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PSELECT(P) PSELECT(P)
NAME
pselect, select - synchronous I/O multiplexing
SYNOPSIS
#include <sys/select.h>
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);
int select(int nfds, fd_set *restrict readfds,
fd_set *restrict writefds, fd_set *restrict errorfds,
struct timeval *restrict timeout);
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 excep-
tional condition pending, respectively.
The select() function shall be equivalent to the pselect() function, except as fol-
lows:
* For the select() function, the timeout period is given in seconds and microsec-
onds 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, STREAMS-based files, FIFOs, pipes, and sockets. The
behavior of pselect() and select() on file descriptors that refer to other types of
file is unspecified.
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 condi-
tions 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 pse-
lect() or select() function shall block until at least one of the requested opera-
tions 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 speci-
fies 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 pse-
lect() 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 implementa-
tion-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 exam-
ining 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 func-
tion 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 con-
dition 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 indi-
cate 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 pres-
ence 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 cur-
rently 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 with-
out 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 con-
dition 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 mem-
ber 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.
RETURN VALUE
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.
ERRORS
Under the following conditions, pselect() and select() shall fail and set errno to:
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-
defined whether the function 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.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
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() func-
tion included the <sys/select.h> header and the <sys/select.h> header defines all
the related definitions for the pselect() and select() functions. Backwards-compat-
ibility to existing XSI implementations is handled by allowing <sys/time.h> to
include <sys/select.h>.
FUTURE DIRECTIONS
None.
SEE ALSO
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>
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 .
POSIX 2003 PSELECT(P)
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