sd_event_now - Retrieve current event loop iteration timestamp
int sd_event_now(sd_event *event, clockid_t clock, uint64_t *usec);
sd_event_now() returns the time when the most recent event loop iteration began. A timestamp is taken right after returning from the event sleep, and before dispatching any event sources. The event parameter specifies the event loop object to retrieve the timestamp from. The clock parameter specifies the clock to retrieve the timestamp for, and is one of CLOCK_REALTIME (or equivalently CLOCK_REALTIME_ALARM), CLOCK_MONOTONIC, or CLOCK_BOOTTIME (or equivalently CLOCK_BOOTTIME_ALARM), see clock_gettime(2) for more information on the various clocks. The retrieved timestamp is stored in the usec parameter, in μs since the clock's epoch. If this function is invoked before the first event loop iteration, the current time is returned, as reported by clock_gettime(). To distinguish this case from a regular invocation the return value will be positive, and zero when the returned timestamp refers to an actual event loop iteration.
If the first event loop iteration has not run yet sd_event_now() writes current time to usec and returns a positive return value. Otherwise, it will write the requested timestamp to usec and return 0. On failure, the call returns a negative errno-style error code.
Returned values may indicate the following problems:
Functions described here are available as a shared library, which can be compiled against and linked to with the libsystemd pkg-config(1) file.
The code described here uses getenv(3), which is declared to be not multi-thread-safe. This means that the code calling the functions described here must not call setenv(3) from a parallel thread. It is recommended to only do calls to setenv() from an early phase of the program when no other threads have been started.