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Diffstat (limited to 'man2/eventfd.2')
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diff --git a/man2/eventfd.2 b/man2/eventfd.2 deleted file mode 100644 index 2de063e..0000000 --- a/man2/eventfd.2 +++ /dev/null @@ -1,446 +0,0 @@ -'\" t -.\" Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com> -.\" starting from a version by Davide Libenzi <davidel@xmailserver.org> -.\" -.\" SPDX-License-Identifier: GPL-2.0-or-later -.\" -.\" 2008-10-10, mtk: describe eventfd2(), and EFD_NONBLOCK and EFD_CLOEXEC -.\" -.TH eventfd 2 2023-10-31 "Linux man-pages 6.7" -.SH NAME -eventfd \- create a file descriptor for event notification -.SH LIBRARY -Standard C library -.RI ( libc ", " \-lc ) -.SH SYNOPSIS -.nf -.B #include <sys/eventfd.h> -.P -.BI "int eventfd(unsigned int " initval ", int " flags ); -.fi -.SH DESCRIPTION -.BR eventfd () -creates an "eventfd object" that can be used as -an event wait/notify mechanism by user-space applications, -and by the kernel to notify user-space applications of events. -The object contains an unsigned 64-bit integer -.RI ( uint64_t ) -counter that is maintained by the kernel. -This counter is initialized with the value specified in the argument -.IR initval . -.P -As its return value, -.BR eventfd () -returns a new file descriptor that can be used to refer to the -eventfd object. -.P -The following values may be bitwise ORed in -.I flags -to change the behavior of -.BR eventfd (): -.TP -.BR EFD_CLOEXEC " (since Linux 2.6.27)" -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. -.TP -.BR EFD_NONBLOCK " (since Linux 2.6.27)" -Set the -.B O_NONBLOCK -file status flag on the open file description (see -.BR open (2)) -referred to by the new file descriptor. -Using this flag saves extra calls to -.BR fcntl (2) -to achieve the same result. -.TP -.BR EFD_SEMAPHORE " (since Linux 2.6.30)" -Provide semaphore-like semantics for reads from the new file descriptor. -See below. -.P -Up to Linux 2.6.26, the -.I flags -argument is unused, and must be specified as zero. -.P -The following operations can be performed on the file descriptor returned by -.BR eventfd (): -.TP -.BR read (2) -Each successful -.BR read (2) -returns an 8-byte integer. -A -.BR read (2) -fails with the error -.B EINVAL -if the size of the supplied buffer is less than 8 bytes. -.IP -The value returned by -.BR read (2) -is in host byte order\[em]that is, -the native byte order for integers on the host machine. -.IP -The semantics of -.BR read (2) -depend on whether the eventfd counter currently has a nonzero value -and whether the -.B EFD_SEMAPHORE -flag was specified when creating the eventfd file descriptor: -.RS -.IP \[bu] 3 -If -.B EFD_SEMAPHORE -was not specified and the eventfd counter has a nonzero value, then a -.BR read (2) -returns 8 bytes containing that value, -and the counter's value is reset to zero. -.IP \[bu] -If -.B EFD_SEMAPHORE -was specified and the eventfd counter has a nonzero value, then a -.BR read (2) -returns 8 bytes containing the value 1, -and the counter's value is decremented by 1. -.IP \[bu] -If the eventfd counter is zero at the time of the call to -.BR read (2), -then the call either blocks until the counter becomes nonzero -(at which time, the -.BR read (2) -proceeds as described above) -or fails with the error -.B EAGAIN -if the file descriptor has been made nonblocking. -.RE -.TP -.BR write (2) -A -.BR write (2) -call adds the 8-byte integer value supplied in its -buffer to the counter. -The maximum value that may be stored in the counter is the largest -unsigned 64-bit value minus 1 (i.e., 0xfffffffffffffffe). -If the addition would cause the counter's