NAME

get_robust_list, set_robust_list - get/set list of robust futexes

LIBRARY

Standard C library (libc, -lc)

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 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.

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).