.\" Copyright (C) 2006 Red Hat, Inc. All Rights Reserved. .\" Written by Ivana Varekova .\" and Copyright (c) 2017, Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" FIXME Something could be added to this page (or exit(2)) .\" about exit_robust_list processing .\" .TH GET_ROBUST_LIST 2 2017-09-15 Linux "Linux System Calls" .SH NAME get_robust_list, set_robust_list \- get/set list of robust futexes .SH SYNOPSIS .nf .B #include .B #include .B #include .PP .BI "long get_robust_list(int " pid ", struct robust_list_head **" head_ptr , .BI " size_t *" len_ptr ); .BI "long set_robust_list(struct robust_list_head *" head ", size_t " len ); .fi .PP .IR Note : There are no glibc wrappers for these system calls; see NOTES. .SH 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 .BR set_robust_list (). The address of a thread's robust futex list can be obtained using .BR get_robust_list (). .PP The purpose of the robust futex list is to ensure that if a thread accidentally fails to unlock a futex before terminating or calling .BR 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 .BR FUTEX_OWNER_DIED bit is set in the futex word, and the kernel performs a .BR futex (2) .BR FUTEX_WAKE operation on one of the threads waiting on the futex. .PP The .BR get_robust_list () system call returns the head of the robust futex list of the thread whose thread ID is specified in .IR pid . If .I 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 .IR head_ptr . The size of the object pointed to by .I **head_ptr is stored in .IR len_ptr . .PP Permission to employ .BR get_robust_list () is governed by a ptrace access mode .B PTRACE_MODE_READ_REALCREDS check; see .BR ptrace (2). .PP The .BR set_robust_list () system call requests the kernel to record the head of the list of robust futexes owned by the calling thread. The .I head argument is the list head to record. The .I len argument should be .IR sizeof(*head) . .SH RETURN VALUE The .BR set_robust_list () and .BR get_robust_list () system calls return zero when the operation is successful, an error code otherwise. .SH ERRORS The .BR set_robust_list () system call can fail with the following error: .TP .B EINVAL .I len does not equal .IR "sizeof(struct\ robust_list_head)" . .PP The .BR get_robust_list () system call can fail with the following errors: .TP .B EPERM The calling process does not have permission to see the robust futex list of the thread with the thread ID .IR pid , and does not have the .BR CAP_SYS_PTRACE capability. .TP .B ESRCH No thread with the thread ID .I pid could be found. .TP .B EFAULT The head of the robust futex list can't be stored at the location .IR head . .SH VERSIONS These system calls were added in Linux 2.6.17. .SH 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 .BR syscall (2). .PP 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. .PP 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, .\" commit 8141c7f3e7aee618312fa1c15109e1219de784a7 notification was extended to include the case where the owner performs an .BR execve (2). .PP The thread IDs mentioned in the main text are .I kernel thread IDs of the kind returned by .BR clone (2) and .BR gettid (2). .SH SEE ALSO .BR futex (2), .BR pthread_mutexattr_setrobust (3) .PP .IR Documentation/robust-futexes.txt and .IR Documentation/robust-futex-ABI.txt in the Linux kernel source tree .\" http://lwn.net/Articles/172149/ .SH COLOPHON This page is part of release 4.16 of the Linux .I 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/.