Ubuntu Manpage: timer_create - create a POSIX per-process timer

NAME

       timer_create - create a POSIX per-process timer

LIBRARY

       Real-time library (librt, -lrt)

SYNOPSIS

       #include <signal.h>           /* Definition of SIGEV_* constants */
       #include <time.h>

       int timer_create(clockid_t clockid,
                        struct sigevent *_Nullable restrict sevp,
                        timer_t *restrict timerid);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       timer_create():
           _POSIX_C_SOURCE >= 199309L

DESCRIPTION

timer_create() creates a new per-process interval timer. The ID of the new timer is
returned in the buffer pointed to by 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 clockid argument specifies the clock that the new timer uses to measure time. It can
be specified as one of the following values:

CLOCK_REALTIME
A settable system-wide real-time clock.

CLOCK_MONOTONIC
A nonsettable monotonically increasing clock that measures time from some
unspecified point in the past that does not change after system startup.

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.

CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
A clock that measures (user and system) CPU time consumed by the calling thread.

CLOCK_BOOTTIME (Since Linux 2.6.39)
Like CLOCK_MONOTONIC, this is a monotonically increasing clock. However, whereas
the CLOCK_MONOTONIC clock does not measure the time while a system is suspended,
the CLOCK_BOOTTIME clock does include the time during which the system is
suspended. This is useful for applications that need to be suspend-aware.
CLOCK_REALTIME is not suitable for such applications, since that clock is affected
by discontinuous changes to the system clock.

CLOCK_REALTIME_ALARM (since Linux 3.0)
This clock is like CLOCK_REALTIME, but will wake the system if it is suspended.
The caller must have the CAP_WAKE_ALARM capability in order to set a timer against
this clock.

CLOCK_BOOTTIME_ALARM (since Linux 3.0)
This clock is like CLOCK_BOOTTIME, but will wake the system if it is suspended.
The caller must have the CAP_WAKE_ALARM capability in order to set a timer against
this clock.

CLOCK_TAI (since Linux 3.10)
A system-wide clock derived from wall-clock time but ignoring leap seconds.

See clock_getres(2) for some further details on the above clocks.

As well as the above values, clockid can be specified as the clockid returned by a call to
clock_getcpuclockid(3) or pthread_getcpuclockid(3).

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

The sevp.sigev_notify field can have the following values:

SIGEV_NONE
Don't asynchronously notify when the timer expires. Progress of the timer can be
monitored using timer_gettime(2).

SIGEV_SIGNAL
Upon timer expiration, generate the signal sigev_signo for the process. See
sigevent(7) for general details. The si_code field of the siginfo_t structure will
be set to SI_TIMER. At any point in time, at most one signal is queued to the
process for a given timer; see timer_getoverrun(2) for more details.

SIGEV_THREAD
Upon timer expiration, invoke sigev_notify_function as if it were the start
function of a new thread. See sigevent(7) for details.

SIGEV_THREAD_ID (Linux-specific)
As for SIGEV_SIGNAL, but the signal is targeted at the thread whose ID is given in
sigev_notify_thread_id, which must be a thread in the same process as the caller.
The sigev_notify_thread_id field specifies a kernel thread ID, that is, the value
returned by clone(2) or gettid(2). This flag is intended only for use by threading
libraries.

Specifying sevp as NULL is equivalent to specifying a pointer to a sigevent structure in
which sigev_notify is SIGEV_SIGNAL, sigev_signo is SIGALRM, and sigev_value.sival_int is
the timer ID.

RETURN VALUE

       On success, timer_create() returns 0, and the ID of the new timer is placed  in  *timerid.
       On failure, -1 is returned, and errno is set to indicate the error.

ERRORS

       EAGAIN Temporary error during kernel allocation of timer structures.

       EINVAL Clock ID, sigev_notify, sigev_signo, or sigev_notify_thread_id is invalid.

       ENOMEM Could not allocate memory.

       ENOTSUP
              The kernel does not support creating a timer against this clockid.

       EPERM  clockid  was  CLOCK_REALTIME_ALARM  or  CLOCK_BOOTTIME_ALARM but the caller did not
              have the CAP_WAKE_ALARM capability.

