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.\" FIXME Something could be added to this page (or exit(2))
.\" about exit_robust_list processing
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.TH get_robust_list 2 2022-10-30 "Linux man-pages 6.03"
.SH NAME
get_robust_list, set_robust_list \- get/set list of robust futexes
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.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 );
.B long syscall(SYS_set_robust_list,
.BI "             struct robust_list_head *" head ", size_t " len );
.fi
.PP
.IR Note :
glibc provides no wrappers for these system calls,
necessitating the use of
.BR syscall (2).
.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
.B FUTEX_OWNER_DIED
bit is set in the futex word, and the kernel performs a
.BR futex (2)
.B 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
.B 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
.I Documentation/robust\-futexes.txt
and
.I Documentation/robust\-futex\-ABI.txt
in the Linux kernel source tree
.\" http://lwn.net/Articles/172149/