CODEWITHSUNDEEP
c++cmultithreading
user3684042
user3684042

Reputation: 671

Ways to share a variable among threads

I have a general question about parallel programming in C and C++ and would appreciate it if you could answer it. As far as I know, we can declare a variable in at least one level higher (parent thread) to share it among children threads. So, I was wondering if there is any other way to share a variable among threads with the same parent thread? Is this API dependant or not?

Upvotes: 1

Views: 2317

Answers (3)

Dr. Debasish Jana
Dr. Debasish Jana

Reputation: 7128

Any global variable is sharable among threads, since threads are light weight processes sharing the same address space. For synchronization, you need to ensure mutual exclusion while updating/accessing those global variables through semaphores or wait notify blocks.

Upvotes: 1

Basile Starynkevitch
Basile Starynkevitch

Reputation: 1

For Posix threads, read some pthread tutorial.

For C++11, read the documentation of its thread library

All threads of the same process share the same address space in virtual memory. As commented by Marco A. consider also thread_local variables.

Notice that you share data or memory (not variables, which exist only in the source code)

In practice, you'll better protect with a mutex the shared data (for synchronization) to avoid data races.

In the simple case, the mutex and the shared data are in some global variables.

You could also use atomic operations.

BTW, you could also develop a parallel application using some message passing paradigm, e.g. using MPI (or simply using some RPC or other messages, e.g. JSON on sockets). You might consider for regular numerical applications to use the GPGPU e.g. using OpenCL. And of course you might mix all the approaches (using OpenCL, with several threads, and having your parallel software running in several such processes communicating with MPI).

Debugging a heavily parallel software can become a nightmare. Performance may depend upon the hardware system and may require tricky tuning. scalability and synchronization may becoming a growing concern.map-reduce is often a useful model.

Upvotes: 12

user3344003
user3344003

Reputation: 21647

In C++ and C any memory location (identified by a variable) can be shared among threads. The memory space is the same across all threads. There is no parent/child thread relationship with memory.

The challenge is to control or synchronize access to the memory location among the threads.

That is implementation dependent.

Upvotes: 6

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