value to exceed -the maximum, then the -.BR write (2) -either blocks until a -.BR read (2) -is performed on the file descriptor, -or fails with the error -.B EAGAIN -if the file descriptor has been made nonblocking. -.IP -A -.BR write (2) -fails with the error -.B EINVAL -if the size of the supplied buffer is less than 8 bytes, -or if an attempt is made to write the value 0xffffffffffffffff. -.TP -.BR poll (2) -.TQ -.BR select (2) -.TQ -(and similar) -The returned file descriptor supports -.BR poll (2) -(and analogously -.BR epoll (7)) -and -.BR select (2), -as follows: -.RS -.IP \[bu] 3 -The file descriptor is readable -(the -.BR select (2) -.I readfds -argument; the -.BR poll (2) -.B POLLIN -flag) -if the counter has a value greater than 0. -.IP \[bu] -The file descriptor is writable -(the -.BR select (2) -.I writefds -argument; the -.BR poll (2) -.B POLLOUT -flag) -if it is possible to write a value of at least "1" without blocking. -.IP \[bu] -If an overflow of the counter value was detected, -then -.BR select (2) -indicates the file descriptor as being both readable and writable, and -.BR poll (2) -returns a -.B POLLERR -event. -As noted above, -.BR write (2) -can never overflow the counter. -However an overflow can occur if 2\[ha]64 -eventfd "signal posts" were performed by the KAIO -subsystem (theoretically possible, but practically unlikely). -If an overflow has occurred, then -.BR read (2) -will return that maximum -.I uint64_t -value (i.e., 0xffffffffffffffff). -.RE -.IP -The eventfd file descriptor also supports the other file-descriptor -multiplexing APIs: -.BR pselect (2) -and -.BR ppoll (2). -.TP -.BR close (2) -When the file descriptor is no longer required it should be closed. -When all file descriptors associated with the same eventfd object -have been closed, the resources for object are freed by the kernel. -.P -A copy of the file descriptor created by -.BR eventfd () -is inherited by the child produced by -.BR fork (2). -The duplicate file descriptor is associated with the same -eventfd object. -File descriptors created by -.BR eventfd () -are preserved across -.BR execve (2), -unless the close-on-exec flag has been set. -.SH RETURN VALUE -On success, -.BR eventfd () -returns a new eventfd file descriptor. -On error, \-1 is returned and -.I errno -is set to indicate the error. -.SH ERRORS -.TP -.B EINVAL -An unsupported value was specified in -.IR flags . -.TP -.B EMFILE -The per-process limit on the number of open file descriptors has been reached. -.TP -.B ENFILE -The system-wide limit on the total number of open files has been -reached. -.TP -.B ENODEV -.\" Note from Davide: -.\" The ENODEV error is basically never going to happen if -.\" the kernel boots correctly. That error happen only if during -.\" the kernel initialization, some error occur in the anonymous -.\" inode source initialization. -Could not mount (internal) anonymous inode device. -.TP -.B ENOMEM -There was insufficient memory to create a new -eventfd file descriptor. -.SH ATTRIBUTES -For an explanation of the terms used in this section, see -.BR attributes (7). -.TS -allbox; -lbx lb lb -l l l. -Interface Attribute Value -T{ -.na -.nh -.BR eventfd () -T} Thread safety MT-Safe -.TE -.SH VERSIONS -.SS C library/kernel differences -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. -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 -reading and writing on an eventfd file descriptor: -.P -.in +4n -.EX -typedef uint64_t eventfd_t; -\& -int eventfd_read(int fd, eventfd_t *value); -int eventfd_write(int fd, eventfd_t value); -.EE -.in -.P -The functions perform the read and write operations on an -eventfd file descriptor, -returning 0 if the correct number of bytes was transferred, -or \-1 otherwise. -.SH STANDARDS -Linux, GNU. -.SH HISTORY -.TP -.BR eventfd () -Linux 2.6.22, -glibc 2.8. -.