get_robust_list

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>
       #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  loca-
       tion  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_ro-
       bust_list().

       The purpose of the robust futex list is to ensure that if a thread  ac-
       cidentally  fails  to  unlock a futex before terminating or calling ex-
       ecve(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 ob-
       ject 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 loca-
              tion 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 pro-
       vided 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
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2019-10-10                GET_ROBUST_LIST(2)