Do forked child processes use the same semaphore?

by

Let’s say I create a semaphore. If I fork a bunch of child processes, will they all still use that same semaphore?

If you are using a SysV IPC semaphore (semctl), then yes. If you are using POSIX semaphores (sem_init), then yes, but only if you pass a true value for the pshared argument on creation and place it in shared memory.

Also, suppose I create a struct with semaphores inside and forked. Do all the child processes still use that same semaphore? If not, would storing that struct+semaphores in shared memory allow the child processes to use the same semaphores?

What do you mean be ‘semaphores inside’? References to SysV IPC semaphores will be shared, because the semaphores don’t belong to any process. If you’re using POSIX semaphores, or constructing something out of pthreads mutexes and condvars, you will need to use shared memory, and the pshared attribute (pthreads has a pshared attribute for condvars and mutexes as well)

Note that anonymous mmaps created with the MAP_SHARED flag count as (anonymous) shared memory for these purposes, so it’s not necessary to actually create a named shared memory segment. Ordinary heap memory will not be shared after a fork.

More Related Contents:

  1. How to use POSIX semaphores on forked processes in C?
  2. How to use shared memory with Linux in C
  3. what the output of this code and why?
  4. Creating child process in C linux [closed]
  5. printf anomaly after “fork()”
  6. How to make child process die after parent exits?
  7. Why does this program print “forked!” 4 times?
  8. Working of fork() in linux gcc [duplicate]
  9. After forking, are global variables shared?
  10. Are file descriptors shared when fork()ing?
  11. What is the difference between using _exit() & exit() in a conventional Linux fork-exec?
  12. Difference between “system” and “exec” in Linux?
  13. Why does forking my process cause the file to be read infinitely
  14. Sharing memory between processes through the use of mmap()
  15. Linux shared memory: shmget() vs mmap()?
  16. Duplicated output using printf() and fork() in C
  17. how to set close-on-exec by default
  18. How to write a signal handler to catch SIGSEGV?
  19. Where is the itoa function in Linux?
  20. Why do we cast sockaddr_in to sockaddr when calling bind()?
  21. How to set the name of a thread in Linux pthreads?
  22. “undefined reference to `pow'” even with math.h and the library link -lm [duplicate]
  23. How Can I debug a C program on Linux?
  24. Pipe buffer size is 4k or 64k?
  25. How to wait for exit of non-children processes
  26. Compiler using local variables without adjusting RSP
  27. How to access mmaped /dev/mem without crashing the Linux kernel?
  28. How can a shared library (.so) call a function that is implemented in its loader code?
  29. How can I include a needed C library using GCC?
  30. When to use pthread condition variables?

Leave a Comment