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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 <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);
#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);
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 corresponding I/O operation (e.g., read(2)) without blocking.
The operation of select() and pselect() 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 argu-
ment.
(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 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 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 use-
ful 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 structure
are zero, then select() returns 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 signal arrived
just after the test but just before the call. By contrast, pselect() allows one to first block signals, handle the sig-
nals 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 precision.
On Linux, select() modifies timeout to reflect the amount of time not slept; most other implementations do not do this.
(POSIX.1-2001 permits 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 descriptors 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. (Perhaps 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, pselect() 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 <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 situa-
tion 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 to 2.2.1 it gives pselect() when _GNU_SOURCE is
defined. Since glibc 2.2.2 the requirements are as shown in the SYNOPSIS.
Linux Notes
The Linux pselect() system call modifies its timeout argument. However, 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 argument; 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 sig-
nal handler writes a byte to a pipe whose other end is monitored 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 behavior, 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 3.25 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 2008-12-05 SELECT(2)
