Adding upstream version 6.05.01.upstream/6.05.01

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+.\" Copyright (C) 2006, 2019 Michael Kerrisk <mtk.manpages@gmail.com>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.\" Additions from Richard Gooch <rgooch@atnf.CSIRO.AU> and aeb, 971207
+.\" 2006-03-13, mtk, Added ppoll() + various other rewordings
+.\" 2006-07-01, mtk, Added POLLRDHUP + various other wording and
+.\"	formatting changes.
+.\"
+.TH poll 2 2023-07-08 "Linux man-pages 6.05.01"
+.SH NAME
+poll, ppoll \- wait for some event on a file descriptor
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
+.SH SYNOPSIS
+.nf
+.B #include <poll.h>
+.PP
+.BI "int poll(struct pollfd *" fds ", nfds_t " nfds ", int " timeout );
+.PP
+.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
+.B #include <poll.h>
+.PP
+.BI "int ppoll(struct pollfd *" fds ", nfds_t " nfds ,
+.BI "          const struct timespec *_Nullable " tmo_p ,
+.BI "          const sigset_t *_Nullable " sigmask );
+.fi
+.SH DESCRIPTION
+.BR poll ()
+performs a similar task to
+.BR select (2):
+it waits for one of a set of file descriptors to become ready
+to perform I/O.
+The Linux-specific
+.BR epoll (7)
+API performs a similar task, but offers features beyond those found in
+.BR poll ().
+.PP
+The set of file descriptors to be monitored is specified in the
+.I fds
+argument, which is an array of structures of the following form:
+.PP
+.in +4n
+.EX
+struct pollfd {
+    int   fd;         /* file descriptor */
+    short events;     /* requested events */
+    short revents;    /* returned events */
+};
+.EE
+.in
+.PP
+The caller should specify the number of items in the
+.I fds
+array in
+.IR nfds .
+.PP
+The field
+.I fd
+contains a file descriptor for an open file.
+If this field is negative, then the corresponding
+.I events
+field is ignored and the
+.I revents
+field returns zero.
+(This provides an easy way of ignoring a
+file descriptor for a single
+.BR poll ()
+call: simply set the
+.I fd
+field to its bitwise complement.)
+.PP
+The field
+.I events
+is an input parameter, a bit mask specifying the events the application
+is interested in for the file descriptor
+.IR fd .
+This field may be specified as zero,
+in which case the only events that can be returned in
+.I revents
+are
+.BR POLLHUP ,
+.BR POLLERR ,
+and
+.B POLLNVAL
+(see below).
+.PP
+The field
+.I revents
+is an output parameter, filled by the kernel with the events that
+actually occurred.
+The bits returned in
+.I revents
+can include any of those specified in
+.IR events ,
+or one of the values
+.BR POLLERR ,
+.BR POLLHUP ,
+or
+.BR POLLNVAL .
+(These three bits are meaningless in the
+.I events
+field, and will be set in the
+.I revents
+field whenever the corresponding condition is true.)
+.PP
+If none of the events requested (and no error) has occurred for any
+of the file descriptors, then
+.BR poll ()
+blocks until one of the events occurs.
+.PP
+The
+.I timeout
+argument specifies the number of milliseconds that
+.BR poll ()
+should block waiting for a file descriptor to become ready.
+The call will block until either:
+.IP \[bu] 3
+a file descriptor becomes ready;
+.IP \[bu]
+the call is interrupted by a signal handler; or
+.IP \[bu]
+the timeout expires.
+.PP
+Being "ready" means that the requested operation will not block; thus,
+.BR poll ()ing
+regular files,
+block devices,
+and other files with no reasonable polling semantic
+.I always
+returns instantly as ready to read and write.
+.PP
+Note that the
+.I timeout
+interval will be rounded up to the system clock granularity,
+and kernel scheduling delays mean that the blocking interval
+may overrun by a small amount.
+Specifying a negative value in
+.I timeout
+means an infinite timeout.
+Specifying a
+.I timeout
+of zero causes
+.BR poll ()
+to return immediately, even if no file descriptors are ready.
+.PP
+The bits that may be set/returned in
+.I events
+and
+.I revents
+are defined in \fI<poll.h>\fP:
+.TP
+.B POLLIN
+There is data to read.
+.TP
+.B POLLPRI
+There is some exceptional condition on the file descriptor.
+Possibilities include:
+.RS
+.IP \[bu] 3
+There is out-of-band data on a TCP socket (see
+.BR tcp (7)).
