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select_tut.2: DESCRIPTION: defer to select(2)
Avoid duplicating the same information in two pages. Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Michael Kerrisk
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.\"
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.TH SELECT_TUT 2 2019-03-06 "Linux" "Linux Programmer's Manual"
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.SH NAME
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select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO \-
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synchronous I/O multiplexing
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select, pselect \- synchronous I/O multiplexing
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.SH SYNOPSIS
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.PP
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See
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@ -46,135 +45,16 @@ to see if any of them is, or becomes, "ready";
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that is, to see whether I/O becomes possible,
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or an "exceptional condition" has occurred on any of the file descriptors.
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.PP
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Its principal arguments are three "sets" of file descriptors:
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\fIreadfds\fP, \fIwritefds\fP, and \fIexceptfds\fP.
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Each set is declared as type
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.IR fd_set ,
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and its contents can be manipulated with the macros
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.BR FD_CLR (),
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.BR FD_ISSET (),
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.BR FD_SET (),
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This page provides background and tutorial information
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on the use of these system calls.
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For details of the arguments and semantics of
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.BR select ()
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and
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.BR FD_ZERO ().
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A newly declared set should first be cleared using
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.BR FD_ZERO ().
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.BR select ()
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modifies the contents of the sets according to the rules
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described below; after calling
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.BR select ()
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you can test if a file descriptor is still present in a set with the
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.BR FD_ISSET ()
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macro.
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.BR FD_ISSET ()
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returns nonzero if a specified file descriptor is present in a set
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and zero if it is not.
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.BR FD_CLR ()
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removes a file descriptor from a set.
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.SS Arguments
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.TP
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\fIreadfds\fP
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This set is watched to see if data is available for reading from any of
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its file descriptors.
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After
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.BR select ()
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has returned, \fIreadfds\fP will be
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cleared of all file descriptors except for those that
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are immediately available for reading.
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.TP
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\fIwritefds\fP
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This set is watched to see if there is space to write data to any of
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its file descriptors.
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After
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.BR select ()
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has returned, \fIwritefds\fP will be
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cleared of all file descriptors except for those that
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are immediately available for writing.
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.TP
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\fIexceptfds\fP
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This set is watched for "exceptional conditions".
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In practice, only one such exceptional condition is common:
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the availability of \fIout-of-band\fP (OOB) data for reading
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from a TCP socket.
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See
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.BR recv (2),
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.BR send (2),
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and
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.BR tcp (7)
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for more details about OOB data.
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(One other less common case where
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.BR select (2)
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indicates an exceptional condition occurs with pseudoterminals
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in packet mode; see
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.BR ioctl_tty (2).)
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After
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.BR select ()
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has returned,
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\fIexceptfds\fP will be cleared of all file descriptors except for those
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for which an exceptional condition has occurred.
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.TP
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\fInfds\fP
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This is an integer one more than the maximum of any file descriptor in
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any of the sets.
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In other words, while adding file descriptors to each of the sets,
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you must calculate the maximum integer value of all of them,
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then increment this value by one, and then pass this as \fInfds\fP.
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.TP
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\fIutimeout\fP
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This is the longest time
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.BR select ()
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may wait before returning, even if nothing interesting happened.
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If this value is passed as NULL, then
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.BR select ()
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blocks indefinitely waiting for a file descriptor to become ready.
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\fIutimeout\fP can be set to zero seconds, which causes
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.BR select ()
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to return immediately, with information about the readiness
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of file descriptors at the time of the call.
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The structure \fIstruct timeval\fP is defined as:
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.IP
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.in +4n
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.EX
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struct timeval {
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time_t tv_sec; /* seconds */
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long tv_usec; /* microseconds */
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};
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.EE
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.in
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.TP
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\fIntimeout\fP
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This argument for
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.BR pselect ()
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has the same meaning as
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.IR utimeout ,
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but
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.I "struct timespec"
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has nanosecond precision as follows:
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.IP
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.in +4n
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.EX
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struct timespec {
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long tv_sec; /* seconds */
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long tv_nsec; /* nanoseconds */
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};
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.EE
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.in
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.TP
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\fIsigmask\fP
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This argument holds a set of signals that the kernel should unblock
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(i.e., remove from the signal mask of the calling thread),
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while the caller is blocked inside the
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.BR pselect ()
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call (see
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.BR sigaddset (3)
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and
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.BR sigprocmask (2)).
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It may be NULL,
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in which case the call does not modify the signal mask on
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entry and exit to the function.
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In this case,
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.BR pselect ()
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will then behave just like
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.BR select ().
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.BR pselect (),
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see
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.BR select (2).
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.PP
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.\"
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.SS Combining signal and data events
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.BR pselect ()
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is useful if you are waiting for a signal as well as
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