get_robust_list, set_robust_list - get/set list of robust futexes
#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.
futex(2), pthread_mutexattr_setrobust(3)
Documentation/robust-futexes.txt
and
Documentation/robust-futex-ABI.txt
in the Linux kernel source
tree