FD_SET (3p) - Linux Manuals
FD_SET: synchronous I/O multiplexing
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.NAME
pselect, select - synchronous I/O multiplexing
SYNOPSIS
#include <sys/select.h>
int pselect(int nfds, fd_set *restrict readfds,
int select(int nfds, fd_set *restrict readfds,
void FD_CLR(int fd, fd_set *fdset);
int FD_ISSET(int fd, fd_set *fdset);
void FD_SET(int fd, fd_set *fdset);
void FD_ZERO(fd_set *fdset);
DESCRIPTION
The pselect() function shall examine the file descriptor sets whose addresses are passed in the readfds, writefds, and errorfds parameters to see whether some of their descriptors are ready for reading, are ready for writing, or have an exceptional condition pending, respectively.
The select() function shall be equivalent to the pselect() function, except as follows:
- *
- 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.
- *
- The select() function has no sigmask argument; it shall behave as pselect() does when sigmask is a null pointer.
- *
- Upon successful completion, the select() function may modify the object pointed to by the timeout argument.
The pselect() and select() functions shall support regular
files, terminal and pseudo-terminal devices,
The nfds argument specifies the range of descriptors to be tested.
The first nfds descriptors shall be checked in
each set; that is, the descriptors from zero through nfds-1
in the descriptor sets shall be 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 pselect() or select()
function shall modify the objects pointed to by the
readfds, writefds, and errorfds arguments to indicate
which file descriptors are ready for reading, ready for
writing, or have an error condition pending, respectively, and shall
return the total number of ready descriptors in all the output
sets. For each file descriptor less than nfds, the corresponding
bit shall 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 pselect() or select() function
shall block 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 pselect()
or select() function shall take 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 shall return. If the
timeout parameter is a null pointer, then the call to pselect()
or select() shall block 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(), ualarm(), or setitimer().
Implementations may place limitations on the maximum timeout interval
supported. All implementations shall support a maximum
timeout interval of at least 31 days. If the timeout argument
specifies a timeout interval greater than the
implementation-defined maximum value, the maximum value shall be used
as the actual timeout value. Implementations may also place
limitations on the granularity of timeout intervals. If the requested
timeout interval requires a finer granularity than the
implementation supports, the actual timeout interval shall be rounded
up to the next supported value.
If sigmask is not a null pointer, then the pselect() function
shall replace the signal mask of the process by the
set of signals pointed to by sigmask before examining the descriptors,
and shall restore the signal mask of the process
before returning.
A descriptor shall be 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 shall be considered ready for
reading.)
A descriptor shall be 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 shall be 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 it is implementation-defined. If the operation
is meaningless for a particular file type,
pselect() or select() shall indicate that the descriptor
is ready for read or write operations, and shall indicate
that the descriptor has no exceptional condition pending.
If a descriptor refers to a socket, the implied input function is
the recvmsg()
function with parameters requesting normal and ancillary data, such
that the presence of either type shall cause the socket to be
marked as readable. The presence of out-of-band data shall be 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 shall be marked as readable if an
incoming connection request has been received, and a call to the accept()
function
shall complete without blocking.
If a descriptor refers to a socket, the implied output function is
the sendmsg()
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 shall be marked as writable.
A socket shall be 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 shall
also be considered to have an exceptional condition pending if
an out-of-band data mark is present in the receive queue. Other circumstances
under which a socket may be considered to have an
exceptional condition pending are protocol-specific and implementation-defined.
If the readfds, writefds, and errorfds arguments
are all null pointers and the timeout argument is
not a null pointer, the pselect() or select() function
shall block 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, the pselect() or select() function
shall block until interrupted by a signal.
File descriptors associated with regular files shall 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 shall not be 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 shall have all bits set to 0.
File descriptor masks of type fd_set can be initialized and
tested with FD_CLR(), FD_ISSET(),
FD_SET(), and FD_ZERO(). It is unspecified whether each
of these is a macro or a function. If a macro definition is
suppressed in order to access an actual function, or a program defines
an external identifier with any of these names, the behavior
is undefined.
FD_CLR(fd, fdsetp) shall remove the file descriptor
fd from the set pointed to by fdsetp. If
fd is not a member of this set, there shall be no effect on
the set, nor will an error be returned.
FD_ISSET(fd, fdsetp) shall evaluate to non-zero
if the file descriptor fd is a member of the set
pointed to by fdsetp, and shall evaluate to zero otherwise.
FD_SET(fd, fdsetp) shall add the file descriptor
fd to the set pointed to by fdsetp. If the
file descriptor fd is already in this set, there shall be no
effect on the set, nor will an error be returned.
FD_ZERO(fdsetp) shall initialize the descriptor set pointed
to by fdsetp to the null set. No error is
returned if the set is not empty at the time FD_ZERO() is invoked.
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.
Upon successful completion, the pselect() and select()
functions shall return the total number of bits set in the
bit masks. Otherwise, -1 shall be returned, and errno shall
be set to indicate the error.
FD_CLR(), FD_SET(), and FD_ZERO() do not return
a value. FD_ISSET() shall return 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.
Under the following conditions, pselect() and select()
shall fail and set errno to:
If SA_RESTART has been set for the interrupting signal, it is implementation-defined
whether the function restarts or returns with
[EINTR].
The following sections are informative.
In previous versions of the Single UNIX Specification, the select()
function was defined in the <sys/time.h> header. This is now
changed to <sys/select.h>. The rationale for this change was
as follows: the introduction of
the pselect() function included the <sys/select.h> header
and the
<sys/select.h> header defines all the related definitions for
the
pselect() and select() functions. Backwards-compatibility
to existing XSI implementations is handled by allowing <sys/time.h>
to include <sys/select.h>.
accept(), alarm(), connect(), fcntl(),
poll(), read(), recvmsg(), sendmsg(),
setitimer(), ualarm(), write(),
the Base Definitions volume of IEEE Std 1003.1-2001, <sys/select.h>,
<sys/time.h>
RETURN VALUE
ERRORS
EXAMPLES
APPLICATION USAGE
RATIONALE
FUTURE DIRECTIONS
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
SEE ALSO