VERSIONS

       This system call is available since Linux 2.6.

STANDARDS

       POSIX.1-2001, POSIX.1-2008.

NOTES

A program may create multiple interval timers using timer_create().

Timers are not inherited by the child of a fork(2), and are disarmed and deleted during an
execve(2).

The kernel preallocates a "queued real-time signal" for each timer created using
timer_create(). Consequently, the number of timers is limited by the RLIMIT_SIGPENDING
resource limit (see setrlimit(2)).

The timers created by timer_create() are commonly known as "POSIX (interval) timers". The
POSIX timers API consists of the following interfaces:

timer_create()
Create a timer.

timer_settime(2)
Arm (start) or disarm (stop) a timer.

timer_gettime(2)
Fetch the time remaining until the next expiration of a timer, along with the
interval setting of the timer.

timer_getoverrun(2)
Return the overrun count for the last timer expiration.

timer_delete(2)
Disarm and delete a timer.

Since Linux 3.10, the /proc/pid/timers file can be used to list the POSIX timers for the
process with PID pid. See proc(5) for further information.

Since Linux 4.10, support for POSIX timers is a configurable option that is enabled by
default. Kernel support can be disabled via the CONFIG_POSIX_TIMERS option.

C library/kernel differences
Part of the implementation of the POSIX timers API is provided by glibc. In particular:

• Much of the functionality for 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 sigev_notify value of SIGEV_THREAD_ID along
with a real-time signal that is reserved by the implementation (see nptl(7)).

• The implementation of the default case where evp is NULL is handled inside glibc, which
invokes the underlying system call with a suitably populated sigevent structure.

• The timer IDs presented at user level are maintained by glibc, which maps these IDs to
the timer IDs employed by the kernel.

The POSIX timers system calls first appeared in Linux 2.6. Prior to this, glibc provided
an incomplete user-space implementation (CLOCK_REALTIME timers only) using POSIX threads,
and before glibc 2.17, the implementation falls back to this technique on systems running
kernels older than Linux 2.6.

EXAMPLES

       The program below takes two arguments: a sleep period in seconds, and a timer frequency in
       nanoseconds.  The program establishes a handler for the signal  it  uses  for  the  timer,
       blocks that signal, creates and arms a timer that expires with the given frequency, sleeps
       for the specified number of seconds, and then unblocks the timer  signal.   Assuming  that
       the  timer  expired  at  least  once  while  the program slept, 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.

       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.

           $ ./a.out 1 100
           Establishing handler for signal 34
           Blocking signal 34
           timer ID is 0x804c008
           Sleeping for 1 seconds
           Unblocking signal 34
           Caught signal 34
               sival_ptr = 0xbfb174f4;     *sival_ptr = 0x804c008
               overrun count = 10004886

   Program source

       #include <signal.h>
       #include <stdint.h>
       #include <stdio.h>
       #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); \
                               } 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\n", (uintmax_t) *tidp);

           or = timer_getoverrun(*tidp);
           if (or == -1)
               errExit("timer_getoverrun");
           else
               printf("    overrun count = %d\n", or);
       }

       static void
       handler(int sig, siginfo_t *si, void *uc)
       {
           /* Note: calling printf() from a signal handler is not safe
              (and should not be done in production programs), since
              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\n", sig);
           print_siginfo(si);
           signal(sig, SIG_IGN);
       }

       int
       main(int argc, char *argv[])
       {
           timer_t            timerid;
           sigset_t           mask;
           long long          freq_nanosecs;
           struct sigevent    sev;
           struct sigaction   sa;
           struct itimerspec  its;

           if (argc != 3) {
               fprintf(stderr, "Usage: %s <sleep-secs> <freq-nanosecs>\n",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

           /* Establish handler for timer signal. */

           printf("Establishing handler for signal %d\n", 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\n", 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\n", (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\n", atoi(argv[1]));
           sleep(atoi(argv[1]));

           /* Unlock the timer signal, so that timer notification
              can be delivered. */

           printf("Unblocking signal %d\n", SIG);
           if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
               errExit("sigprocmask");

           exit(EXIT_SUCCESS);
       }