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:

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

       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.

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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