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[thirdparty/man-pages.git] / man2 / timer_create.2

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.TH TIMER_CREATE 2 2017-07-13 Linux "Linux Programmer's Manual"
.SH NAME
timer_create \- create a POSIX per-process timer
.SH SYNOPSIS
.nf
.B  #include <signal.h>
.B  #include <time.h>
.PP
.BI "int timer_create(clockid_t " clockid ", struct sigevent *" sevp ,
.BI "                 timer_t *" timerid );
.fi
.PP
Link with \fI\-lrt\fP.
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.PP
.BR timer_create ():
_POSIX_C_SOURCE\ >=\ 199309L
.SH DESCRIPTION
.BR timer_create ()
creates a new per-process interval timer.
The ID of the new timer is returned in the buffer pointed to by
.IR timerid ,
which must be a non-null pointer.
This ID is unique within the process, until the timer is deleted.
The new timer is initially disarmed.

The
.I clockid
argument specifies the clock that the new timer uses to measure time.
It can be specified as one of the following values:
.TP
.B CLOCK_REALTIME
A settable system-wide real-time clock.
.TP
.B CLOCK_MONOTONIC
A nonsettable monotonically increasing clock that measures time
from some unspecified point in the past that does not change
after system startup.
.\" Note: the CLOCK_MONOTONIC_RAW clock added for clock_gettime()
.\" in 2.6.28 is not supported for POSIX timers -- mtk, Feb 2009
.TP
.BR CLOCK_PROCESS_CPUTIME_ID " (since Linux 2.6.12)"
A clock that measures (user and system) CPU time consumed by
(all of the threads in) the calling process.
.TP
.BR CLOCK_THREAD_CPUTIME_ID " (since Linux 2.6.12)"
A clock that measures (user and system) CPU time consumed by
the calling thread.
.\" The CLOCK_MONOTONIC_RAW that was added in 2.6.28 can't be used
.\" to create a timer -- mtk, Feb 2009
.TP
.BR CLOCK_BOOTTIME " (Since Linux 2.6.39)"
.\" commit 70a08cca1227dc31c784ec930099a4417a06e7d0
Like
.BR CLOCK_MONOTONIC ,
this is a monotonically increasing clock.
However, whereas the
.BR CLOCK_MONOTONIC
clock does not measure the time while a system is suspended, the
.BR CLOCK_BOOTTIME
clock does include the time during which the system is suspended.
This is useful for applications that need to be suspend-aware.
.BR CLOCK_REALTIME
is not suitable for such applications, since that clock is affected
by discontinuous changes to the system clock.
.TP
.BR CLOCK_REALTIME_ALARM " (since Linux 3.0)"
.\" commit 9a7adcf5c6dea63d2e47e6f6d2f7a6c9f48b9337
This clock is like
.BR CLOCK_REALTIME ,
but will wake the system if it is suspended.
The caller must have the
.B CAP_WAKE_ALARM
capability in order to set a timer against this clock.
.TP
.BR CLOCK_BOOTTIME_ALARM " (since Linux 3.0)"
.\" commit 9a7adcf5c6dea63d2e47e6f6d2f7a6c9f48b9337
This clock is like
.BR CLOCK_BOOTTIME ,
but will wake the system if it is suspended.
The caller must have the
.B CAP_WAKE_ALARM
capability in order to set a timer against this clock.
.PP
As well as the above values,
.I clockid
can be specified as the
.I clockid
returned by a call to
.BR clock_getcpuclockid (3)
or
.BR pthread_getcpuclockid (3).

The
.I sevp
argument points to a
.I sigevent
structure that specifies how the caller
should be notified when the timer expires.
For the definition and general details of this structure, see
.BR sigevent (7).

The
.I sevp.sigev_notify
field can have the following values:
.TP
.BR SIGEV_NONE
Don't asynchronously notify when the timer expires.
Progress of the timer can be monitored using
.BR timer_gettime (2).
.TP
.BR SIGEV_SIGNAL
Upon timer expiration, generate the signal
.I sigev_signo
for the process.
See
.BR sigevent (7)
for general details.
The
.I si_code
field of the
.I siginfo_t
structure will be set to
.BR SI_TIMER .
At any point in time,
at most one signal is queued to the process for a given timer; see
.BR timer_getoverrun (2)
for more details.
.TP
.BR SIGEV_THREAD
Upon timer expiration, invoke
.I sigev_notify_function
as if it were the start function of a new thread.
See
.BR sigevent (7)
for details.
.TP
.BR SIGEV_THREAD_ID " (Linux-specific)"
As for
.BR SIGEV_SIGNAL ,
but the signal is targeted at the thread whose ID is given in
.IR sigev_notify_thread_id ,
which must be a thread in the same process as the caller.
The
.IR sigev_notify_thread_id
field specifies a kernel thread ID, that is, the value returned by
.BR clone (2)
or
.BR gettid (2).
This flag is intended only for use by threading libraries.
.PP
Specifying
.I sevp
as NULL is equivalent to specifying a pointer to a
.I sigevent
structure in which
.I sigev_notify
is
.BR SIGEV_SIGNAL ,
.I sigev_signo
is
.BR SIGALRM ,
and
.I sigev_value.sival_int
is the timer ID.
.SH RETURN VALUE
On success,
.BR timer_create ()
returns 0, and the ID of the new timer is placed in
.IR *timerid .
On failure, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EAGAIN
Temporary error during kernel allocation of timer structures.
.TP
.B EINVAL
Clock ID,
.IR sigev_notify ,
.IR sigev_signo ,