Question

I’m looking for an open source timer library written in C. The library should have Timer callback functions, etc.

On searching I see POSIX timers or setitimer(), which use a signal based approach that may lead to problems in multithreaded code.

Say if I use POSIX Timers inside threaded code, the signals won’t be reaching to the correct place. If I use several timers in a process then each one should use different signals. Are there any other alternatives?

Answer 1

Since you are running Linux, I would recommend using the built in POSIX timer API’s.

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

Here is a link to some documentation showing how to use POSIX timers which provide support for callback functions.

Regarding multiple timers in a process, the documentation says this:

   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)).

Note that POSIX timers can be used in a threaded application by setting up notification using SIGEV_THREAD_ID as shown below:

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 only intended for use by threading libraries.