Ubuntu Manpage: timer_getoverrun - get overrun count for a POSIX per-process timer

NAME

       timer_getoverrun - get overrun count for a POSIX per-process timer

LIBRARY

       Real-time library (librt, -lrt)

SYNOPSIS

       #include <time.h>

       int timer_getoverrun(timer_t timerid);

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

       timer_getoverrun():
           _POSIX_C_SOURCE >= 199309L

DESCRIPTION

timer_getoverrun() returns the "overrun count" for the timer referred to by timerid. An
application can use the overrun count to accurately calculate the number of timer
expirations that would have occurred over a given time interval. Timer overruns can occur
both when receiving expiration notifications via signals (SIGEV_SIGNAL), and via threads
(SIGEV_THREAD).

When expiration notifications are delivered via a signal, overruns can occur as follows.
Regardless of whether or not a real-time signal is used for timer notifications, the
system queues at most one signal per timer. (This is the behavior specified by POSIX.1.
The alternative, queuing one signal for each timer expiration, could easily result in
overflowing the allowed limits for queued signals on the system.) Because of system
scheduling delays, or because the signal may be temporarily blocked, there can be a delay
between the time when the notification signal is generated and the time when it is
delivered (e.g., caught by a signal handler) or accepted (e.g., using sigwaitinfo(2)). In
this interval, further timer expirations may occur. The timer overrun count is the number
of additional timer expirations that occurred between the time when the signal was
generated and when it was delivered or accepted.

Timer overruns can also occur when expiration notifications are delivered via invocation
of a thread, since there may be an arbitrary delay between an expiration of the timer and
the invocation of the notification thread, and in that delay interval, additional timer
expirations may occur.

RETURN VALUE

       On success, timer_getoverrun() returns the overrun count  of  the  specified  timer;  this
       count may be 0 if no overruns have occurred.  On failure, -1 is returned, and errno is set
       to indicate the error.

ERRORS

       EINVAL timerid is not a valid timer ID.

VERSIONS

       This system call is available since Linux 2.6.

STANDARDS

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

NOTES

       When timer notifications are delivered via signals (SIGEV_SIGNAL), on  Linux  it  is  also
       possible  to  obtain the overrun count via the si_overrun field of the siginfo_t structure
       (see sigaction(2)).  This allows an application to avoid the overhead of making  a  system
       call to obtain the overrun count, but is a nonportable extension to POSIX.1.

       POSIX.1 discusses timer overruns only in the context of timer notifications using signals.

BUGS

       POSIX.1  specifies  that  if  the  timer  overrun  count  is  equal  to or greater than an
       implementation-defined maximum,  DELAYTIMER_MAX,  then  timer_getoverrun()  should  return
       DELAYTIMER_MAX.   However,  before  Linux  4.19,  if  the  timer overrun value exceeds the
       maximum representable integer, the counter cycles, starting once  more  from  low  values.
       Since  Linux  4.19,  timer_getoverrun()  returns  DELAYTIMER_MAX  (defined  as  INT_MAX in
       <limits.h>) in this case (and the overrun value is reset to 0).

EXAMPLES

       See timer_create(2).