eventfd2 - Linux
Overview
eventfd2 is a Linux system call that creates an eventfd object, a type of file descriptor that can be used to efficiently notify userspace processes of events. It provides a more flexible and efficient way to signal events compared to traditional methods like pipes or signals.
Syntax
int eventfd2(unsigned int initval, int flags);
Options/Flags
- initval: Initial value for the eventfd object.
- flags: Flags to control the behavior of the eventfd object. They can be any combination of the following:
- EFD_CLOEXEC: Close the eventfd on exec.
- EFD_NONBLOCK: Make the eventfd non-blocking.
Examples
Simple event signaling:
#include <sys/eventfd.h>
int main() {
int efd = eventfd(0, 0); // Create eventfd with initial value 0
uint64_t val;
write(efd, &val, sizeof(val)); // Signal an event
read(efd, &val, sizeof(val)); // Wait for and read the event
return 0;
}
Non-blocking event signaling:
#include <sys/eventfd.h>
#include <fcntl.h>
int main() {
int efd = eventfd(0, EFD_NONBLOCK); // Create non-blocking eventfd
uint64_t val = 1;
int ret = write(efd, &val, sizeof(val)); // Signal an event
if (ret == -1 && errno == EAGAIN) {
// Non-blocking write failed, handle accordingly
}
return 0;
}
Common Issues
- Non-blocking write error: If the eventfd is non-blocking and the buffer is full, the
write()
operation will fail withEAGAIN
. - Unexpected event count: Ensure that the eventfd is only signaled when intended, as multiple signals can accumulate.
Integration
eventfd2 can be combined with other commands or tools for advanced tasks, such as:
- epoll: Monitor eventfds for events using
epoll
. - inotify: Use eventfds to efficiently signal file system events.
Related Commands
- eventfd: Legacy eventfd system call with limited features.
- epoll_create1: Create an epoll instance that can monitor eventfds.