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NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O
multiplexing
SYNOPSIS
/* 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);
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. 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 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 <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.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.
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.
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).
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 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.
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(2) 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 <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);
}
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
ture_test_macros(7)