.\" Copyright (c) 2009 Linux Foundation, written by Michael Kerrisk .\" .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .TH timer_create 2 2023-03-30 "Linux man-pages 6.04" .SH NAME timer_create \- create a POSIX per-process timer .SH LIBRARY Real-time library .RI ( librt ", " \-lrt ) .SH SYNOPSIS .nf .BR "#include " " /* Definition of " SIGEV_* " constants */" .B #include .PP .BI "int timer_create(clockid_t " clockid , .BI " struct sigevent *_Nullable restrict " sevp , .BI " timer_t *restrict " timerid ); .fi .PP .RS -4 Feature Test Macro Requirements for glibc (see .BR feature_test_macros (7)): .RE .PP .BR timer_create (): .nf _POSIX_C_SOURCE >= 199309L .fi .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. .PP 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 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 (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 Linux 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 .B CLOCK_MONOTONIC clock does not measure the time while a system is suspended, the .B CLOCK_BOOTTIME clock does include the time during which the system is suspended. This is useful for applications that need to be suspend-aware. .B 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. .TP .BR CLOCK_TAI " (since Linux 3.10)" A system-wide clock derived from wall-clock time but ignoring leap seconds. .PP See .BR clock_getres (2) for some further details on the above clocks. .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). .PP 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). .PP The .I sevp.sigev_notify field can have the following values: .TP .B SIGEV_NONE Don't asynchronously notify when the timer expires. Progress of the timer can be monitored using .BR timer_gettime (2). .TP .B 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 .B 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 .I 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 , or .I sigev_notify_thread_id is invalid. .TP .B ENOMEM .\" glibc layer: malloc() Could not allocate memory. .TP .B ENOTSUP The kernel does not support creating a timer against this .IR clockid . .TP .B EPERM .I clockid was .B CLOCK_REALTIME_ALARM or .B CLOCK_BOOTTIME_ALARM but the caller did not have the .B CAP_WAKE_ALARM capability. .SH VERSIONS .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.1-2008. .SH HISTORY Linux 2.6. POSIX.1-2001. .PP 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 Timers are not inherited by the child of a .BR fork (2), and are disarmed and deleted during an .BR execve (2). .PP The kernel preallocates a "queued real-time signal" for each timer created using .BR timer_create (). Consequently, the number of timers is limited by the .B RLIMIT_SIGPENDING resource limit (see .BR setrlimit (2)). .PP The timers created by .BR timer_create () are commonly known as "POSIX (interval) timers". The POSIX timers API consists of the following interfaces: .TP .BR timer_create () Create a timer. .TP .BR timer_settime (2) Arm (start) or disarm (stop) a timer. .TP .BR timer_gettime (2) Fetch the time remaining until the next expiration of a timer, along with the interval setting of the timer. .TP .BR timer_getoverrun (2) Return the overrun count for the last timer expiration. .TP .BR timer_delete (2) Disarm and delete a timer. .PP Since Linux 3.10, the .IR /proc/ pid /timers file can be used to list the POSIX timers for the process with PID .IR pid . See .BR proc (5) for further information. .PP 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. .SH 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. .PP 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 .in +4n .EX $ \fB./a.out 1 100\fP 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 .EE .in .SS Program source \& .\" SRC BEGIN (timer_create.c) .EX #include #include #include #include #include #include #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) { /* 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\en", 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 \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 .\" SRC END .SH SEE ALSO .ad l .nh .BR clock_gettime (2), .BR setitimer (2), .BR timer_delete (2), .BR timer_getoverrun (2), .BR timer_settime (2), .BR timerfd_create (2), .BR clock_getcpuclockid (3), .BR pthread_getcpuclockid (3), .BR pthreads (7), .BR sigevent (7), .BR signal (7), .BR time (7)