Originální popis anglicky:
pselect, select - synchronous I/O multiplexing
Návod, kniha: POSIX Programmer's Manual
#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 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:
- 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.
None.
None.
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>.
None.
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>
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
.
