Guide: Threads and Processes with clone()

Let’s go back to our initial demos that used threads and processes. We’ll see that in order to create both threads and processes, the underlying Linux syscall is clone. For this, we’ll run both sum_array_threads and sum_array_processes under strace. As we’ve already established, we’re only interested in the clone syscall:

student@os:~/.../sum-array/support/c$ strace -e clone,clone3 ./sum_array_threads 2
clone(child_stack=0x7f60b56482b0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tid=[1819693], tls=0x7f60b5649640, child_tidptr=0x7f60b5649910) = 1819693
clone(child_stack=0x7f60b4e472b0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tid=[1819694], tls=0x7f60b4e48640, child_tidptr=0x7f60b4e48910) = 1819694

student@os:~/.../sum-array/support/c$ strace -e clone,clone3 ./sum_array_processes 2
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f7a4e346650) = 1820599
clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f7a4e346650) = 1820600

We ran each program with an argument of 2, so we have 2 calls to clone. Notice that in the case of threads, the clone3 syscall receives more arguments. The relevant flags passed as arguments when creating threads are documented in clone’s man page:

  • CLONE_VM: the child and the parent process share the same VAS
  • CLONE_{FS,FILES,SIGHAND}: the new thread shares the filesystem information, file and signal handlers with the one that created it. The syscall also receives valid pointers to the new thread’s stack and TLS, i.e. the only parts of the VAS that are distinct between threads (although they are technically accessible from all threads).

By contrast, when creating a new process, the arguments of the clone syscall are simpler (i.e. fewer flags are present). Remember that in both cases clone creates a new thread. When creating a process, clone creates this new thread within a new separate address space.