# sd_event_source_set_priority

## 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.
    


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