get_robust_list(2) - Linux manual page (original) (raw)

getrobustlist(2) System Calls Manual getrobustlist(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** _headptr_**, size_t ***_sizep_**);**
   **long syscall(SYS_set_robust_list,**
                **struct robust_list_head ***_head_**, size_t** _size_**);**

   _Note_: glibc provides no wrappers for these system calls,
   necessitating the use of [syscall(2)](../man2/syscall.2.html).

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)](../man2/execve.2.html), 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)](../man2/futex.2.html)
   **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 _headptr_.
   The size of the object pointed to by _**headptr_ is stored in
   _sizep_.

   Permission to employ **get_robust_list**() is governed by a ptrace
   access mode **PTRACE_MODE_READ_REALCREDS** check; see [ptrace(2)](../man2/ptrace.2.html).

   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 _size_
   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
   error:

   **EINVAL** _size_ does not equal _sizeof(struct robustlisthead)_.

   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 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)](../man2/execve.2.html).

   The thread IDs mentioned in the main text are _kernel_ thread IDs of
   the kind returned by [clone(2)](../man2/clone.2.html) and [gettid(2)](../man2/gettid.2.html).

SEE ALSO top

   [futex(2)](../man2/futex.2.html), [pthread_mutexattr_setrobust(3)](../man3/pthread%5Fmutexattr%5Fsetrobust.3.html)

   _Documentation/robust-futexes.txt_ and
   _Documentation/robust-futex-ABI.txt_ in the Linux kernel source tree

COLOPHON top

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Linux man-pages 6.10 2024-11-17 getrobustlist(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)