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sd_event_add_signal(3) — Linux manual page

SD_EVENT_ADD_SIGNAL(3)     sd_event_add_signalSD_EVENT_ADD_SIGNAL(3)

NAME        top

       sd_event_add_signal, sd_event_source_get_signal,       sd_event_signal_handler_t, SD_EVENT_SIGNAL_PROCMASK - Add a UNIX       process signal event source to an event loop

SYNOPSIS        top

#include <systemd/sd-event.h>typedef struct sd_event_source sd_event_source;SD_EVENT_SIGNAL_PROCMASKtypedef int (*sd_event_signal_handler_t)(sd_event_source *s,const struct signalfd_siginfo *si,void *userdata);int sd_event_add_signal(sd_event *event, sd_event_source **source,intsignal,sd_event_signal_handler_thandler,void *userdata);int sd_event_source_get_signal(sd_event_source *source);

DESCRIPTION        top

sd_event_add_signal()adds a new UNIX process signal event source       to an event loop. The event loop object is specified in theevent       parameter, and the event source object is returned in thesource       parameter. Thesignal parameter specifies the numeric signal to be       handled (seesignal(7)).       Thehandler parameter is a function to call when the signal is       received orNULL. The handler function will be passed theuserdata       pointer, which may be chosen freely by the caller. The handler       also receives a pointer to a signalfd_siginfo structure containing       information about the received signal. Seesignalfd(2) for further       information. The handler may return negative to signal an error       (see below), other return values are ignored. Ifhandler isNULL,       a default handler that callssd_event_exit(3) will be used.       Only a single handler may be installed for a specific signal. The       signal must be blocked in all threads before this function is       called (usingsigprocmask(2) orpthread_sigmask(3)). For       convenience, if the special flagSD_EVENT_SIGNAL_PROCMASKis ORed       into the specified signal the signal will be automatically masked       as necessary, for the calling thread. Note that this only works       reliably if the signal is already masked in all other threads of       the process, or if there are no other threads at the moment of       invocation.       By default, the event source is enabled permanently (SD_EVENT_ON),       but this may be changed withsd_event_source_set_enabled(3). If       the handler function returns a negative error code, it will either       be disabled after the invocation, even if theSD_EVENT_ONmode was       requested before, or it will cause the loop to terminate, seesd_event_source_set_exit_on_failure(3).       To destroy an event source object usesd_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 if it is still       referenced, disable the event source usingsd_event_source_set_enabled(3) withSD_EVENT_OFF.       If the second parameter ofsd_event_add_signal()isNULLno       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 thehandler parameter tosd_event_add_signal()isNULL, and the       event source fires, this will be considered a request to exit the       event loop. In this case, theuserdata parameter, cast to an       integer, is passed as the exit code parameter tosd_event_exit(3).sd_event_source_get_signal()returns the configured signal number       of an event source created previously withsd_event_add_signal().       It takes the event source object as thesource parameter.

RETURN VALUE        top

       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.-EBUSY           A handler is already installed for this signal or the signal           was not blocked previously.-ESTALE           The event loop is already terminated.-ECHILD           The event loop has been created in a different process,           library or module instance.-EDOM           The passed event source is not a signal event source.

NOTES        top

       Functions described here are available as a shared library, which       can be compiled against and linked to with thelibsystemd pkg-config(1) file.       The code described here usesgetenv(3), which is declared to be       not multi-thread-safe. This means that the code calling the       functions described here must not callsetenv(3) from a parallel       thread. It is recommended to only do calls tosetenv()from an       early phase of the program when no other threads have been       started.

HISTORY        top

sd_event_add_signal(),sd_event_signal_handler_t(), andsd_event_source_get_signal()were added in version 217.

SEE ALSO        top

systemd(1),sd-event(3),sd_event_new(3),sd_event_now(3),sd_event_add_io(3),sd_event_add_time(3),sd_event_add_child(3),sd_event_add_inotify(3),sd_event_add_defer(3),sd_event_source_set_enabled(3),sd_event_source_set_description(3),sd_event_source_set_userdata(3),sd_event_source_set_floating(3),signal(7),signalfd(2),sigprocmask(2),pthread_sigmask(3)

COLOPHON        top

       This page is part of thesystemd (systemd system and service       manager) project.  Information about the project can be found at       ⟨http://www.freedesktop.org/wiki/Software/systemd⟩.  If you have a       bug report for this manual page, see       ⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.       This page was obtained from the project's upstream Git repository       ⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11.  (At that       time, the date of the most recent commit that was found in the       repository was 2025-08-11.)  If you discover any rendering       problems in this HTML version of the page, or you believe there is       a better or more up-to-date source for the page, or you have       corrections or improvements to the information in this COLOPHON       (which isnot part of the original manual page), send a mail to       man-pages@man7.orgsystemd 258~rc2SD_EVENT_ADD_SIGNAL(3)

Pages that refer to this page:sd-event(3), sd_event_add_child(3), sd_event_add_defer(3), sd_event_add_inotify(3), sd_event_add_io(3), sd_event_add_time(3), sd_event_exit(3), sd_event_new(3), sd_event_run(3), sd_event_set_signal_exit(3), sd_event_set_watchdog(3), sd_event_source_get_event(3), sd_event_source_get_pending(3), sd_event_source_set_description(3), sd_event_source_set_destroy_callback(3), sd_event_source_set_enabled(3), sd_event_source_set_exit_on_failure(3), sd_event_source_set_floating(3), sd_event_source_set_prepare(3), sd_event_source_set_priority(3), sd_event_source_set_ratelimit(3), sd_event_source_set_userdata(3), sd_event_source_unref(3), sd_event_wait(3), systemd.directives(7), systemd.index(7)

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