The alarm() function is a Unix system call used to set a timer that will send a SIGALRM signal when it expires.
The prototype of the alarm() function is: unsigned int alarm(unsigned int seconds), where the seconds parameter specifies the time for the timer in seconds.
If a timer has already been set before calling the alarm() function, it will be cancelled and replaced with a new timer.
If the seconds parameter is set to 0, the current timer will be canceled without setting a new one.
The alarm() function returns the remaining time of the previously set timer, or 0 if no timer was set before.
When the timer expires, the system sends a SIGALRM signal to the process. You can use the signal() function to register a signal handler function to handle the signal.
In the signal handling function, the alarm() function can be used to reset the timer to achieve a periodic timer.
When using the alarm() function, it’s important to consider the reentrancy of the signal handling function to avoid calling non-reentrant functions within the signal handling function.
When using the alarm() function in a multi-threaded environment, it is important to be cautious of thread safety to prevent multiple threads from simultaneously calling the alarm() function and causing a race condition.
In some systems, the alarm() function does not support millisecond timers and can only set timers in seconds. If a more precise timer is needed, other methods like using timed events or timed threads can be used.
