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eventfd.2: Describe eventfd2() and EFD_NONBLOCK and EFD_CLOEXEC 

Linux 2.6.27 added eventfd(), which supports a flags argument
that eventfd() did not provide.  The flags so far implemented
are EFD_NONBLOCK and EFD_CLOEXEC,

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2008-10-10 16:34:15 +02:00
parent b25c5b2388
commit e64b5482ad
1 changed files with 53 additions and 13 deletions
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66
man2/eventfd.2
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@ -16,7 +16,7 @@
.\" Foundation, Inc., 59 Temple Place, Suite 330, Boston,
.\" MA 02111-1307 USA
.\"
.TH EVENTFD 2 2008-08-19 Linux "Linux Programmer's Manual"
.TH EVENTFD 2 2008-10-10 Linux "Linux Programmer's Manual"
.SH NAME
eventfd \- create a file descriptor for event notification
.SH SYNOPSIS
@ -34,10 +34,32 @@ counter that is maintained by the kernel.
This counter is initialized with the value specified in the argument
.IR initval .
The
Starting with Linux 2.6.27, the following values may be bitwise ORed in
.IR flags
to change the behaviour of
.BR eventfd ():
.TP 14
.B EFD_NONBLOCK
Set the
.BR O_NONBLOCK
file status flag on the new open file description.
Using this flag saves extra calls to
.BR fcntl (2)
to achieve the same result.
.TP
.B EFD_CLOEXEC
Set the close-on-exec
.RB ( FD_CLOEXEC )
flag on the new file descriptor.
See the description of the
.B O_CLOEXEC
flag in
.BR open (2)
for reasons why this may be useful.
.PP
In Linux up to version 2.6.26, the
.I flags
argument is currently unused, and must be specified as zero.
In the future, it may be used to request additional functionality.
argument is unused, and must be specified as zero.
As its return value,
.BR eventfd ()
@ -58,13 +80,7 @@ If the counter is zero at the time of the
then the call either blocks until the counter becomes non-zero,
or fails with the error
.B EAGAIN
if the file descriptor has been made non-blocking
(via the use of the
.BR fcntl (2)
.B F_SETFL
operation to set the
.B O_NONBLOCK
flag).
if the file descriptor has been made non-blocking.
.IP
A
.BR read (2)
@ -180,8 +196,10 @@ is set to indicate the error.
.TP
.B EINVAL
.I flags
is invalid;
or, in Linux 2.6.26 or earlier,
.I flags
is non-zero.
.\" Eventually glibc may support some flags
.TP
.B EMFILE
The per-process limit on open file descriptors has been reached.
@ -206,9 +224,14 @@ eventfd file descriptor.
is available on Linux since kernel 2.6.22.
Working support is provided in glibc since version 2.8.
.\" eventfd() is in glibc 2.7, but reportedly does not build
The
.BR eventfd2 ()
system call (see NOTES) is available on Linux since kernel 2.6.27.
.SH CONFORMING TO
.BR eventfd ()
is Linux-specific.
and
.BR eventfd ()
are Linux-specific.
.SH NOTES
Applications can use an eventfd file descriptor instead of a pipe (see
.BR pipe (2))
@ -247,6 +270,23 @@ argument is a glibc addition to the underlying system call,
which takes only the
.I initval
argument.
.SS Underlying Linux system calls
There are two underlying Linux system calls:
.BR eventfd ()
and the more recent
.BR eventfd2 ().
The former system call does not implement a
.I flags
argument.
The latter system call implements the
.I flags
values described above.
.\" FIXME as at 2008-10-10, the glibc snapshot doesn't seem
.\" to make use of eventfd2() yet. Check later to see what
.\" glibc version adds support for this syscall.
The glibc wrapper function will use
.BR eventfd2 ()
where it is available.
.SS Additional glibc features
The GNU C library defines an additional type,
and two functions that attempt to abstract some of the details of