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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,
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 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, 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 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 upon
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() 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 caller by the set of signals pointed to by sigmask before examining the
descriptors, and shall restore the signal mask of the calling thread 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.
If a thread gets canceled during a pselect() call, the signal mask in effect when
executing the registered cleanup functions is either the original signal mask or the
signal mask installed as part of the pselect() call.
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 while blocked waiting for any of the selected
descriptors to become ready 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 earlier 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>.
Code which wants to avoid the ambiguity of the signal mask for thread cancellation
handlers can install an additional cancellation handler which resets the signal mask to
the expected value.
void cleanup(void *arg)
{
sigset_t *ss = (sigset_t *) arg;
pthread_sigmask(SIG_SETMASK, ss, NULL);
}
int call_pselect(int nfds, fd_set *readfds, fd_set *writefds,
fd_set errorfds, const struct timespec *timeout,
const sigset_t *sigmask)
{
sigset_t oldmask;
int result;
pthread_sigmask(SIG_SETMASK, NULL, &oldmask);
pthread_cleanup_push(cleanup, &oldmask);
result = pselect(nfds, readfds, writefds, errorfds, timeout, sigmask);
pthread_cleanup_pop(0);
return result;
}
FUTURE DIRECTIONS
None.
SEE ALSO
accept(), alarm(), connect(), fcntl(), getitimer(), poll(), read(), recvmsg(), sendmsg(),
write()
The Base Definitions volume of POSIX.1‐2017, <sys_select.h>, <sys_time.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .