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glibc  2.9
Functions | Variables
fork.c File Reference
#include <assert.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sysdep.h>
#include <libio/libioP.h>
#include <tls.h>
#include "fork.h"
#include <hp-timing.h>
#include <ldsodefs.h>
#include <bits/stdio-lock.h>
#include <atomic.h>
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static void fresetlockfiles (void)
pid_t __libc_fork (void)


unsigned long int__fork_generation_pointer
struct fork_handler__fork_handlers

Function Documentation

pid_t __libc_fork ( void  )

Definition at line 53 of file fork.c.

  pid_t pid;
  struct used_handler
    struct fork_handler *handler;
    struct used_handler *next;
  } *allp = NULL;

  /* Run all the registered preparation handlers.  In reverse order.
     While doing this we build up a list of all the entries.  */
  struct fork_handler *runp;
  while ((runp = __fork_handlers) != NULL)
      /* Make sure we read from the current RUNP pointer.  */
      atomic_full_barrier ();

      unsigned int oldval = runp->refcntr;

      if (oldval == 0)
       /* This means some other thread removed the list just after
          the pointer has been loaded.  Try again.  Either the list
          is empty or we can retry it.  */

      /* Bump the reference counter.  */
      if (atomic_compare_and_exchange_bool_acq (&__fork_handlers->refcntr,
                                          oldval + 1, oldval))
       /* The value changed, try again.  */

      /* We bumped the reference counter for the first entry in the
        list.  That means that none of the following entries will
        just go away.  The unloading code works in the order of the

         While executing the registered handlers we are building a
         list of all the entries so that we can go backward later on.  */
      while (1)
         /* Execute the handler if there is one.  */
         if (runp->prepare_handler != NULL)
           runp->prepare_handler ();

         /* Create a new element for the list.  */
         struct used_handler *newp
           = (struct used_handler *) alloca (sizeof (*newp));
         newp->handler = runp;
         newp->next = allp;
         allp = newp;

         /* Advance to the next handler.  */
         runp = runp->next;
         if (runp == NULL)

         /* Bump the reference counter for the next entry.  */
         atomic_increment (&runp->refcntr);

      /* We are done.  */

  _IO_list_lock ();

#ifndef NDEBUG
  pid_t ppid = THREAD_GETMEM (THREAD_SELF, tid);

  /* We need to prevent the getpid() code to update the PID field so
     that, if a signal arrives in the child very early and the signal
     handler uses getpid(), the value returned is correct.  */
  pid_t parentpid = THREAD_GETMEM (THREAD_SELF, pid);
  THREAD_SETMEM (THREAD_SELF, pid, -parentpid);

#ifdef ARCH_FORK
  pid = ARCH_FORK ();
# error "ARCH_FORK must be defined so that the CLONE_SETTID flag is used"
  pid = INLINE_SYSCALL (fork, 0);

  if (pid == 0)
      struct pthread *self = THREAD_SELF;

      assert (THREAD_GETMEM (self, tid) != ppid);

      if (__fork_generation_pointer != NULL)
       *__fork_generation_pointer += 4;

      /* Adjust the PID field for the new process.  */
      THREAD_SETMEM (self, pid, THREAD_GETMEM (self, tid));

      /* The CPU clock of the thread and process have to be set to zero.  */
      hp_timing_t now;
      HP_TIMING_NOW (now);
      THREAD_SETMEM (self, cpuclock_offset, now);
      GL(dl_cpuclock_offset) = now;

      /* Reset the file list.  These are recursive mutexes.  */
      fresetlockfiles ();

      /* Reset locks in the I/O code.  */
      _IO_list_resetlock ();

      /* Reset the lock the dynamic loader uses to protect its data.  */
      __rtld_lock_initialize (GL(dl_load_lock));

      /* Run the handlers registered for the child.  */
      while (allp != NULL)
         if (allp->handler->child_handler != NULL)
           allp->handler->child_handler ();

         /* Note that we do not have to wake any possible waiter.
            This is the only thread in the new process.  The count
            may have been bumped up by other threads doing a fork.
            We reset it to 1, to avoid waiting for non-existing
            thread(s) to release the count.  */
         allp->handler->refcntr = 1;

         /* XXX We could at this point look through the object pool
            and mark all objects not on the __fork_handlers list as
            unused.  This is necessary in case the fork() happened
            while another thread called dlclose() and that call had
            to create a new list.  */

         allp = allp->next;

      /* Initialize the fork lock.  */
      __fork_lock = LLL_LOCK_INITIALIZER;
      assert (THREAD_GETMEM (THREAD_SELF, tid) == ppid);

      /* Restore the PID value.  */
      THREAD_SETMEM (THREAD_SELF, pid, parentpid);

      /* We execute this even if the 'fork' call failed.  */
      _IO_list_unlock ();

      /* Run the handlers registered for the parent.  */
      while (allp != NULL)
         if (allp->handler->parent_handler != NULL)
           allp->handler->parent_handler ();

         if (atomic_decrement_and_test (&allp->handler->refcntr)
             && allp->handler->need_signal)
           lll_futex_wake (allp->handler->refcntr, 1, LLL_PRIVATE);

         allp = allp->next;

  return pid;

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static void fresetlockfiles ( void  ) [static]

Definition at line 43 of file fork.c.

  _IO_ITER i;

  for (i = _IO_iter_begin(); i != _IO_iter_end(); i = _IO_iter_next(i))
    _IO_lock_init (*((_IO_lock_t *) _IO_iter_file(i)->_lock));

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Variable Documentation

Definition at line 34 of file fork.c.

Definition at line 39 of file fork.c.