| EVENTFD(2) | Linux Programmer's Manual | EVENTFD(2) |
eventfd - create a file descriptor for event notification
#include <sys/eventfd.h>
int eventfd(unsigned int initval, int flags);
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 (uint64_t) counter that is maintained by the kernel. This counter is initialized with the value specified in the argument initval.
As its return value, eventfd() returns a new file descriptor that can be used to refer to the eventfd object.
The following values may be bitwise ORed in flags to change the behavior of eventfd():
In Linux up to version 2.6.26, the flags argument is unused, and must be specified as zero.
The following operations can be performed on the file descriptor returned by eventfd():
A copy of the file descriptor created by eventfd() is inherited by the child produced by fork(2). The duplicate file descriptor is associated with the same eventfd object. File descriptors created by eventfd() are preserved across execve(2), unless the close-on-exec flag has been set.
On success, eventfd() returns a new eventfd file descriptor. On error, -1 is returned and errno is set to indicate the error.
eventfd() is available on Linux since kernel 2.6.22. Working support is provided in glibc since version 2.8. The eventfd2() system call (see NOTES) is available on Linux since kernel 2.6.27. Since version 2.9, the glibc eventfd() wrapper will employ the eventfd2() system call, if it is supported by the kernel.
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
| eventfd () | Thread safety | MT-Safe |
eventfd() and eventfd2() are Linux-specific.
Applications can use an eventfd file descriptor instead of a pipe (see 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).
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) to signal to a file descriptor that some operation is complete.
A key point about an eventfd file descriptor is that it can be monitored just like any other file descriptor using select(2), poll(2), or 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 eventfd() interface, these mechanisms could not be multiplexed via select(2), poll(2), or epoll(7).)
The current value of an eventfd counter can be viewed via the entry for the corresponding file descriptor in the process's /proc/[pid]/fdinfo directory. See proc(5) for further details.
There are two underlying Linux system calls: eventfd() and the more recent eventfd2(). The former system call does not implement a flags argument. The latter system call implements the flags values described above. The glibc wrapper function will use eventfd2() where it is available.
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:
typedef uint64_t eventfd_t; int eventfd_read(int fd, eventfd_t *value); int eventfd_write(int fd, eventfd_t value);
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.
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.
The following shell session shows a sample run of the program:
$ ./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
#include <sys/eventfd.h> #include <unistd.h> #include <inttypes.h> /* Definition of PRIu64 & PRIx64 */ #include <stdlib.h> #include <stdio.h> #include <stdint.h> /* Definition of uint64_t */ #define handle_error(msg) \
do { perror(msg); exit(EXIT_FAILURE); } while (0) int main(int argc, char *argv[]) {
int efd;
uint64_t u;
ssize_t s;
if (argc < 2) {
fprintf(stderr, "Usage: %s <num>...\n", argv[0]);
exit(EXIT_FAILURE);
}
efd = eventfd(0, 0);
if (efd == -1)
handle_error("eventfd");
switch (fork()) {
case 0:
for (int j = 1; j < argc; j++) {
printf("Child writing %s to efd\n", argv[j]);
u = strtoull(argv[j], NULL, 0);
/* strtoull() allows various bases */
s = write(efd, &u, sizeof(uint64_t));
if (s != sizeof(uint64_t))
handle_error("write");
}
printf("Child completed write loop\n");
exit(EXIT_SUCCESS);
default:
sleep(2);
printf("Parent about to read\n");
s = read(efd, &u, sizeof(uint64_t));
if (s != sizeof(uint64_t))
handle_error("read");
printf("Parent read %"PRIu64" (%#"PRIx64") from efd\n", u, u);
exit(EXIT_SUCCESS);
case -1:
handle_error("fork");
} }
futex(2), pipe(2), poll(2), read(2), select(2), signalfd(2), timerfd_create(2), write(2), epoll(7), sem_overview(7)
This page is part of release 5.10 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
| 2020-11-01 | Linux |