibc2.0 has a function with this name, that however does not take a
.I sigmask
parameter.
.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
.B 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
.B 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 BUGS
.B pselect
is currently emulated with a user-space wrapper that has a race condition.
For reliable (and more portable) signal trapping, use the self-pipe trick.
(Where a signal handler writes to a pipe whose other end is read by the
main loop.)

Under Linux,
.B 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)