+.IP \[bu]
+A pseudoterminal master in packet mode has seen a state change on the slave
+(see
+.BR ioctl_tty (2)).
+.IP \[bu]
+A
+.I cgroup.events
+file has been modified (see
+.BR cgroups (7)).
+.RE
+.TP
+.B POLLOUT
+Writing is now possible, though a write larger than the available space
+in a socket or pipe will still block (unless
+.B O_NONBLOCK
+is set).
+.TP
+.BR POLLRDHUP " (since Linux 2.6.17)"
+Stream socket peer closed connection,
+or shut down writing half of connection.
+The
+.B _GNU_SOURCE
+feature test macro must be defined
+(before including
+.I any
+header files)
+in order to obtain this definition.
+.TP
+.B POLLERR
+Error condition (only returned in
+.IR revents ;
+ignored in
+.IR events ).
+This bit is also set for a file descriptor referring
+to the write end of a pipe when the read end has been closed.
+.TP
+.B POLLHUP
+Hang up (only returned in
+.IR revents ;
+ignored in
+.IR events ).
+Note that when reading from a channel such as a pipe or a stream socket,
+this event merely indicates that the peer closed its end of the channel.
+Subsequent reads from the channel will return 0 (end of file)
+only after all outstanding data in the channel has been consumed.
+.TP
+.B POLLNVAL
+Invalid request:
+.I fd
+not open (only returned in
+.IR revents ;
+ignored in
+.IR events ).
+.PP
+When compiling with
+.B _XOPEN_SOURCE
+defined, one also has the following,
+which convey no further information beyond the bits listed above:
+.TP
+.B POLLRDNORM
+Equivalent to
+.BR POLLIN .
+.TP
+.B POLLRDBAND
+Priority band data can be read (generally unused on Linux).
+.\" POLLRDBAND is used in the DECnet protocol.
+.TP
+.B POLLWRNORM
+Equivalent to
+.BR POLLOUT .
+.TP
+.B POLLWRBAND
+Priority data may be written.
+.PP
+Linux also knows about, but does not use
+.BR POLLMSG .
+.SS ppoll()
+The relationship between
+.BR poll ()
+and
+.BR ppoll ()
+is analogous to the relationship between
+.BR select (2)
+and
+.BR pselect (2):
+like
+.BR pselect (2),
+.BR ppoll ()
+allows an application to safely wait until either a file descriptor
+becomes ready or until a signal is caught.
+.PP
+Other than the difference in the precision of the
+.I timeout
+argument, the following
+.BR ppoll ()
+call:
+.PP
+.in +4n
+.EX
+ready = ppoll(&fds, nfds, tmo_p, &sigmask);
+.EE
+.in
+.PP
+is nearly equivalent to
+.I atomically
+executing the following calls:
+.PP
+.in +4n
+.EX
+sigset_t origmask;
+int timeout;
+\&
+timeout = (tmo_p == NULL) ? \-1 :
+          (tmo_p\->tv_sec * 1000 + tmo_p\->tv_nsec / 1000000);
+pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
+ready = poll(&fds, nfds, timeout);
+pthread_sigmask(SIG_SETMASK, &origmask, NULL);
+.EE
+.in
+.PP
+The above code segment is described as
+.I nearly
+equivalent because whereas a negative
+.I timeout
+value for
+.BR poll ()
+is interpreted as an infinite timeout, a negative value expressed in
+.I *tmo_p
+results in an error from
+.BR ppoll ().
+.PP
+See the description of
+.BR pselect (2)
+for an explanation of why
+.BR ppoll ()
+is necessary.
+.PP
+If the
+.I sigmask
+argument is specified as NULL, then
+no signal mask manipulation is performed
+(and thus
+.BR ppoll ()
+differs from
+.BR poll ()
+only in the precision of the
+.I timeout
+argument).
+.PP
+The
+.I tmo_p
+argument specifies an upper limit on the amount of time that
+.BR ppoll ()
+will block.
+This argument is a pointer to a
+.BR timespec (3)
+structure.
+.PP
+If
+.I tmo_p
+is specified as NULL, then
+.BR ppoll ()
+can block indefinitely.
+.SH RETURN VALUE
+On success,
+.BR poll ()
+returns a nonnegative value which is the number of elements in the
+.I pollfds
+whose
+.I revents
+fields have been set to a nonzero value (indicating an event or an error).
