select(2) System Calls Manual select(2)
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 applica-
tions 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 consid-
ered 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. Accord-
ing to POSIX, the maximum number of file descriptors in an fd_set struc-
ture 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 fol-
lowing macros:
FD_ZERO()
This macro clears (removes all file descriptors from) set. It
should be employed as the first step in initializing a file de-
scriptor set.
FD_SET()
This macro adds the file descriptor fd to set. Adding a file de-
scriptor 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 descrip-
tor 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 condi-
tion has occurred.
nfds This argument should be set to the highest-numbered file descrip-
tor in any of the three sets, plus 1. The indicated file de-
scriptors 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 ei-
ther:
• 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 ei-
ther 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 re-
stores 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 ei-
ther 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 fol-
lowed by a call of select() could hang indefinitely if the signal ar-
rived just after the test but just before the call. By contrast, pse-
lect() 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 ei-
ther 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 se-
lect() 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 num-
ber of bits that are set in readfds, writefds, exceptfds). The return
value may be zero if the timeout expired before any file descriptors be-
came 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. (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
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.
STANDARDS
POSIX.1-2008.
HISTORY
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 vari-
able 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.
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_NON-
BLOCK 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() un-
blocks and returns, with an indication that the file descriptor is ready
(a subsequent I/O operation will likely fail with an error, unless an-
other process reopens the file descriptor between the time select() re-
turned 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 be-
havior 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 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 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.
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 cur-
rently 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 de-
signed to prevent. Modern versions of glibc use the (race-free) pse-
lect() 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).
Linux man-pages 6.9.1 2024-06-15 select(2)
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