\" eventfd() is in glibc 2.7, but reportedly does not build -.TP -.BR eventfd2 () -Linux 2.6.27 (see VERSIONS). -Since glibc 2.9, the -.BR eventfd () -wrapper will employ the -.BR eventfd2 () -system call, if it is supported by the kernel. -.SH NOTES -Applications can use an eventfd file descriptor instead of a pipe (see -.BR pipe (2)) -in all cases where a pipe is used simply to signal events. -The kernel overhead of an eventfd file descriptor -is much lower than that of a pipe, -and only one file descriptor is -required (versus the two required for a pipe). -.P -When used in the kernel, an eventfd -file descriptor can provide a bridge from kernel to user space, allowing, -for example, functionalities like KAIO (kernel AIO) -.\" or eventually syslets/threadlets -to signal to a file descriptor that some operation is complete. -.P -A key point about an eventfd file descriptor is that it can be -monitored just like any other file descriptor using -.BR select (2), -.BR poll (2), -or -.BR epoll (7). -This means that an application can simultaneously monitor the -readiness of "traditional" files and the readiness of other -kernel mechanisms that support the eventfd interface. -(Without the -.BR eventfd () -interface, these mechanisms could not be multiplexed via -.BR select (2), -.BR poll (2), -or -.BR epoll (7).) -.P -The current value of an eventfd counter can be viewed -via the entry for the corresponding file descriptor in the process's -.IR /proc/ pid /fdinfo -directory. -See -.BR proc (5) -for further details. -.SH EXAMPLES -The following program creates an eventfd file descriptor -and then forks to create a child process. -While the parent briefly sleeps, -the child writes each of the integers supplied in the program's -command-line arguments to the eventfd file descriptor. -When the parent has finished sleeping, -it reads from the eventfd file descriptor. -.P -The following shell session shows a sample run of the program: -.P -.in +4n -.EX -.RB "$" " ./a.out 1 2 4 7 14" -Child writing 1 to efd -Child writing 2 to efd -Child writing 4 to efd -Child writing 7 to efd -Child writing 14 to efd -Child completed write loop -Parent about to read -Parent read 28 (0x1c) from efd -.EE -.in -.SS Program source -\& -.\" SRC BEGIN (eventfd.c) -.EX -#include <err.h> -#include <inttypes.h> -#include <stdio.h> -#include <stdlib.h> -#include <sys/eventfd.h> -#include <unistd.h> -\& -int -main(int argc, char *argv[]) -{ - int efd; - uint64_t u; - ssize_t s; -\& - if (argc < 2) { - fprintf(stderr, "Usage: %s <num>...\en", argv[0]); - exit(EXIT_FAILURE); - } -\& - efd = eventfd(0, 0); - if (efd == \-1) - err(EXIT_FAILURE, "eventfd"); -\& - switch (fork()) { - case 0: - for (size_t j = 1; j < argc; j++) { - printf("Child writing %s to efd\en", argv[j]); - u = strtoull(argv[j], NULL, 0); - /* strtoull() allows various bases */ - s = write(efd, &u, sizeof(uint64_t)); - if (s != sizeof(uint64_t)) - err(EXIT_FAILURE, "write"); - } - printf("Child completed write loop\en"); -\& - exit(EXIT_SUCCESS); -\& - default: - sleep(2); -\& - printf("Parent about to read\en"); - s = read(efd, &u, sizeof(uint64_t)); - if (s != sizeof(uint64_t)) - err(EXIT_FAILURE, "read"); - printf("Parent read %"PRIu64" (%#"PRIx64") from efd\en", u, u); - exit(EXIT_SUCCESS); -\& - case \-1: - err(EXIT_FAILURE, "fork"); - } -} -.EE -.\" SRC END -.SH SEE ALSO -.BR futex (2), -.BR pipe (2), -.BR poll (2), -.BR read (2), -.BR select (2), -.BR signalfd (2), -.BR timerfd_create (2), -.BR write (2), -.BR epoll (7), -.BR sem_overview (7) |