+A return value of zero indicates that the system call timed out
+before any file descriptors became ready.
+.PP
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EFAULT
+.I fds
+points outside the process's accessible address space.
+The array given as argument was not contained in the calling program's
+address space.
+.TP
+.B EINTR
+A signal occurred before any requested event; see
+.BR signal (7).
+.TP
+.B EINVAL
+The
+.I nfds
+value exceeds the
+.B RLIMIT_NOFILE
+value.
+.TP
+.B EINVAL
+.RB ( ppoll ())
+The timeout value expressed in
+.I *tmo_p
+is invalid (negative).
+.TP
+.B ENOMEM
+Unable to allocate memory for kernel data structures.
+.SH VERSIONS
+On some other UNIX systems,
+.\" Darwin, according to a report by Jeremy Sequoia, relayed by Josh Triplett
+.BR poll ()
+can fail with the error
+.B EAGAIN
+if the system fails to allocate kernel-internal resources, rather than
+.B ENOMEM
+as Linux does.
+POSIX permits this behavior.
+Portable programs may wish to check for
+.B EAGAIN
+and loop, just as with
+.BR EINTR .
+.PP
+Some implementations define the nonstandard constant
+.B INFTIM
+with the value \-1 for use as a
+.I timeout
+for
+.BR poll ().
+This constant is not provided in glibc.
+.SS C library/kernel differences
+The Linux
+.BR ppoll ()
+system call modifies its
+.I tmo_p
+argument.
+However, the glibc wrapper function hides this behavior
+by using a local variable for the timeout argument that
+is passed to the system call.
+Thus, the glibc
+.BR ppoll ()
+function does not modify its
+.I tmo_p
+argument.
+.PP
+The raw
+.BR ppoll ()
+system call has a fifth argument,
+.IR "size_t sigsetsize" ,
+which specifies the size in bytes of the
+.I sigmask
+argument.
+The glibc
+.BR ppoll ()
+wrapper function specifies this argument as a fixed value
+(equal to
+.IR sizeof(kernel_sigset_t) ).
+See
+.BR sigprocmask (2)
+for a discussion on the differences between the kernel and the libc
+notion of the sigset.
+.SH STANDARDS
+.TP
+.BR poll ()
+POSIX.1-2008.
+.TP
+.BR ppoll ()
+Linux.
+.\" FIXME .
+.\" ppoll() is proposed for inclusion in POSIX:
+.\" https://www.austingroupbugs.net/view.php?id=1263
+.\" NetBSD 3.0 has a pollts() which is like Linux ppoll().
+.SH HISTORY
+.TP
+.BR poll ()
+POSIX.1-2001.
+Linux 2.1.23.
+.IP
+On older kernels that lack this system call,
+the glibc
+.BR poll ()
+wrapper function provides emulation using
+.BR select (2).
+.TP
+.BR ppoll ()
+Linux 2.6.16,
+glibc 2.4.
+.SH NOTES
+The operation of
+.BR poll ()
+and
+.BR ppoll ()
+is not affected by the
+.B O_NONBLOCK
+flag.
+.PP
+For a discussion of what may happen if a file descriptor being monitored by
+.BR poll ()
+is closed in another thread, see
+.BR select (2).
+.SH BUGS
+See the discussion of spurious readiness notifications under the
+BUGS section of
+.BR select (2).
+.SH EXAMPLES
+The program below opens each of the files named in its command-line
+arguments and monitors the resulting file descriptors for readiness to read
+.RB ( POLLIN ).
+The program loops, repeatedly using
+.BR poll ()
+to monitor the file descriptors,
+printing the number of ready file descriptors on return.
+For each ready file descriptor, the program:
+.IP \[bu] 3
+displays the returned
+.I revents
+field in a human-readable form;
+.IP \[bu]
+if the file descriptor is readable, reads some data from it,
+and displays that data on standard output; and
+.IP \[bu]
+if the file descriptor was not readable,
+but some other event occurred (presumably
+.BR POLLHUP ),
+closes the file descriptor.
