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, POSIX.1-2008 */
#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
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 a corre-
sponding I/O operation (e.g., read(2) without blocking, or a suffi-
ciently small write(2)).
select() can monitor only file descriptors numbers that are less than
FD_SETSIZE; poll(2) does not have this limitation. See BUGS.
The operation of select() and pselect() is identical, other than these
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 file descriptors are watched. The file
descriptors 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).
The file descriptors in writefds will be watched to see if space is
available for write (though a large write may still block). The file
descriptors in exceptfds will be watched for exceptional conditions.
(For examples of some exceptional conditions, see the discussion of
POLLPRI in poll(2).)
On exit, each of the file descriptor sets is modified in place to indi-
cate which file descriptors actually changed status. (Thus, if using
select() within a loop, the sets must be reinitialized before each
call.)
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 should be set to the highest-numbered file descriptor in any of
the three sets, plus 1. The indicated file descriptors in each set are
checked, up to this limit (but see BUGS).
The timeout argument specifies the interval that select() should block
waiting for a file descriptor to become ready. The call will block
until either:
* a file descriptor becomes ready;
* the call is interrupted by a signal handler; or
* the timeout expires.
Note that the timeout interval will be rounded up to the system clock
granularity, and kernel scheduling delays mean that the blocking inter-
val may overrun by a small amount. 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;
pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
pthread_sigmask(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 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 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 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 to indicate the
error; the file descriptor sets are unmodified, and timeout becomes
undefined.
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.) However, see BUGS.
EINTR A signal was caught; see signal(7).
EINVAL nfds is negative or exceeds the RLIMIT_NOFILE resource limit
(see getrlimit(2)).
EINVAL 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).
CONFORMING TO
select() conforms to POSIX.1-2001, POSIX.1-2008, 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 vari-
ants). 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 and POSIX.1-2008.
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.
On some other UNIX systems, select() can fail with the error EAGAIN if
the system fails to allocate kernel-internal resources, rather than
ENOMEM as Linux does. POSIX specifies this error for poll(2), but not
for select(). Portable programs may wish to check for EAGAIN and loop,
just as with EINTR.
On systems that lack pselect(), reliable (and more portable) signal
trapping can be achieved using the self-pipe trick. In this technique,
a signal handler writes a byte to a pipe whose other end is monitored
by select() in the main program. (To avoid possibly blocking when
writing to a pipe that may be full or reading from a pipe that may be
empty, nonblocking I/O is used when reading from and writing to the
pipe.)
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 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 situation is that one
should include <sys/select.h> for select() and pselect().
Under glibc 2.0, <sys/select.h> gives the wrong prototype for pse-
lect(). 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.
Correspondence between select() and poll() notifications
Within the Linux kernel source, we find the following definitions which
show the correspondence between the readable, writable, and exceptional
condition notifications of select() and the event notifications pro-
vided by poll(2) (and epoll(7)):
#define POLLIN_SET (POLLRDNORM | POLLRDBAND | POLLIN | POLLHUP |
POLLERR)
/* Ready for reading */
#define POLLOUT_SET (POLLWRBAND | POLLWRNORM | POLLOUT | POLLERR)
/* Ready for writing */
#define POLLEX_SET (POLLPRI)
/* Exceptional condition */
Multithreaded applications
If a file descriptor being monitored by select() is closed in another
thread, the result is unspecified. On some UNIX systems, select()
unblocks and returns, with an indication that the file descriptor is
ready (a subsequent I/O operation will likely fail with an error,
unless another the file descriptor reopened between the time select()
returned and the I/O operations was performed). On Linux (and some
other systems), closing the file descriptor in another thread has no
effect on select(). In summary, any application that relies on a par-
ticular behavior in this scenario must be considered buggy.
C library/kernel differences
The Linux kernel allows file descriptor sets of arbitrary size, deter-
mining the length of the sets to be checked from the value of nfds.
However, in the glibc implementation, the fd_set type is fixed in size.
See also BUGS.
The pselect() interface described in this page is implemented by glibc.
The underlying Linux system call is named pselect6(). This system call
has somewhat different behavior from the glibc wrapper function.
The Linux pselect6() 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.
The final argument of the pselect6() system call is not a sigset_t *
pointer, but is instead a structure of the form:
struct {
const kernel_sigset_t *ss; /* Pointer to signal set */
size_t ss_len; /* Size (in bytes) of object
pointed to by 'ss' */
};
This allows the system call to obtain both a pointer to the signal set
and its size, while allowing for the fact that most architectures sup-
port a maximum of 6 arguments to a system call. See sigprocmask(2) for
a discussion of the difference between the kernel and libc notion of
the signal set.
BUGS
POSIX allows an implementation to define an upper limit, advertised via
the constant FD_SETSIZE, on the range of file descriptors that can be
specified in a file descriptor set. The Linux kernel imposes no fixed
limit, but the glibc implementation makes fd_set a fixed-size type,
with FD_SETSIZE defined as 1024, and the FD_*() macros operating
according to that limit. To monitor file descriptors greater than
1023, use poll(2) instead.
According to POSIX, select() should check all specified file descrip-
tors in the three file descriptor sets, up to the limit nfds-1. How-
ever, the current implementation ignores any file descriptor in these
sets that is greater than the maximum file descriptor number that the
process currently has open. According to POSIX, any such file descrip-
tor that is specified in one of the sets should result in the error
EBADF.
Glibc 2.0 provided a version of pselect() that did not take a sigmask
argument.
Starting with version 2.1, glibc provided an emulation of pselect()
that was implemented using sigprocmask(2) and select(). This implemen-
tation remained vulnerable to the very race condition that pselect()
was designed to prevent. Modern versions of glibc use the (race-free)
pselect() system call on kernels where it is provided.
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. 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
accept(2), connect(2), poll(2), read(2), recv(2), restart_syscall(2),
send(2), sigprocmask(2), write(2), epoll(7), time(7)
For a tutorial with discussion and examples, see select_tut(2).
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
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