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What is the purpose of condition variables in Monitors? Why really do we need the condition variables? It may sound something stupid to ask, but never mind I am really new at this. Thanks in advance
A condition variable is basically a container of threads that are waiting for a certain condition. Monitors provide a mechanism for threads to temporarily give up exclusive access in order to wait for some condition to be met, before regaining exclusive access and resuming their task.Reference link here.
Conceptually a condition variable is a queue of threads, associated with a monitor, on which a thread may wait for some condition to become true.
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What is a main thread in multithreading in a c program?
I need to create m threads and to execute different operations on main thread and on the created m threads. Is the main thread the main function maybe?
The main thread is the thread on which main() is called at program startup. Never end the main thread: on most platforms this ends the process quite rapidly.
There's a reason the alternative to multi-threaded programming is called single-threaded programming, not threadless programming. You always have at least one thread.
While some platforms allow you to create a process without any threads at all, that process isn't going to do anything.
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I am building a very tiny shell in c,
I have the option on running programs in the background.
I keep a list of all my jobs meaning all the ones in the bg.
Now if i want to go and update this list, how can i check is a process is finished or if its still running.
ps
if i waited with waitpid for some process, will i still be able to check if the process is done? (i mean if i used waitpid it took the process of zombie state.
You should be able to call waitpid, passing it the process id and the WNOHANG option and call the WIFEXITED macro on the integer returned through status argument. See Just check status process in c.
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I want to create a timer that counts seconds and display them, but in parallel I want to do other actions. Can someone explain me how can I do that or if this is possible (in Visual Studio)?
This is one way to handle your desired actions.
in main thread,
-fork a child process. passing
1) a desired time interval
2) a callback function ptr
3) indication of oneShot or repeating timer
into the child process,
in the child process:
-begin loop
-sleep the passed-in-time-interval
-execute the call back function
-if non repeating timer
-then
-exit child process
-endif
-end loop
in main thread, in the call back function:
-get current time
-display the current time to user
-return
in main thread, when ready to exit, kill child thread
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I am trying to add instrumentation into my code that will print out something like
'Thread 1 forks Thread 2'
Any suggestions on how I can achieve this?
Terminology correction: one thread may create another thread, not fork, which is usually
used to mention one process forking another.
No, a thread has no way to get another thread's identifier. On Linux, you can check if gettid() == getpid() to find if it's the main thread. Solaris has thr_main() to identify if the caller is main thread or not. FreeBSD has pthread_main_np() for the same purpose.
But there's no way to identify parent-child relationship between any threads. Any thread can create more threads. You'll have to use pass the thread identifiers around when creating threads or use global data structure to maintain this information.
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I don't really understand re-entrant code.
Can somebody show me an algorithm for re-entrant code so I can look at it? An explanation of how it works would be nice too. Thanks.
If you, as a developer of some function, can be sure that your function is not called during its execution time, then you may not to pay attention to make it reentrant.
However if your function can be called from different task, threads, or just by interruption signal, there can be a situation, when the function reenters itself during its execution, if it is not reentrant-compliant , not protected from multiple entrances it may lead to unpredictable behaviour, such as data corruption\overwriting or corruption of chronology of the actions in the function.
Here may be a number of approaches. Sometimes it is enough to write a function, for example, working only with copies of the external variables. However sometimes using of mutexes is inevitable. When mutexes are used, the caller of the function will be waiting for mutex unlocking. You just place mutex locking before the critical section and mutex unlocking after the critical section. Whole function body can be wrapped between lock and unlock couple.