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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 1003.1-2001 */
#include <sys/select.h>
/* According to earlier standards */
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>
int select(int n, 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 n, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, const struct timespec *timeout,
const sigset_t *sigmask);
DESCRIPTION
select() and pselect() wait for one or more file descriptors to change
status.
Their operation 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 descriptors are watched. Those listed in
readfds will be watched to see if characters become available for read-
ing (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 descriptors actually changed status.
Four macros are provided to manipulate the sets. FD_ZERO() clears a
set. FD_SET() and FD_CLR() respectively add and remove a given
descriptor from a set. FD_ISSET() tests to see if a descriptor is part
of the set; this is useful after select() returns.
n is the highest-numbered 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(n, &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 */
};
and
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
(However, see below on the POSIX 1003.1-2001 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-
sion.
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.
RETURN VALUE
On success, select() and pselect() return the number of descriptors
contained in the three returned descriptor sets (that is, the total
number of one bits 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.
EINTR A non blocked signal was caught.
EINVAL n is negative or the value contained within timeout is invalid.
ENOMEM select() was unable to allocate memory for internal tables.
EXAMPLE
#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;
}
CONFORMING TO
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 IEEE Std 1003.1g-2000 (POSIX.1g), and in POSIX
1003.1-2001.
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 struct timeval are longs (as shown above), and the struct
is defined in <sys/time.h>. The POSIX 1003.1-2001 situation is
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
where the struct 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 1003.1-2001 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 1003.1-2001, it should give the prototype by default.
VERSIONS
pselect() was added to Linux in kernel 2.6.16.
Prior to this, pselect() was emulated in glibc (but see BUGS).
LINUX NOTES
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.
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.
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)
Linux 2.6.16 2006-03-11 SELECT(2)
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This page was generated on Tue Feb 13 02:17:41 GMT 2007
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