How to use timer in redhat9
 

I want do somthing periodically.
I want use timer but how to do it.
would you give me some idea or examples.
anything will be helpful.

You can use cron to start your program.

that's good and the program will be start periodically.

but I want do it in the program, that means the program will be start only once.

This might interst you as well.
of course i'm assuming that you use C

I want use somthing like timers.
I give it a period and a function to invoke, then when the
time specified by period comes to the end, the function
will be invoked automatically.

I like this way ,because it make the program logic clear.

do you know How to achieve this purpose.

man alarm

...and...

man setitimer

you can use threads.for example "i=1 to 1000" block takes 1 seconds to complete so that in a thread function,you can control the value (it is i) if equals to 1000,if equals to 1000 you can start a code block you want.
i actually didnt tried this example cos i just learned threads.but i think it will work ; ).(and also i will try by now : )

Are you suggesting busy waiting (or whatever the term is)? This will eat up cpu even as it does nothing. Doing sleep and alarm are less expensive.