Ubuntu Manpage: sd_event_source_set_priority, sd_event_source_get_priority, SD_EVENT_PRIORITY

Provided by: libsystemd-dev_256.5-2ubuntu3.1_amd64

NAME

       sd_event_source_set_priority, sd_event_source_get_priority, SD_EVENT_PRIORITY_IMPORTANT,
       SD_EVENT_PRIORITY_NORMAL, SD_EVENT_PRIORITY_IDLE - Set or retrieve the priority of event
       sources

SYNOPSIS

       #include <systemd/sd-event.h>

       enum {
               SD_EVENT_PRIORITY_IMPORTANT = -100,
               SD_EVENT_PRIORITY_NORMAL = 0,
               SD_EVENT_PRIORITY_IDLE = 100,
       };

       int sd_event_source_set_priority(sd_event_source *source, int64_t priority);

       int sd_event_source_get_priority(sd_event_source *source, int64_t *priority);

DESCRIPTION

sd_event_source_set_priority() may be used to set the priority for the event source object
specified as source. The priority is specified as an arbitrary signed 64-bit integer. The
priority is initialized to SD_EVENT_PRIORITY_NORMAL (0) when the event source is allocated
with a call such as sd_event_add_io(3) or sd_event_add_time(3), and may be changed with
this call. If multiple event sources have seen events at the same time, they are
dispatched in the order indicated by the event sources' priorities. Event sources with
smaller priority values are dispatched first. As well-known points of reference, the
constants SD_EVENT_PRIORITY_IMPORTANT (-100), SD_EVENT_PRIORITY_NORMAL (0) and
SD_EVENT_PRIORITY_IDLE (100) may be used to indicate event sources that shall be
dispatched early, normally or late. It is recommended to specify priorities based on these
definitions, and relative to them — however, the full 64-bit signed integer range is
available for ordering event sources.

Priorities define the order in which event sources that have seen events are dispatched.
Care should be taken to ensure that high-priority event sources (those with negative
priority values assigned) do not cause starvation of low-priority event sources (those
with positive priority values assigned).

The order in which event sources with the same priority are dispatched is undefined, but
the event loop generally tries to dispatch them in the order it learnt about events on
them. As the backing kernel primitives do not provide accurate information about the order
in which events occurred this is not necessarily reliable. However, it is guaranteed that
if events are seen on multiple same-priority event sources at the same time, each one is
not dispatched again until all others have been dispatched once. This behavior guarantees
that within each priority particular event sources do not starve or dominate the event
loop.

The priority of event sources may be changed at any time of their lifetime, with the
exception of inotify event sources (i.e. those created with sd_event_add_inotify(3)) whose
priority may only be changed in the time between their initial creation and the first
subsequent event loop iteration.

sd_event_source_get_priority() may be used to query the current priority assigned to the
event source object source.

RETURN VALUE

       On success, sd_event_source_set_priority() and sd_event_source_get_priority() return a
       non-negative integer. On failure, they return a negative errno-style error code.

   Errors
       Returned errors may indicate the following problems:

       -EINVAL
           source is not a valid pointer to an sd_event_source object.

       -ENOMEM
           Not enough memory.

       -ESTALE
           The event loop is already terminated.

       -ECHILD
           The event loop has been created in a different process, library or module instance.

NOTES

       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.

HISTORY