Tribblix: manual page: select.3c

SELECT(3C) Standard C Library Functions SELECT(3C)

NAME

select, pselect, FD_SET, FD_CLR, FD_ISSET, FD_ZERO - synchronous I/O
multiplexing

SYNOPSIS

#include <sys/time.h>

int
select(int nfds, fd_set *restrict readfds, fd_set *restrict writefds,
fd_set *restrict errorfds, struct timeval *restrict timeout);

int
pselect(int nfds, fd_set *restrict readfds, fd_set *restrict writefds,
fd_set *restrict errorfds, const struct timespec *restrict timeout,
const sigset_t *restrict sigmask);

void
FD_SET(int fd, fd_set *fdset);

void
FD_CLR(int fd, fd_set *fdset);

int
FD_ISSET(int fd, fd_set *fd_set);

void
FD_ZERO(fd_set *fdset);

DESCRIPTION

The pselect() function examines the file descriptor sets whose
addresses are passed in the readfds, writefds, and errorfds parameters
to see if some of their descriptors are ready for reading, are ready
for writing, or have an exceptional condition pending, respectively.

The select() function is equivalent to the pselect() function, except
as follows:

+o For the select() function, the timeout period is given in seconds
and microseconds in an argument of type struct timeval, whereas for
the pselect() function the timeout period is given in seconds and
nanoseconds in an argument of type struct timespec

+o The select() function has no sigmask argument. It behaves as
pselect() does when sigmask is a null pointer.

+o Upon successful completion, the select() function might modify the
object pointed to by the Itimeout argument.

The select() and pselect() functions support regular files, terminal
and pseudo-terminal devices, STREAMS-based files, FIFOs, pipes, and
sockets. The behavior of select() and pselect() on file descriptors
that refer to other types of file is unspecified.

The nfds argument specifies the range of file descriptors to be tested.
The first nfds descriptors are checked in each set; that is, the
descriptors from zero through "nfds - 1" in the descriptor sets are
examined.

If the readfds argument is not a null pointer, it points to an object
of type fd_set that on input specifies the file descriptors to be
checked for being ready to read, and on output indicates which file
descriptors are ready to read.

If the writefds argument is not a null pointer, it points to an object
of type fd_set that on input specifies the file descriptors to be
checked for being ready to write, and on output indicates which file
descriptors are ready to write.

If the errorfds argument is not a null pointer, it points to an object
of type fd_set that on input specifies the file descriptors to be
checked for error conditions pending, and on output indicates which
file descriptors have error conditions pending.

Upon successful completion, the objects pointed to by the readfds,
writefds, and errorfds arguments are modified to indicate which file
descriptors are ready for reading, ready for writing, or have an error
condition pending, respectively, and return the total number of ready
descriptors in all the output sets. For each file descriptor less than
nfds, the corresponding bit will be set on successful completion if it
was set on input and the associated condition is true for that file
descriptor.

If none of the selected descriptors are ready for the requested
operation, the select() or pselect() function blocks until at least one
of the requested operations becomes ready, until the timeout occurs, or
until interrupted by a signal. The timeout parameter controls how long
the select() or pselect() function takes before timing out. If the
timeout parameter is not a null pointer, it specifies a maximum
interval to wait for the selection to complete. If the specified time
interval expires without any requested operation becoming ready, the
function returns. If the timeout parameter is a null pointer, then the
call to select() or pselect() blocks indefinitely until at least one
descriptor meets the specified criteria. To effect a poll, the timeout
parameter should not be a null pointer, and should point to a zero-
valued timespec structure.

The use of a timeout does not affect any pending timers set up by
alarm(2), ualarm(3C), or setitimer(2).

If sigmask is not a null pointer, then the pselect() function replaces
the signal mask of the process by the set of signals pointed to by
sigmask before examining the descriptors, and restores the signal mask
of the process before returning.

A descriptor is considered ready for reading when a call to an input
function with O_NONBLOCK clear would not block, whether or not the
function would transfer data successfully. (The function might return
data, an end-of-file indication, or an error other than one indicating
that it is blocked, and in each of these cases the descriptor will be
considered ready for reading.)

A descriptor is considered ready for writing when a call to an output
function with O_NONBLOCK clear would not block, whether or not the
function would transfer data successfully.

If a socket has a pending error, it is considered to have an
exceptional condition pending. Otherwise, what constitutes an
exceptional condition is file type-specific. For a file descriptor for
use with a socket, it is protocol-specific except as noted below. For
other file types, if the operation is meaningless for a particular file
type, select() or pselect() indicates that the descriptor is ready for
read or write operations and indicates that the descriptor has no
exceptional condition pending.

