Basic "Threading 101"

[Cliff Nichols]

As not to "Hijack" a question brought about from Kevin (last names held to protect the "Guilty" (or were they innocent?) ) I thought I would open my similar question (if not the same question)

Many EXCELLENT points were brought up at http://www.powerbasic.com/support/pb...429#post305429

Just a rough background to my limited thought of Threading

1. Threads are great up to their point of "Diminishing Returns" of execution and speeding up your apps
2. Globals are a "No-No" because of context switching and synchronization and such
3. Welllllll hopefully this question will satisfy understanding the idiosyncrasy

For my example of "A base Question" I will use the idea of enumerating all drives on a system. (Theoretically a possible 26 letters A-Z, and hence 26 threads, which is MORE than I have ever used, and should be WELL BELOW that point of "Diminishing Returns", but nice example)

Ok, now I have my list of drives, (could be 3 could be 26), so I decide to thread to get info of each one to speed things up for the following reasons

1. A:\ has no floppy, but default times out 3 seconds before telling me there is no floppy
2. Z:\ Has a CD or DVD that spends time spinning up and spinning down before telling me its info
3. Typical response....1 and 2 take MUCH longer than "Just give me the facts Mammm" and would save time if they could be found out during any "Down Time" I have
4. Down Time to my understanding is "Managed by Windows OS" and could theoretically never occure (defeating my thread attempt)
   

So that said...I create a UDT

TYPE MYTYPE
     DriveName as ASCIIZ * %MAX_PATH
     DriveInfo as WHATEVER
END TYPE

so each of my local threads, I create a variable as MYTYPE
Question....Is the variable local to thread?, Can another thread corrupt the data?, Is my local really a global pointed at the same data??? and hence could be seen in the midst of changing???

Since I am getting long in my question of questions I will keep it short, but also hazzard to guess that misuse of sleep, and misuse of tight loops will also change outcomes, and become more of a "Depends on your purpose" on what the answers are????

[Michael Mattias]

It' does not 'depend on' anything.

First thing, though...."LOCAL" is compile-time concept; a thread of execution is a run-time concept.

That said, in the compiler's implementation, a LOCAL variable is unique to each entry to the procedure in which it is defined.

If that procedure is a THREAD FUNCTION which is entered as a result of a THREAD CREATE statement, then each thread of execution will have a unique copy of that variable.

If that procedure is a window or dialog procedure, then each notification message sent to that procedure is a unique entry, meaning the subject variable is unique to that particular message-processing code. This is what allows one window procedure to service many windows accurately, or what allows you to "send a message to yourself" whilst processing another message and have unique 'working' variables for each set of message-processing code in that procedure.


MCM