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GET_ROBUST_LIST(2)                                                         Linux System Calls                                                         GET_ROBUST_LIST(2)

NAME
       get_robust_list, set_robust_list - get/set list of robust futexes

SYNOPSIS
       #include <linux/futex.h>   /* Definition of struct robust_list_head */
       #include <sys/syscall.h>   /* Definition of SYS_* constants */
       #include <unistd.h>

       long syscall(SYS_get_robust_list, int pid,
                    struct robust_list_head **head_ptr, size_t *len_ptr);
       long syscall(SYS_set_robust_list,
                    struct robust_list_head *head, size_t len);

       Note: glibc provides no wrappers for these system calls, necessitating the use of syscall(2).

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  ob‐
       tained 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 capa‐
              bility.

       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.

       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, noti‐
       fication 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

Linux                                                                          2021-03-22                                                             GET_ROBUST_LIST(2)