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After forking, are global variables shared?

Posted by: admin November 29, 2017 Leave a comment

Questions:

Consider this simple code:

 int myvar = 0;
 int main() {
     if (fork()>0) {
       myvar++;
     } else {
       // father do nothing
     }
 }

When child increments myvar, is the value shared with the father (like pthread)?

Answers:

No and yes.

No, they are not shared in any way which is visible to the programmer; the processes can modify their own copies of the variables independently and they will change without any noticable effect on the other process(es) which are fork() parents, siblings or descendents.

But yes, the OS actually does share the pages initially, because fork implements copy-on-write which means that provided none of the processes modifies the pages, they are shared. This is, however, an optimisation which can be ignored.

If you wanted to have shared variables, put them in an anonymous shared mapping (see mmap()) in which case they really will get shared, with all the caveats which come with that.

Questions:
Answers:

fork()ing creates an exact copy of the parent process at the time of forking. However, after the fork() is completed, the child has a completely different existence, and will not report back to the parent.

In other words, no, the parent’s global variables will not be altered by changes in the child.

Questions:
Answers:

After fork(), the entire process, including all global variables, is duplicated. The child is an exact replica of the parent, except that it has a different PID(Process Id), a different parent, and fork() returned 0. Global variables are still global within its own process. So the answer is no, global variables are not shared between processes after a call to fork().

Questions:
Answers:

No, since global variables are not shared between processes unless some IPC mechanism is implemented. The memory space will be copied. As a consequence, the global variable in both processes will have the same value inmediately after fork, but if one changes it, the other wont see it changed.

Threads on the other hand do share global variables.