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| .\" FIXME Something could be added to this page (or exit(2)) |
| .\" about exit_robust_list processing |
| .\" |
| .TH GET_ROBUST_LIST 2 2021-03-22 Linux "Linux System Calls" |
| .SH NAME |
| get_robust_list, set_robust_list \- get/set list of robust futexes |
| .SH SYNOPSIS |
| .nf |
| .BR "#include <linux/futex.h>" \ |
| " /* Definition of " "struct robust_list_head" " */" |
| .BR "#include <sys/syscall.h>" " /* Definition of " SYS_* " constants */" |
| .B #include <unistd.h> |
| .PP |
| .BI "long syscall(SYS_get_robust_list, int " pid , |
| .BI " struct robust_list_head **" head_ptr ", size_t *" len_ptr ); |
| .BI "long syscall(SYS_set_robust_list," |
| .BI " struct robust_list_head *" head ", size_t " len ); |
| .fi |
| .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 EFAULT |
| The head of the robust futex list can't be stored at the location |
| .IR head . |
| .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. |
| .SH VERSIONS |
| These system calls were added in Linux 2.6.17. |
| .SH NOTES |
| These system calls are not needed by normal applications. |
| .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/ |