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.
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, SignalGeneration 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
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
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.
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.
The timer_create() function shall fail if:
EAGAIN The system lacks sufficient signal queuing resources to honor
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.
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.
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
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()
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
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 TIMER_CREATE(3P)