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NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO, fd_set - synchronous I/O multiplexing
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
DESCRIPTION
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.
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). 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.
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.)
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 in Linux 2.6.16. Prior to this, pselect() was emulated in glibc (but
see BUGS).
STANDARDS
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 variants). However, note that the System V variant typically
sets the timeout variable before returning, but the BSD variant does not.
pselect() is defined in POSIX.1g, and in POSIX.1-2001 and POSIX.1-2008.
fd_set is defined in POSIX.1-2001 and later.
NOTES
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.
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.
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>.
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) 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.
EXAMPLES
#include <stdio.h>
#include <stdlib.h>
#include <sys/select.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);
}
SEE ALSO
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).