# sd_event_add_defer

## NAME



           sd_event_add_defer, sd_event_add_post, sd_event_add_exit,
           sd_event_handler_t - Add static event sources to an event loop
    


## SYNOPSIS



           **#include <systemd/sd-event.h>**
    
           **typedef struct sd_event_source sd_event_source;**
    
           **typedef int (*sd_event_handler_t)(sd_event_source ***_s_**,**
                                             **void ***_userdata_**);**
    
           **int sd_event_add_defer(sd_event ***_event_**, sd_event_source ****_source_**,**
                                  **sd_event_handler_t** _handler_**,**
                                  **void ***_userdata_**);**
    
           **int sd_event_add_post(sd_event ***_event_**, sd_event_source ****_source_**,**
                                 **sd_event_handler_t** _handler_**, void ***_userdata_**);**
    
           **int sd_event_add_exit(sd_event ***_event_**, sd_event_source ****_source_**,**
                                 **sd_event_handler_t** _handler_**, void ***_userdata_**);**
    


## DESCRIPTION



           These three functions add new static event sources to an event
           loop. The event loop object is specified in the _event_ parameter,
           the event source object is returned in the _source_ parameter. The
           event sources are enabled statically and will "fire" when the
           event loop is run and the conditions described below are met.
    
           The _handler_ is a function to call or **NULL**. The handler function
           will be passed the _userdata_ pointer, which may be chosen freely by
           the caller. The handler may return negative to signal an error
           (see below), other return values are ignored. If _handler_ is **NULL** ,
           a default handler that calls sd_event_exit(3) will be used.
    
           **sd_event_add_defer()** adds a new event source that will be
           dispatched instantly, before the event loop goes to sleep again
           and waits for new events. By default, the handler will be called
           once (**SD_EVENT_ONESHOT**). Note that if the event source is set to
           **SD_EVENT_ON** the event loop will never go to sleep again, but
           continuously call the handler, possibly interleaved with other
           event sources.
    
           **sd_event_add_post()** adds a new event source that is run before the
           event loop will sleep and wait for new events, but only after at
           least one other non-post event source was dispatched. By default,
           the source is enabled permanently (**SD_EVENT_ON**). Note that this
           event source type will still allow the event loop to go to sleep
           again, even if set to **SD_EVENT_ON** , as long as no other event
           source is ever triggered.
    
           **sd_event_add_exit()** adds a new event source that will be
           dispatched when the event loop is terminated with
           sd_event_exit(3).
    
           The sd_event_source_set_enabled(3) function may be used to enable
           the event source permanently (**SD_EVENT_ON**) or to make it fire just
           once (**SD_EVENT_ONESHOT**).
    
           If the handler function returns a negative error code, it will
           either be disabled after the invocation, even if the **SD_EVENT_ON**
           mode was requested before, or it will cause the loop to terminate,
           see sd_event_source_set_exit_on_failure(3).
    
           To destroy an event source object use sd_event_source_unref(3),
           but note that the event source is only removed from the event loop
           when all references to the event source are dropped. To make sure
           an event source does not fire anymore, even when there's still a
           reference to it kept, consider setting the event source to
           **SD_EVENT_OFF** with sd_event_source_set_enabled(3).
    
           If the second parameter of these functions is passed as **NULL** no
           reference to the event source object is returned. In this case,
           the event source is considered "floating", and will be destroyed
           implicitly when the event loop itself is destroyed.
    
           If the _handler_ parameter to **sd_event_add_defer()** or
           **sd_event_add_post()** is **NULL** , and the event source fires, this will
           be considered a request to exit the event loop. In this case, the
           _userdata_ parameter, cast to an integer, is passed as the exit code
           parameter to sd_event_exit(3). Similar functionality is not
           available for **sd_event_add_exit()** , as these types of event sources
           are only dispatched when exiting anyway.
    


## RETURN VALUE



           On success, these functions return 0 or a positive integer. On
           failure, they return a negative errno-style error code.
    
       **Errors**
           Returned errors may indicate the following problems:
    
           **-ENOMEM**
               Not enough memory to allocate an object.
    
           **-EINVAL**
               An invalid argument has been passed.
    
           **-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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