If a descriptor refers to a socket, the implied input function is the
recvmsg(3XNET) function with parameters requesting normal and ancillary
data, such that the presence of either type causes the socket to be
marked as readable. The presence of out-of-band data is checked if the
socket option SO_OOBINLINE has been enabled, as out-of-band data is
enqueued with normal data. If the socket is currently listening, then
it is marked as readable if an incoming connection request has been
received, and a call to the accept function completes without blocking.

If a descriptor refers to a socket, the implied output function is the
sendmsg(3XNET) function supplying an amount of normal data equal to the
current value of the SO_SNDLOWAT option for the socket. If a non-
blocking call to the connect function has been made for a socket, and
the connection attempt has either succeeded or failed leaving a pending
error, the socket is marked as writable.

A socket is considered to have an exceptional condition pending if a
receive operation with O_NONBLOCK clear for the open file description
and with the MSG_OOB flag set would return out-of-band data without
blocking. (It is protocol-specific whether the MSG_OOB flag would be
used to read out-of-band data.) A socket will also be considered to
have an exceptional condition pending if an out-of-band data mark is
present in the receive queue.

A file descriptor for a socket that is listening for connections will
indicate that it is ready for reading, when connections are available.
A file descriptor for a socket that is connecting asynchronously will
indicate that it is ready for writing, when a connection has been
established.

Selecting true for reading on a socket descriptor upon which a
listen(3XNET) call has been performed indicates that a subsequent
accept(3XNET) call on that descriptor will not block.

If the timeout argument is not a null pointer, it points to an object
of type struct timeval that specifies a maximum interval to wait for
the selection to complete. If the timeout argument points to an object
of type struct timeval whose members are 0, select() does not block.
If the timeout argument is a null pointer, select() blocks until an
event causes one of the masks to be returned with a valid (non-zero)
value. If the time limit expires before any event occurs that would
cause one of the masks to be set to a non-zero value, select()
completes successfully and returns 0.

If the readfds, writefds, and errorfds arguments are all null pointers
and the timeout argument is not a null pointer, select() or pselect()
blocks for the time specified, or until interrupted by a signal. If
the readfds, writefds, and errorfds arguments are all null pointers and
the timeout argument is a null pointer, select() blocks until
interrupted by a signal.

File descriptors associated with regular files always select true for
ready to read, ready to write, and error conditions.

On failure, the objects pointed to by the readfds, writefds, and
errorfds arguments are not modified. If the timeout interval expires
without the specified condition being true for any of the specified
file descriptors, the objects pointed to by the readfds, writefds, and
errorfds arguments have all bits set to 0.

File descriptor masks of type fd_set can be initialized and tested with
the macros FD_CLR(), FD_ISSET(), FD_SET(), and FD_ZERO().

FD_CLR(fd, &fdset)
Clears the bit for the file descriptor fd in the file
descriptor set fdset.

FD_ISSET(fd, &fdset)
Returns a non-zero value if the bit for the file descriptor fd
is set in the file descriptor set pointed to by fdset, and 0
otherwise.

FD_SET(fd, &fdset)
Sets the bit for the file descriptor fd in the file descriptor
set fdset

FD_ZERO(&fdset)
Initializes the file descriptor set fdset to have zero bits for
all file descriptors.

The behavior of these macros is undefined if the fd argument is less
than 0 or greater than or equal to FD_SETSIZE, or if fd is not a valid
file descriptor, or if any of the arguments are expressions with side
effects.

RETURN VALUES

On successful completion, select() and pselect() return the total
number of bits set in the bit masks. Otherwise, -1 is returned and
errno is set to indicate the error.

The FD_CLR(), FD_SET(), and FD_ZERO(), macros return no value. The
FD_ISSET() macro returns a non-zero value if the bit for the file
descriptor fd is set in the file descriptor set pointed to by fdset,
and 0 otherwise.

ERRORS

The select() and pselect() functions will fail if:

EBADF One or more of the file descriptor sets specified a file
descriptor that is not a valid open file descriptor.

EINTR The function was interrupted before any of the selected events
occurred and before the timeout interval expired.

If SA_RESTART has been set for the interrupting signal, it is
implementation-dependent whether select() restarts or returns
with EINTR

EINVAL An invalid timeout interval was specified.

EINVAL The nfds argument is less than 0 or greater than FD_SETSIZE.

EINVAL One of the specified file descriptors refers to a STREAM or
multiplexer that is linked (directly or indirectly) downstream
from a multiplexer.

EINVAL A component of the pointed-to time limit is outside the
acceptable range: t_sec must be between 0 and 10^8, inclusive.
t_usec must be greater than or equal to 0, and less than 10^6.

USAGE

The poll(2) function is preferred over this function.

The use of a timeout does not affect any pending timers set up by
alarm(2), ualarm(3C), or setitimer(2).

On successful completion, the object pointed to by the timeout argument
may be modified.

INTERFACE STABILITY

Standard

MT Level
MT-Safe