% I need to share data between thread 1 and thread 2.
% I tried doing this by using a static variable in thread 1 and thread 2.
% works OK, but all other client connections then use the same value for
% variable. I thought the scope of a static variable was from parent
% to client process ?
Static variables are not shared between processes, and threads are not
processes, anyway. Also, posix doesn't have the concept of parent and
child relationships between threads.
Threads share all the resources of a process. In particular, every
memory address within a process's address space is available to all
threads in the process, so there are lots of ways to share memory
between two threads. For instance, you can use
the problem here is that the global variable address will be the same for
every thread in the system. You can get around this by making an array
and working out a scheme of allocating array indices. Ie, for connection
1, you use array index 0, for connection 2, you use index 1, etc.
static variable, with address passed in pthread_create call:
this is exactly like a global variable, except the scope of the
variable name is limited either to a function or a compilation unit.
dynamic allocation (malloc/new/mmap):
allocate a chunk of memory in thread A and pass its address to thread
B. You have to be very careful that both threads are finished with it
before you free it.
don't do this, because you'll probably have a problem and then everyone
will blame me for suggesting it, but you can pass the address of a local
variable from the thread function of thread A, and use it in both threads.
You _must_ ensure that thread B finishes before thread A (ie, using
pthread_join) or your program will crash.
don't do this, because it's ridiculous, but you could allocate shared
memory using shmat or mmap and treat it as if it had been allocated
In any case, make sure you use mutexes and/or condition variables to
control access to the shared data.
Patrick TJ McPhee
East York Canada