Multithreading in C and Java

Summary about mutlti-threading in C and Java
1. If the main thread terminates, then the whole program dies, too. So if you
want to run some tasks in a sub thread, you must guarantee that sub thread

terminates before main thread.

2. If you want to execute some task in a separate thread and wait until its
termination, you should use pthread_join rather than condition and mutex.
pthread_join is like waitpid, it suspend the calling thread until another
thread terminates and you can retrieve thread's termination status through

pthread_join which is much more difficult to do using condition and mutex.

3. PTHREAD_MUTEX_INITIALIZER and PTHREAD_COND_INITIALIZER are implemented as
macros. So when you use them incorrectly, you will get very confucious
compiling error messages. For example,
...
static pthread_mutex_t lock;
...
43 lock = PTHREAD_MUTEX_INITIALIZER;
...
If you compile, you will get:
43: error: expected expression before token '{'
The most weird thing is that there is no such token '{'. We can imagine that
PTHREAD_MUTEX_INITIALIZER is implemented as macro.
In addition we have two ways to initialize mutex and conditions,
1. initialize them when declaring them.

        static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
        static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

2. use initializing functions after declaration.

        pthread_mutex_init( &lock, NULL );
        pthread_cond_init( &cond, NULL );

4. When you wait a condition, you must lock its mutex, when you signal a
condition, you MUST NOT lock the mutex. MAKE sure you change the condition

before signaling.

5. Thread has its local stack frame, just like a function. Any local
variable(auto variables, declared in the thread) becomes invalid when thread
exits. So you should NEVER return a auto variable or put an auto variable
into global data strutures.