.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
-.TH TIMER_CREATE 2 (date) "Linux man-pages (unreleased)"
+.TH timer_create 2 (date) "Linux man-pages (unreleased)"
.SH NAME
timer_create \- create a POSIX per-process timer
.SH LIBRARY
.nf
.BR "#include <signal.h>" " /* Definition of " SIGEV_* " constants */"
.B #include <time.h>
-.PP
-.BI "int timer_create(clockid_t " clockid ", struct sigevent *restrict " sevp ,
+.P
+.BI "int timer_create(clockid_t " clockid ,
+.BI " struct sigevent *_Nullable restrict " sevp ,
.BI " timer_t *restrict " timerid );
.fi
-.PP
+.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
-.PP
+.P
.BR timer_create ():
.nf
_POSIX_C_SOURCE >= 199309L
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.
-.PP
+.P
The
.I clockid
argument specifies the clock that the new timer uses to measure 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
+.\" in Linux 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
.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
+.\" The CLOCK_MONOTONIC_RAW that was added in Linux 2.6.28 can't be used
.\" to create a timer -- mtk, Feb 2009
.TP
.BR CLOCK_BOOTTIME " (Since Linux 2.6.39)"
.TP
.BR CLOCK_TAI " (since Linux 3.10)"
A system-wide clock derived from wall-clock time but ignoring leap seconds.
-.PP
+.P
See
.BR clock_getres (2)
for some further details on the above clocks.
-.PP
+.P
As well as the above values,
.I clockid
can be specified as the
.BR clock_getcpuclockid (3)
or
.BR pthread_getcpuclockid (3).
-.PP
+.P
The
.I sevp
argument points to a
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).
-.PP
+.BR sigevent (3type).
+.P
The
.I sevp.sigev_notify
field can have the following values:
.I sigev_signo
for the process.
See
-.BR sigevent (7)
+.BR sigevent (3type)
for general details.
The
.I si_code
.I sigev_notify_function
as if it were the start function of a new thread.
See
-.BR sigevent (7)
+.BR sigevent (3type)
for details.
.TP
.BR SIGEV_THREAD_ID " (Linux-specific)"
or
.BR gettid (2).
This flag is intended only for use by threading libraries.
-.PP
+.P
Specifying
.I sevp
as NULL is equivalent to specifying a pointer to a
.B CAP_WAKE_ALARM
capability.
.SH VERSIONS
-This system call is available since Linux 2.6.
+.SS C library/kernel differences
+Part of the implementation of the POSIX timers API is provided by glibc.
+.\" See nptl/sysdeps/unix/sysv/linux/timer_create.c
+In particular:
+.IP \[bu] 3
+Much of the functionality for
+.B SIGEV_THREAD
+is implemented within glibc, rather than the kernel.
+(This is necessarily so,
+since the thread involved in handling the notification is one
+that must be managed by the C library POSIX threads implementation.)
+Although the notification delivered to the process is via a thread,
+internally the NPTL implementation uses a
+.I sigev_notify
+value of
+.B SIGEV_THREAD_ID
+along with a real-time signal that is reserved by the implementation (see
+.BR nptl (7)).
+.IP \[bu]
+The implementation of the default case where
+.I evp
+is NULL is handled inside glibc,
+which invokes the underlying system call with a suitably populated
+.I sigevent
+structure.
+.IP \[bu]
+The timer IDs presented at user level are maintained by glibc,
+which maps these IDs to the timer IDs employed by the kernel.
+.\" See the glibc source file kernel-posix-timers.h for the structure
+.\" that glibc uses to map user-space timer IDs to kernel timer IDs
+.\" The kernel-level timer ID is exposed via siginfo.si_tid.
.SH STANDARDS
-POSIX.1-2001, POSIX.1-2008.
+POSIX.1-2008.
+.SH HISTORY
+Linux 2.6.
+POSIX.1-2001.
+.P
+Prior to Linux 2.6,
+glibc provided an incomplete user-space implementation
+.RB ( CLOCK_REALTIME
+timers only) using POSIX threads,
+and before glibc 2.17,
+.\" glibc commit 93a78ac437ba44f493333d7e2a4b0249839ce460
+the implementation falls back to this technique on systems
+running kernels older than Linux 2.6.
.SH NOTES
A program may create multiple interval timers using
.BR timer_create ().
-.PP
+.P
Timers are not inherited by the child of a
.BR fork (2),
and are disarmed and deleted during an
.BR execve (2).
-.PP
+.P
The kernel preallocates a "queued real-time signal"
for each timer created using
.BR timer_create ().
.B RLIMIT_SIGPENDING
resource limit (see
.BR setrlimit (2)).
-.PP
+.P
The timers created by
.BR timer_create ()
are commonly known as "POSIX (interval) timers".
.TP
.BR timer_delete (2)
Disarm and delete a timer.
-.PP
+.P
Since Linux 3.10, the
.IR /proc/ pid /timers
file can be used to list the POSIX timers for the process with PID
See
.BR proc (5)
for further information.
-.PP
+.P
Since Linux 4.10,
.\" baa73d9e478ff32d62f3f9422822b59dd9a95a21
support for POSIX timers is a configurable option that is enabled by default.
Kernel support can be disabled via the
.B CONFIG_POSIX_TIMERS
option.
-.\"
-.SS C library/kernel differences
-Part of the implementation of the POSIX timers API is provided by glibc.
-.\" See nptl/sysdeps/unix/sysv/linux/timer_create.c
-In particular:
-.IP \(bu 3
-Much of the functionality for
-.B SIGEV_THREAD
-is implemented within glibc, rather than the kernel.
