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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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PSELECT(3P) POSIX Programmer's Manual PSELECT(3P)
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.
pselect, select — synchronous I/O multiplexing
#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);
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.
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:
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.
None.
None.
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;
}
None.
accept(3p), alarm(3p), connect(3p), fcntl(3p), getitimer(3p),
poll(3p), read(3p), recvmsg(3p), sendmsg(3p), write(3p)
The Base Definitions volume of POSIX.1‐2017, sys_select.h(0p),
sys_time.h(0p)
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 .
IEEE/The Open Group 2017 PSELECT(3P)
Pages that refer to this page: sys_select.h(0p), sys_time.h(0p), connect(3p), FD_CLR(3p), poll(3p), recv(3p), recvfrom(3p), recvmsg(3p), select(3p), send(3p), sendmsg(3p), sendto(3p), shutdown(3p), sockatmark(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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