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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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select(2) System Calls Manual select(2)
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO, fd_set -
synchronous I/O multiplexing
Standard C library (libc, -lc)
#include <sys/select.h>
typedef /* ... */ fd_set;
int select(int nfds, fd_set *_Nullable restrict readfds,
fd_set *_Nullable restrict writefds,
fd_set *_Nullable restrict exceptfds,
struct timeval *_Nullable restrict 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);
int pselect(int nfds, fd_set *_Nullable restrict readfds,
fd_set *_Nullable restrict writefds,
fd_set *_Nullable restrict exceptfds,
const struct timespec *_Nullable restrict timeout,
const sigset_t *_Nullable restrict sigmask);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
pselect():
_POSIX_C_SOURCE >= 200112L
WARNING: select() can monitor only file descriptors numbers that
are less than FD_SETSIZE (1024)—an unreasonably low limit for many
modern applications—and this limitation will not change. All
modern applications should instead use poll(2) or epoll(7), which
do not suffer this limitation.
select() allows 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
corresponding I/O operation (e.g., read(2), or a sufficiently
small write(2)) without blocking.
fd_set
A structure type that can represent a set of file descriptors.
According to POSIX, the maximum number of file descriptors in an
fd_set structure is the value of the macro FD_SETSIZE.
File descriptor sets
The principal arguments of select() are three "sets" of file
descriptors (declared with the type fd_set), which allow the
caller to wait for three classes of events on the specified set of
file descriptors. Each of the fd_set arguments may be specified
as NULL if no file descriptors are to be watched for the
corresponding class of events.
Note well: Upon return, each of the file descriptor sets is
modified in place to indicate which file descriptors are currently
"ready". Thus, if using select() within a loop, the sets must be
reinitialized before each call.
The contents of a file descriptor set can be manipulated using the
following macros:
FD_ZERO()
This macro clears (removes all file descriptors from) set.
It should be employed as the first step in initializing a
file descriptor set.
FD_SET()
This macro adds the file descriptor fd to set. Adding a
file descriptor that is already present in the set is a no-
op, and does not produce an error.
FD_CLR()
This macro removes the file descriptor fd from set.
Removing a file descriptor that is not present in the set
is a no-op, and does not produce an error.
FD_ISSET()
select() modifies the contents of the sets according to the
rules described below. After calling select(), the
FD_ISSET() macro can be used to test if a file descriptor
is still present in a set. FD_ISSET() returns nonzero if
the file descriptor fd is present in set, and zero if it is
not.
Arguments
The arguments of select() are as follows:
readfds
The file descriptors in this set are watched to see if they
are ready for reading. A file descriptor is ready for
reading if a read operation will not block; in particular,
a file descriptor is also ready on end-of-file.
After select() has returned, readfds will be cleared of all
file descriptors except for those that are ready for
reading.
writefds
The file descriptors in this set are watched to see if they
are ready for writing. A file descriptor is ready for
writing if a write operation will not block. However, even
if a file descriptor indicates as writable, a large write
may still block.
After select() has returned, writefds will be cleared of
all file descriptors except for those that are ready for
writing.
exceptfds
The file descriptors in this set are watched for
"exceptional conditions". For examples of some exceptional
conditions, see the discussion of POLLPRI in poll(2).
After select() has returned, exceptfds will be cleared of
all file descriptors except for those for which an
exceptional condition has occurred.
nfds This argument 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).
timeout
The timeout argument is a timeval structure (shown below)
that 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 interval 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 specified as NULL, select() blocks
indefinitely waiting for a file descriptor to become ready.
pselect()
The pselect() system call allows an application to safely wait
until either a file descriptor becomes ready or until a signal is
caught.
The operation of select() and pselect() is identical, other than
these three differences:
• select() uses a timeout that is a struct timeval (with seconds
and microseconds), while pselect() uses a struct timespec (with
seconds and nanoseconds).
• select() may update the timeout argument to indicate how much
time was left. pselect() does not change this argument.
• select() has no sigmask argument, and behaves as pselect()
called with NULL sigmask.
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. (If sigmask is NULL,
the signal mask is not modified during the pselect() call.)
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 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 timeout argument for select() is a structure of the following
type:
struct timeval {
time_t tv_sec; /* seconds */
suseconds_t tv_usec; /* microseconds */
};
The corresponding argument for pselect() is a timespec(3)
structure.
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.
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). The return value may be zero if the timeout expired
before any file descriptors became ready.
On error, -1 is returned, and errno is set to indicate the error;
the file descriptor sets are unmodified, and timeout becomes
undefined.
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.) 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.
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.
POSIX.1-2008.
select()
POSIX.1-2001, 4.4BSD (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 returning, but
the BSD variant does not.
pselect()
Linux 2.6.16. POSIX.1g, POSIX.1-2001.
Prior to this, it was emulated in glibc (but see BUGS).
fd_set POSIX.1-2001.
The following header also provides the fd_set type: <sys/time.h>.
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.
The operation of select() and pselect() is not affected by the
O_NONBLOCK flag.
The self-pipe trick
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.)
Emulating usleep(3)
Before the advent of usleep(3), some code employed a call to
select() with all three sets empty, nfds zero, and a non-NULL
timeout as a fairly portable way to sleep with subsecond
precision.
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 provided by poll(2) and epoll(7):
#define POLLIN_SET (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN |
EPOLLHUP | EPOLLERR)
/* Ready for reading */
#define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT |
EPOLLERR)
/* Ready for writing */
#define POLLEX_SET (EPOLLPRI)
/* 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 process reopens the file descriptor
between the time select() returned and the I/O operation is
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 particular behavior in
this scenario must be considered buggy.
C library/kernel differences
The Linux kernel allows file descriptor sets of arbitrary size,
determining 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.
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.
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 support 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.
Historical glibc details
glibc 2.0 provided an incorrect version of pselect() that did not
take a sigmask argument.
From glibc 2.1 to glibc 2.2.1, one must define _GNU_SOURCE in
order to obtain the declaration of pselect() from <sys/select.h>.
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) or
epoll(7) instead.
The implementation of the fd_set arguments as value-result
arguments is a design error that is avoided in poll(2) and
epoll(7).
According to POSIX, select() should check all specified file
descriptors in the three file descriptor sets, up to the limit
nfds-1. However, 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 descriptor that is specified in one of the
sets should result in the error EBADF.
Starting with glibc 2.1, glibc provided an emulation of pselect()
that was implemented using sigprocmask(2) and select(). This
implementation 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.
On 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 the 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.
#include <stdio.h>
#include <stdlib.h>
#include <sys/select.h>
#include <sys/time.h>
int
main(void)
{
int retval;
fd_set rfds;
struct timeval tv;
/* 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);
}
accept(2), connect(2), poll(2), read(2), recv(2),
restart_syscall(2), send(2), sigprocmask(2), write(2),
timespec(3), epoll(7), time(7)
For a tutorial with discussion and examples, see select_tut(2).
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 select(2)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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