mirror of https://github.com/mkerrisk/man-pages
Jamie Lokier / mtk
Improved discussion of F_SETOWN and F_SETSIG with respect to multi-threaded programs. Generally cleaned up the discussion of F_SETOWN.
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Michael Kerrisk
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87
man2/fcntl.2
87
man2/fcntl.2
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@ -45,6 +45,10 @@
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.\" Added notes on capability requirements
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.\" Modified 2004-12-08, added O_NOATIME after note from Martin Pool
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.\" 2004-12-10, mtk, noted F_GETOWN bug after suggestion from aeb.
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.\" 2005-04-08 Jamie Lokier <jamie@shareable.org>, mtk
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.\" Described behaviour of F_SETOWN/F_SETSIG in
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.\" multi-threaded processes, and generally cleaned
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.\" up the discussion of F_SETOWN.
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.\"
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.TH FCNTL 2 2004-12-08 "Linux 2.6.9" "Linux Programmer's Manual"
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.SH NAME
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@ -334,40 +338,85 @@ are used to manage I/O availability signals:
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Get the process ID or process group currently receiving SIGIO
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and SIGURG signals for events on file descriptor
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.IR fd .
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Process groups are returned as negative values (but see BUGS below).
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Process IDs are returned as positive values;
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process group IDs are returned as negative values (but see BUGS below).
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.TP
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.B F_SETOWN
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Set the process ID or process group that will receive SIGIO
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Set the process ID or process group ID that will receive SIGIO
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and SIGURG signals for events on file descriptor
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.IR fd .
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Process groups are specified using negative values.
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.RB ( F_SETSIG
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can be used to specify a different signal instead of SIGIO).
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A process ID is specified as a positive value;
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a process group ID is specified as a negative value.
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.\" From glibc.info:
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If you set the
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.B O_ASYNC
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status flag on a file descriptor (either by providing this flag with the
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.IR open (2)
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.BR open (2)
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.\" FIXME setting O_ASYNC via open() does not seem to be effective -- MTK
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call, or by using the
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.B F_SETFL
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command of
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.BR fcntl ),
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a SIGIO signal is sent whenever input or output becomes possible
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on that file descriptor.
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.B F_SETSIG
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can be used to obtain delivery of a signal other than SIGIO.
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.sp
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The process or process group to receive the signal can be selected by
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using the
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If the file descriptor
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.I fd
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refers to a socket,
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.B F_SETOWN
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command to the
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.B fcntl
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function. If the file descriptor is a socket, this also selects
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also selects
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the recipient of SIGURG signals that are delivered when out-of-band
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data arrives on that socket. (SIGURG is sent in any situation where
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.BR select (2)
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would report the socket as having an "exceptional condition".)
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If the file descriptor corresponds to a terminal device, then SIGIO
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signals are sent to the foreground process group of the terminal.
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.\" The following appears to be rubbish. It doesn't seem to
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.\" be true according to the kernel source, and I can write
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.\" a program that gets a terminal-generated SIGIO even though
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.\" it is not the foreground process group of the terminal.
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.\" -- MTK, 8 Apr 05
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.\"
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.\" If the file descriptor
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.\" .I fd
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.\" refers to a terminal device, then SIGIO
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.\" signals are sent to the foreground process group of the terminal.
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If a non-zero value is given to
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.B F_SETSIG
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in a multi-threaded process, then a positive value value given to
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.B F_SETOWN
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has a different meaning:
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.\" The relevant place in the (2.6) kernel source is the
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.\" 'switch' in fs/fcntl.c::send_sigio_to_task() -- MTK, Apr 2005
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instead of being a process ID identifying a whole process,
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it is a thread ID identifying a specific thread within a process.
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Consequently, it may be necessary to pass
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.B F_SETOWN
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the result of
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.BR gettid ()
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instead of
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.BR getpid ()
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to get sensible results when
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.B F_SETSIG
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is used.
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(In current Linux threading implementations,
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a main thread's thread ID is the same as its process ID.
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This means that a single-threaded program can equally use
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.BR gettid ()
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or
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.BR getpid ()
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in this scenario.)
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Note, however, that the statements in this paragraph do not apply
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to the SIGURG signal generated for out-of-band data on a socket:
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this signal is always sent to either a process or a process group,
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depending on the value given to
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.BR F_SETOWN .
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.\" send_sigurg()/send_sigurg_to_task() bypasses
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.\" kill_fasync()/send_sigio()/send_sigio_to_task()
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.\" to directly call send_group_sig_info()
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.\" -- MTK, Apr 2005 (kernel 2.6.11)
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.TP
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.B F_GETSIG
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Get the signal sent when input or output becomes possible. A value of
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@ -381,7 +430,17 @@ zero means to send the default SIGIO signal. Any other value (including
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SIGIO) is the signal to send instead, and in this case additional info
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is available to the signal handler if installed with SA_SIGINFO.
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.sp
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By using F_SETSIG with a non-zero value, and setting SA_SIGINFO for the
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Additionally, passing a non-zero value to
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.B F_SETSIG
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changes the signal recipient from a whole process to a specific thread
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within a process.
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See the desciption of
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.B F_SETOWN
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for more details.
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.sp
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By using
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.B F_SETSIG
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with a non-zero value, and setting SA_SIGINFO for the
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signal handler (see
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.BR sigaction (2)),
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extra information about I/O events is passed to
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