+.PP
+Suppose we run the program in one terminal, asking it to open a FIFO:
+.PP
+.in +4n
+.EX
+$ \fBmkfifo myfifo\fP
+$ \fB./poll_input myfifo\fP
+.EE
+.in
+.PP
+In a second terminal window, we then open the FIFO for writing,
+write some data to it, and close the FIFO:
+.PP
+.in +4n
+.EX
+$ \fBecho aaaaabbbbbccccc > myfifo\fP
+.EE
+.in
+.PP
+In the terminal where we are running the program, we would then see:
+.PP
+.in +4n
+.EX
+Opened "myfifo" on fd 3
+About to poll()
+Ready: 1
+  fd=3; events: POLLIN POLLHUP
+    read 10 bytes: aaaaabbbbb
+About to poll()
+Ready: 1
+  fd=3; events: POLLIN POLLHUP
+    read 6 bytes: ccccc
+\&
+About to poll()
+Ready: 1
+  fd=3; events: POLLHUP
+    closing fd 3
+All file descriptors closed; bye
+.EE
+.in
+.PP
+In the above output, we see that
+.BR poll ()
+returned three times:
+.IP \[bu] 3
+On the first return, the bits returned in the
+.I revents
+field were
+.BR POLLIN ,
+indicating that the file descriptor is readable, and
+.BR POLLHUP ,
+indicating that the other end of the FIFO has been closed.
+The program then consumed some of the available input.
+.IP \[bu]
+The second return from
+.BR poll ()
+also indicated
+.B POLLIN
+and
+.BR POLLHUP ;
+the program then consumed the last of the available input.
+.IP \[bu]
+On the final return,
+.BR poll ()
+indicated only
+.B POLLHUP
+on the FIFO,
+at which point the file descriptor was closed and the program terminated.
+.\"
+.SS Program source
+\&
+.\" SRC BEGIN (poll_input.c)
+.EX
+/* poll_input.c
+\&
+   Licensed under GNU General Public License v2 or later.
+*/
+#include <fcntl.h>
+#include <poll.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+\&
+#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \e
+                        } while (0)
+\&
+int
+main(int argc, char *argv[])
+{
+    int            ready;
+    char           buf[10];
+    nfds_t         num_open_fds, nfds;
+    ssize_t        s;
+    struct pollfd  *pfds;
+\&
+    if (argc < 2) {
+       fprintf(stderr, "Usage: %s file...\en", argv[0]);
+       exit(EXIT_FAILURE);
+    }
+\&
+    num_open_fds = nfds = argc \- 1;
+    pfds = calloc(nfds, sizeof(struct pollfd));
+    if (pfds == NULL)
+        errExit("malloc");
+\&
+    /* Open each file on command line, and add it to \[aq]pfds\[aq] array. */
+\&
+    for (nfds_t j = 0; j < nfds; j++) {
+        pfds[j].fd = open(argv[j + 1], O_RDONLY);
+        if (pfds[j].fd == \-1)
+            errExit("open");
+\&
+        printf("Opened \e"%s\e" on fd %d\en", argv[j + 1], pfds[j].fd);
+\&
+        pfds[j].events = POLLIN;
+    }
+\&
+    /* Keep calling poll() as long as at least one file descriptor is
+       open. */
+\&
+    while (num_open_fds > 0) {
+        printf("About to poll()\en");
+        ready = poll(pfds, nfds, \-1);
+        if (ready == \-1)
+            errExit("poll");
+\&
+        printf("Ready: %d\en", ready);
+\&
+        /* Deal with array returned by poll(). */
+\&
+        for (nfds_t j = 0; j < nfds; j++) {
+            if (pfds[j].revents != 0) {
+                printf("  fd=%d; events: %s%s%s\en", pfds[j].fd,
+                       (pfds[j].revents & POLLIN)  ? "POLLIN "  : "",
+                       (pfds[j].revents & POLLHUP) ? "POLLHUP " : "",
+                       (pfds[j].revents & POLLERR) ? "POLLERR " : "");
+\&
+                if (pfds[j].revents & POLLIN) {
+                    s = read(pfds[j].fd, buf, sizeof(buf));
+                    if (s == \-1)
+                        errExit("read");
+                    printf("    read %zd bytes: %.*s\en",
+                           s, (int) s, buf);
+                } else {                /* POLLERR | POLLHUP */
+                    printf("    closing fd %d\en", pfds[j].fd);
+                    if (close(pfds[j].fd) == \-1)
+                        errExit("close");
+                    num_open_fds\-\-;
+                }
+            }
+        }
+    }
+\&
+    printf("All file descriptors closed; bye\en");
+    exit(EXIT_SUCCESS);
+}
+.EE
+.\" SRC END
+.SH SEE ALSO
+.BR restart_syscall (2),
+.BR select (2),
+.BR select_tut (2),
+.BR timespec (3),
+.BR epoll (7),
+.BR time (7)