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union sigval {          /* Data passed with notification */
    int     sival_int;         /* Integer value */
    void   *sival_ptr;         /* Pointer value */
};
struct sigevent {
    int          sigev_notify; /* Notification method */
    int          sigev_signo;  /* Notification signal */
    union sigval sigev_value;  /* Data passed with
                                  notification */
    void       (*sigev_notify_function) (union sigval);
                     /* Function used for thread
                        notification (SIGEV_THREAD) */
    void        *sigev_notify_attributes;
                     /* Attributes for notification thread
                        (SIGEV_THREAD) */
    pid_t        sigev_notify_thread_id;
                     /* ID of thread to signal (SIGEV_THREAD_ID) */
};

Description

The sigevent structure is used by various APIs to describe the way a process is to be notified about an event (e.g., completion of an asynchronous request, expiration of a timer, or the arrival of a message).

The definition shown in the SYNOPSIS is approximate: some of the fields in the sigevent structure may be defined as part of a union. Programs should employ only those fields relevant to the value specified in sigev_notify.

The sigev_notify field specifies how notification is to be performed. This field can have one of the following values:

SIGEV_NONE

A "null" notification: don’t do anything when the event occurs.

SIGEV_SIGNAL

Notify the process by sending the signal specified in sigev_signo.

If the signal is caught with a signal handler that was registered using the

sigaction(2) SA_SIGINFO flag, then the following fields are set in the siginfo_t structure that is passed as the second argument of the handler:

si_code

This field is set to a value that depends on the API delivering the notification.

si_signo

This field is set to the signal number (i.e., the same value as in sigev_signo).

si_value

This field is set to the value specified in sigev_value.

Depending on the API, other fields may also be set in the

siginfo_t structure.

The same information is also available if the signal is accepted using

sigwaitinfo(2) .

SIGEV_THREAD

Notify the process by invoking sigev_notify_function "as if" it were the start function of a new thread. (Among the implementation possibilities here are that each timer notification could result in the creation of a new thread, or that a single thread is created to receive all notifications.) The function is invoked with sigev_value as its sole argument. If sigev_notify_attributes is not NULL, it should point to a pthread_attr_t structure that defines attributes for the new thread (see pthread_attr_init(3) ).

SIGEV_THREAD_ID (Linux-specific)

Currently used only by POSIX timers; see timer_create(2) .

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