timerfd API Reference

Header

#include <sys/timerfd.h>

Core APIs

int timerfd_create(int clockid, int flags);
int timerfd_settime(int fd, int flags,
                    const struct itimerspec *new_value,
                    struct itimerspec *old_value);
int timerfd_gettime(int fd, struct itimerspec *curr_value);

Common Flags

Flag	Purpose
TFD_NONBLOCK	Create non-blocking timerfd
TFD_CLOEXEC	Close fd across exec()
TFD_TIMER_ABSTIME	Use absolute expiration time

Expiration Read Format

uint64_t expirations;
read(timer_fd, &expirations, sizeof(expirations));

The read value is the number of expirations since the previous read.

Common Pitfalls

• Always read 8 bytes from the timerfd.
• Forgetting to read causes repeated readiness events.
• read() returns expiration count, not timestamp.
• Prefer one global timerfd plus software state for many similar timeout checks.

Related Topic

• timerfd