get_robust_list, set_robust_list - get/set list of robust futexes
Standard C library (
libc,
-lc)
#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).
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).
The
set_robust_list() and
get_robust_list() system calls return
zero when the operation is successful, an error code otherwise.
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.
These system calls were added in Linux 2.6.17.
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).
futex(2),
pthread_mutexattr_setrobust(3)
Documentation/robust-futexes.txt and
Documentation/robust-futex-ABI.txt in the Linux kernel source
tree