pselect6 (2) - Linux Man Pages
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 corresponding I/O operation (e.g., read(2), or a sufficiently small write(2)) without blocking.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 indicate 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() 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 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 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,
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.)
struct timeval {
and
struct timespec {
(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 precision.
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.
pselect()
is defined in POSIX.1g, and in
POSIX.1-2001 and POSIX.1-2008.
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.
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 {
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
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.
#define POLLIN_SET (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN |
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 {
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
The timeout
The time structures involved are defined in
<sys/time.h>
and look like
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 to indicate the error;
the file descriptor sets are unmodified,
and
timeout
becomes undefined.
ERRORS
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, 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 exit, but the BSD variant does not.
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.
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):
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 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.