select(2) - Linux manual page (original) (raw)
select(2) System Calls Manual select(2)
NAME top
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO, fd_set -
synchronous I/O multiplexingLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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 >= 200112LDESCRIPTION top
**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)](../man2/poll.2.html) or [epoll(7)](../man7/epoll.7.html), 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)](../man2/read.2.html), or a sufficiently
small [write(2)](../man2/write.2.html)) 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 fdset 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 fdset), which allow the caller to wait for three classes of events on the specified set of file descriptors. Each of the fdset 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)](../man2/poll.2.html).
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)](../man2/sigprocmask.2.html)); 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 top
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](../man3/errno.3.html)_ is set to indicate the error;
the file descriptor sets are unmodified, and _timeout_ becomes
undefined.ERRORS top
**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)](../man7/signal.7.html).
**EINVAL** _nfds_ is negative or exceeds the **RLIMIT_NOFILE** resource
limit (see [getrlimit(2)](../man2/getrlimit.2.html)).
**EINVAL** The value contained within _timeout_ is invalid.
**ENOMEM** Unable to allocate memory for internal tables.VERSIONS top
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)](../man2/poll.2.html), but not for **select**(). Portable programs may wish to
check for **EAGAIN** and loop, just as with **EINTR**.STANDARDS top
POSIX.1-2008.HISTORY top
**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.NOTES top
The following header also provides the _fdset_ type: _<sys/time.h>_.
An _fdset_ 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 fdset 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
_sigsett *_ 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)](../man2/sigprocmask.2.html) 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 top
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 _fdset_ 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)](../man2/poll.2.html) or
[epoll(7)](../man7/epoll.7.html) instead.
The implementation of the _fdset_ arguments as value-result
arguments is a design error that is avoided in [poll(2)](../man2/poll.2.html) and
[epoll(7)](../man7/epoll.7.html).
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)](../man2/sigprocmask.2.html) 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 top
#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);
}SEE ALSO top
[accept(2)](../man2/accept.2.html), [connect(2)](../man2/connect.2.html), [poll(2)](../man2/poll.2.html), [read(2)](../man2/read.2.html), [recv(2)](../man2/recv.2.html),
[restart_syscall(2)](../man2/restart%5Fsyscall.2.html), [send(2)](../man2/send.2.html), [sigprocmask(2)](../man2/sigprocmask.2.html), [write(2)](../man2/write.2.html),
**timespec**(3), [epoll(7)](../man7/epoll.7.html), [time(7)](../man7/time.7.html)
For a tutorial with discussion and examples, see [select_tut(2)](../man2/select%5Ftut.2.html).COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-03 select(2)
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