get_robust_list(2) — Linux manual page


get_robust_list(2)         System Calls Manual        get_robust_list(2)

NAME         top

       get_robust_list, set_robust_list - get/set list of robust futexes

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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         top

       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         top

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

ERRORS         top

       The set_robust_list() system call can fail with the following

       EINVAL len does not equal sizeof(struct robust_list_head).

       The get_robust_list() system call can fail with the following

       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         top

       These system calls were added in Linux 2.6.17.

NOTES         top

       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, 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         top

       futex(2), pthread_mutexattr_setrobust(3)

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02             get_robust_list(2)

Pages that refer to this page: execve(2)futex(2)gettid(2)ptrace(2)syscalls(2)exit(3)pthread_mutexattr_setrobust(3)capabilities(7)futex(7)