таймер процесса (process timer)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
timer_create — create a per-process timer
Синопсис (Synopsis)
#include <signal.h>
#include <time.h>
int timer_create(clockid_t clockid, struct sigevent *restrict evp,
timer_t *restrict timerid);
Описание (Description)
The timer_create() function shall create a per-process timer
using the specified clock, clock_id, as the timing base. The
timer_create() function shall return, in the location referenced
by timerid, a timer ID of type timer_t used to identify the timer
in timer requests. This timer ID shall be unique within the
calling process until the timer is deleted. The particular clock,
clock_id, is defined in <time.h>. The timer whose ID is returned
shall be in a disarmed state upon return from timer_create().
The evp argument, if non-NULL, points to a sigevent structure.
This structure, allocated by the application, defines the
asynchronous notification to occur as specified in Section 2.4.1,
Signal Generation and Delivery when the timer expires. If the evp
argument is NULL, the effect is as if the evp argument pointed to
a sigevent structure with the sigev_notify member having the
value SIGEV_SIGNAL, the sigev_signo having a default signal
number, and the sigev_value member having the value of the timer
ID.
Each implementation shall define a set of clocks that can be used
as timing bases for per-process timers. All implementations shall
support a clock_id of CLOCK_REALTIME. If the Monotonic Clock
option is supported, implementations shall support a clock_id of
CLOCK_MONOTONIC.
Per-process timers shall not be inherited by a child process
across a fork() and shall be disarmed and deleted by an exec.
If _POSIX_CPUTIME is defined, implementations shall support
clock_id values representing the CPU-time clock of the calling
process.
If _POSIX_THREAD_CPUTIME is defined, implementations shall
support clock_id values representing the CPU-time clock of the
calling thread.
It is implementation-defined whether a timer_create() function
will succeed if the value defined by clock_id corresponds to the
CPU-time clock of a process or thread different from the process
or thread invoking the function.
If evp->sigev_sigev_notify is SIGEV_THREAD and
sev->sigev_notify_attributes is not NULL, if the attribute
pointed to by sev->sigev_notify_attributes has a thread stack
address specified by a call to pthread_attr_setstack(), the
results are unspecified if the signal is generated more than
once.
Возвращаемое значение (Return value)
If the call succeeds, timer_create() shall return zero and update
the location referenced by timerid to a timer_t, which can be
passed to the per-process timer calls. If an error occurs, the
function shall return a value of -1 and set errno to indicate the
error. The value of timerid is undefined if an error occurs.
Ошибки (Error)
The timer_create() function shall fail if:
EAGAIN The system lacks sufficient signal queuing resources to
honor the request.
EAGAIN The calling process has already created all of the timers
it is allowed by this implementation.
EINVAL The specified clock ID is not defined.
ENOTSUP
The implementation does not support the creation of a
timer attached to the CPU-time clock that is specified by
clock_id and associated with a process or thread different
from the process or thread invoking timer_create().
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
If a timer is created which has evp->sigev_sigev_notify set to
SIGEV_THREAD and the attribute pointed to by
evp->sigev_notify_attributes has a thread stack address specified
by a call to pthread_attr_setstack(), the memory dedicated as a
thread stack cannot be recovered. The reason for this is that the
threads created in response to a timer expiration are created
detached, or in an unspecified way if the thread attribute's
detachstate is PTHREAD_CREATE_JOINABLE. In neither case is it
valid to call pthread_join(), which makes it impossible to
determine the lifetime of the created thread which thus means the
stack memory cannot be reused.
Обоснование (Rationale)
Periodic Timer Overrun and Resource Allocation
The specified timer facilities may deliver realtime signals (that
is, queued signals) on implementations that support this option.
Since realtime applications cannot afford to lose notifications
of asynchronous events, like timer expirations or asynchronous
I/O completions, it must be possible to ensure that sufficient
resources exist to deliver the signal when the event occurs. In
general, this is not a difficulty because there is a one-to-one
correspondence between a request and a subsequent signal
generation. If the request cannot allocate the signal delivery
resources, it can fail the call with an [EAGAIN] error.
Periodic timers are a special case. A single request can generate
an unspecified number of signals. This is not a problem if the
requesting process can service the signals as fast as they are
generated, thus making the signal delivery resources available
for delivery of subsequent periodic timer expiration signals.
But, in general, this cannot be assured—processing of periodic
timer signals may ``overrun''; that is, subsequent periodic timer
expirations may occur before the currently pending signal has
been delivered.
Also, for signals, according to the POSIX.1‐1990 standard, if
subsequent occurrences of a pending signal are generated, it is
implementation-defined whether a signal is delivered for each
occurrence. This is not adequate for some realtime applications.
So a mechanism is required to allow applications to detect how
many timer expirations were delayed without requiring an
indefinite amount of system resources to store the delayed
expirations.
The specified facilities provide for an overrun count. The
overrun count is defined as the number of extra timer expirations
that occurred between the time a timer expiration signal is
generated and the time the signal is delivered. The signal-
catching function, if it is concerned with overruns, can retrieve
this count on entry. With this method, a periodic timer only
needs one ``signal queuing resource'' that can be allocated at
the time of the timer_create() function call.
A function is defined to retrieve the overrun count so that an
application need not allocate static storage to contain the
count, and an implementation need not update this storage
asynchronously on timer expirations. But, for some high-frequency
periodic applications, the overhead of an additional system call
on each timer expiration may be prohibitive. The functions, as
defined, permit an implementation to maintain the overrun count
in user space, associated with the timerid. The
timer_getoverrun() function can then be implemented as a macro
that uses the timerid argument (which may just be a pointer to a
user space structure containing the counter) to locate the
overrun count with no system call overhead. Other
implementations, less concerned with this class of applications,
can avoid the asynchronous update of user space by maintaining
the count in a system structure at the cost of the extra system
call to obtain it.
Timer Expiration Signal Parameters
The Realtime Signals Extension option supports an application-
specific datum that is delivered to the extended signal handler.
This value is explicitly specified by the application, along with
the signal number to be delivered, in a sigevent structure. The
type of the application-defined value can be either an integer
constant or a pointer. This explicit specification of the value,
as opposed to always sending the timer ID, was selected based on
existing practice.
It is common practice for realtime applications (on non-POSIX
systems or realtime extended POSIX systems) to use the parameters
of event handlers as the case label of a switch statement or as a
pointer to an application-defined data structure. Since timer_ids
are dynamically allocated by the timer_create() function, they
can be used for neither of these functions without additional
application overhead in the signal handler; for example, to
search an array of saved timer IDs to associate the ID with a
constant or application data structure.
Будущие направления (Future directions)
None.
Смотри также (See also)
clock_getres(3p), timer_delete(3p), timer_getoverrun(3p)
The Base Definitions volume of POSIX.1‐2017, signal.h(0p),
time.h(0p)