-(This is necessarily so,
-since the thread involved in handling the notification is one
-that must be managed by the C library POSIX threads implementation.)
-Although the notification delivered to the process is via a thread,
-internally the NPTL implementation uses a
-.I sigev_notify
-value of
-.B SIGEV_THREAD_ID
-along with a real-time signal that is reserved by the implementation (see
-.BR nptl (7)).
-.IP \(bu
-The implementation of the default case where
-.I evp
-is NULL is handled inside glibc,
-which invokes the underlying system call with a suitably populated
-.I sigevent
-structure.
-.IP \(bu
-The timer IDs presented at user level are maintained by glibc,
-which maps these IDs to the timer IDs employed by the kernel.
-.\" See the glibc source file kernel-posix-timers.h for the structure
-.\" that glibc uses to map user-space timer IDs to kernel timer IDs
-.\" The kernel-level timer ID is exposed via siginfo.si_tid.
-.PP
-The POSIX timers system calls first appeared in Linux 2.6.
-Prior to this,
-glibc provided an incomplete user-space implementation
-.RB ( CLOCK_REALTIME
-timers only) using POSIX threads,
-and in glibc versions before 2.17,
-.\" glibc commit 93a78ac437ba44f493333d7e2a4b0249839ce460
-the implementation falls back to this technique on systems
-running pre-2.6 Linux kernels.
.SH EXAMPLES
The program below takes two arguments: a sleep period in seconds,
and a timer frequency in nanoseconds.
the signal handler will be invoked,
and the handler displays some information about the timer notification.
The program terminates after one invocation of the signal handler.
-.PP
+.P
In the following example run, the program sleeps for 1 second,
after creating a timer that has a frequency of 100 nanoseconds.
By the time the signal is unblocked and delivered,
there have been around ten million overruns.
-.PP
+.P
.in +4n
.EX
$ \fB./a.out 1 100\fP
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
-
+\&
#define CLOCKID CLOCK_REALTIME
#define SIG SIGRTMIN
-
+\&
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \e
} while (0)
-
+\&
static void
print_siginfo(siginfo_t *si)
{
int or;
timer_t *tidp;
-
+\&
tidp = si\->si_value.sival_ptr;
-
+\&
printf(" sival_ptr = %p; ", si\->si_value.sival_ptr);
printf(" *sival_ptr = %#jx\en", (uintmax_t) *tidp);
-
+\&
or = timer_getoverrun(*tidp);
if (or == \-1)
errExit("timer_getoverrun");
else
printf(" overrun count = %d\en", or);
}
-
+\&
static void
handler(int sig, siginfo_t *si, void *uc)
{
printf() is not async\-signal\-safe; see signal\-safety(7).
Nevertheless, we use printf() here as a simple way of
showing that the handler was called. */
-
+\&
printf("Caught signal %d\en", sig);
print_siginfo(si);
signal(sig, SIG_IGN);
}
-
+\&
int
main(int argc, char *argv[])
{
struct sigevent sev;
struct sigaction sa;
struct itimerspec its;
-
+\&
if (argc != 3) {
fprintf(stderr, "Usage: %s <sleep\-secs> <freq\-nanosecs>\en",
argv[0]);
exit(EXIT_FAILURE);
}
-
+\&
/* Establish handler for timer signal. */
-
+\&
printf("Establishing handler for signal %d\en", SIG);
sa.sa_flags = SA_SIGINFO;
sa.sa_sigaction = handler;
sigemptyset(&sa.sa_mask);
if (sigaction(SIG, &sa, NULL) == \-1)
errExit("sigaction");
-
+\&
/* Block timer signal temporarily. */
-
+\&
printf("Blocking signal %d\en", SIG);
sigemptyset(&mask);
sigaddset(&mask, SIG);
if (sigprocmask(SIG_SETMASK, &mask, NULL) == \-1)
errExit("sigprocmask");
-
+\&
/* Create the timer. */
-
+\&
sev.sigev_notify = SIGEV_SIGNAL;
sev.sigev_signo = SIG;
sev.sigev_value.sival_ptr = &timerid;
if (timer_create(CLOCKID, &sev, &timerid) == \-1)
errExit("timer_create");
-
+\&
printf("timer ID is %#jx\en", (uintmax_t) timerid);
-
+\&
/* Start the timer. */
-
+\&
freq_nanosecs = atoll(argv[2]);
its.it_value.tv_sec = freq_nanosecs / 1000000000;
its.it_value.tv_nsec = freq_nanosecs % 1000000000;
its.it_interval.tv_sec = its.it_value.tv_sec;
its.it_interval.tv_nsec = its.it_value.tv_nsec;
-
+\&
if (timer_settime(timerid, 0, &its, NULL) == \-1)
errExit("timer_settime");
-
+\&
/* Sleep for a while; meanwhile, the timer may expire
multiple times. */
-
+\&
printf("Sleeping for %d seconds\en", atoi(argv[1]));
sleep(atoi(argv[1]));
-
+\&
/* Unlock the timer signal, so that timer notification
can be delivered. */
-
+\&
printf("Unblocking signal %d\en", SIG);
if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == \-1)
errExit("sigprocmask");
-
+\&
exit(EXIT_SUCCESS);
}
.EE
.BR clock_getcpuclockid (3),
.BR pthread_getcpuclockid (3),
.BR pthreads (7),
-.BR sigevent (7),
+.BR sigevent (3type),
.BR signal (7),
.BR time (7)
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