The IEEE Std 1003.1-2004 (“POSIX.1”) standard extends traditional UNIX signal semantics by providing facilities for realtime signal generation and delivery.
Please note that this manual describes an interface that is not yet fully functional in NetBSD: neither realtime signals nor SIGEV_THREAD are currently supported.
Realtime functions that can generate realtime signals include:
1.
Completion of asynchronous I/O; see
aio(3).
3.
Arrival of a message to an empty message queue; see
mq_notify(3).
The
<sys/signal.h> header, included by
<signal.h>, defines a
sigevent structure, which is the cornerstone in asynchronous delivery of realtime signals. This structure is defined as:
struct sigevent {
int sigev_notify;
int sigev_signo;
union sigval sigev_value;
void (*sigev_notify_function)(union sigval);
void *sigev_notify_attributes;
};
The included union is further defined in
<siginfo.h> as:
typedef union sigval {
int sival_int;
void *sival_ptr;
} sigval_t;
The
sigev_notify integer defines the action taken when a notification such as timer expiration occurs. The possiblue values are:
SIGEV_NONE
This constant specifies a “null” handler: when a notification arrives, nothing happens.
SIGEV_SIGNAL
The
SIGEV_SIGNAL constant specifies that notifications are delivered by signals. When a notification arrives, the kernel sends the signal specified in
sigev_signo.
In the signal handler the ‘si_value' of
siginfo_t is set to the value specified by the
sigev_value. In another words, the
sigev_value member is an application-defined value to be passed to a particular signal handler at the time of signal delivery. Depending whether the specified value is an integer or a pointer, the delivered value can be either
sigval_intr or
sigval_ptr.
SIGEV_THREAD
This constant specifies a thread-driven notification mechanism. When a notification occurs, the kernel creates a new thread that starts executing the function specified in the function pointer
sigev_notify_function. The single argument passed to the function is specified in
sigev_value.
If
sigev_notify_attributes is not
NULL, the provided attribute specifies the behavior of the thread; see
pthread_attr(3). (Note that although a pointer to void is specified for
sigev_notify_attributes, the type is
pthread_attr_t in practice.)
The threads are created as detached, or in an unspecified way if
pthread_attr_setdetachstate(3) is used with
sigev_notify_attributes to set
PTHREAD_CREATE_JOINABLE. It is not valid to call
pthread_join(3) in either case. Hence, it is impossible to determine the lifetime of the created thread. This in turn means that it is neither possible to recover the memory nor the address of the memory possibly dedicated as thread stack via
pthread_attr_setstack() or
pthread_attr_setstackaddr().