After forking, are global variables shared?

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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)?

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.

fork() and memory shared b/w processes created using it , program since memory allocated in the heap remains in existence for static local variables are on the heap (this is how they keep their The FORK() System Call. Processes in Each child process inherents the shared memory allocated by. Prerequisite : fork() in C. So when we do a fork() what are the sections the two process actually share ? Is the heap memory per process ? Are the global variables shared ? Will malloc return the same address to both ? Let us run below program and take look at the output of it to clear the questions above .

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.

[PDF] Processes, Problem is these Global Variables are not getting updated when I tried Fork:: Shared and Dat::Dumper, but ther issue is they are not able to save here is that your child will also have a hangover after only you got drunk. Before fork call in my code, the parent process has a global variable declared. So after the fork call does child process get a separate copy of the global variable on its own thread stack or shares the existing parent instance of global variable. so i guess there are three possibilities here 1) child process gets separate instance of global variable declared in parent process 2) child process shares the global variable with parent thread.

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().

Global Variables Not Updated in Child Process, As what i know when you fork () a process, parent process and child process will be able to access the same global variables. But i'm having the issue that when i changes some values in the child process, parent process isn't taking the changed values instead it still getting the same value as i initialized at first?

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.

Parallel::ForkManager uses fork() (hence the name). This means that each version running in parallel is actually a separate process (a separate invocation of the perl interpreter) and thus separate memory. The variables will have the same name in each process, but they won't point to the same place in memory.

When the processes fork any global variables are passed but if you change the value of that variable in the forked process the change is only visible to the process that changed it. It sounds to me like you should be looking at a threaded solution since threads can share variables but you will need to use semaphors or some equivalent to protect the variable from multiple concurrent writes to it.

Once the excel macro runs with a global variable’s value of the variable is the same across all the Sub procedures. It is better to maintain a particular module to declare global variables in VBA and have all the variables in one module.

If static or global variables are used then mutexes must be applied or the functions must be re-written to avoid the use of these variables. In C, local variables are dynamically allocated on the stack. Therefore, any function that does not use static data or other shared resources is thread-safe.

Comments
  • I agree with Tom. You could have just output the value from both processes and seen for yourself what the output was. Though remember to sleep in the one that is not expecting an increase in myvar so that it doesn't execute before the other process would (though it won't) increment it. But yeah, just run it and see.
  • Note the misleading comment: actually, the parent process increments the counter (unless the fork failed) and the child process does nothing except exit immediately.
  • First, you aren’t checking for failure. Second, only file descriptors are shared.
  • Are you sure about this part global variable can be shared among parent and child process ?
  • +1 Although I would argue that "completely different existence" may be misleading as some objects, such as file-handles, may be shared under different circumstances, IIRC.
  • True, each process will have its own handle to any shared resources (files, pipes, etc), as a consequence of the "exact copy". Not much different, in fact, from two separate processes opening the same file.
  • There's one crucial difference between two processes opening the same file and two processes sharing a file descriptor created before forking: the forked processes share the same open file description and offset; if one process moves the file pointer (by reading or writing or seeking), it affects the other too. This does not happen with separate opens because they have separate open file descriptions. See: open(), fork(), lseek(), etc.
  • Thanks, I didn't know that. Or, if I did know it, I forgot. It's been a long time since I've fork()ed.
  • It doesn’t matter whether a variable is global or not.
  • @tchrist I know, I mention globals because it seems what the op is worried about.