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NAME

       get_robust_list, set_robust_list - get/set list of robust futexes

SYNOPSIS

       #include <linux/futex.h>
       #include <sys/types.h>
       #include <syscall.h>

       long get_robust_list(int pid, struct robust_list_head **head_ptr,
                            size_t *len_ptr);
       long set_robust_list(struct robust_list_head *head, size_t len);

       Note: There are no glibc wrappers for these system calls; see NOTES.

DESCRIPTION

       These  system  calls  deal with per-thread robust futex lists.  These lists are managed in
       user space: the kernel knows only about the location of the head of the  list.   A  thread
       can  inform  the  kernel of the location of its robust futex list using set_robust_list().
       The address of a thread's robust futex list can be obtained using get_robust_list().

       The purpose of the robust futex list is to ensure that if a thread accidentally  fails  to
       unlock  a futex before terminating or calling execve(2), another thread that is waiting on
       that futex is notified that the former owner of the futex  has  died.   This  notification
       consists  of two pieces: the FUTEX_OWNER_DIED bit is set in the futex word, and the kernel
       performs a futex(2) FUTEX_WAKE operation on one of the threads waiting on the futex.

       The get_robust_list() system call returns the head of the robust futex list of the  thread
       whose  thread  ID  is specified in pid.  If pid is 0, the head of the list for the calling
       thread is returned.  The list head is stored in the location pointed to by head_ptr.   The
       size of the object pointed to by **head_ptr is stored in len_ptr.

       Permission   to   employ   get_robust_list()   is   governed   by  a  ptrace  access  mode
       PTRACE_MODE_READ_REALCREDS check; see ptrace(2).

       The set_robust_list() system call requests the kernel to record the head of  the  list  of
       robust futexes owned by the calling thread.  The head argument is the list head to record.
       The len argument should be sizeof(*head).

RETURN VALUE

       The set_robust_list() and get_robust_list() system calls return zero when the operation is
       successful, an error code otherwise.

ERRORS

       The set_robust_list() system call can fail with the following error:

       EINVAL len does not equal sizeof(struct robust_list_head).

       The get_robust_list() system call can fail with the following errors:

       EPERM  The  calling  process  does not have permission to see the robust futex list of the
              thread with the thread ID pid, and does not have the CAP_SYS_PTRACE capability.

       ESRCH  No thread with the thread ID pid could be found.

       EFAULT The head of the robust futex list can't be stored at the location head.

VERSIONS

       These system calls were added in Linux 2.6.17.

NOTES

       These system calls are not needed by normal applications.  No support for them is provided
       in glibc.  In the unlikely event that you want to call them directly, use syscall(2).

       A thread can have only one robust futex list; therefore applications that wish to use this
       functionality should use the robust mutexes provided by glibc.

       In the initial implementation, a thread waiting on a futex was notified that the owner had
       died  only if the owner terminated.  Starting with Linux 2.6.28, notification was extended
       to include the case where the owner performs an execve(2).

       The thread IDs mentioned in the main text are kernel thread IDs of the  kind  returned  by
       clone(2) and gettid(2).

SEE ALSO

       futex(2), pthread_mutexattr_setrobust(3)

       Documentation/robust-futexes.txt   and  Documentation/robust-futex-ABI.txt  in  the  Linux
       kernel source tree

COLOPHON

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