man2/get_robust_list.2

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  27 .\" FIXME Something could be added to this page (or exit(2))
  28 .\" about exit_robust_list processing
  29 .\"
  30 .TH GET_ROBUST_LIST 2 2017-09-15 Linux "Linux System Calls"
  31 .SH NAME
  32 get_robust_list, set_robust_list \- get/set list of robust futexes
  33 .SH SYNOPSIS
  34 .nf
  35 .B #include <linux/futex.h>
  36 .B #include <sys/types.h>
  37 .B #include <syscall.h>
  38 .PP
  39 .BI "long get_robust_list(int " pid ", struct robust_list_head **" head_ptr ,
  40 .BI "                     size_t *" len_ptr );
  41 .BI "long set_robust_list(struct robust_list_head *" head ", size_t " len );
  42 .fi
  43 .PP
  44 .IR Note :
  45 There are no glibc wrappers for these system calls; see NOTES.
  46 .SH DESCRIPTION
  47 These system calls deal with per-thread robust futex lists.
  48 These lists are managed in user space:
  49 the kernel knows only about the location of the head of the list.
  50 A thread can inform the kernel of the location of its robust futex list using
  51 .BR set_robust_list ().
  52 The address of a thread's robust futex list can be obtained using
  53 .BR get_robust_list ().
  54 .PP
  55 The purpose of the robust futex list is to ensure that if a thread
  56 accidentally fails to unlock a futex before terminating or calling
  57 .BR execve (2),
  58 another thread that is waiting on that futex is notified that
  59 the former owner of the futex has died.
  60 This notification consists of two pieces: the
  61 .BR FUTEX_OWNER_DIED
  62 bit is set in the futex word, and the kernel performs a
  63 .BR futex (2)
  64 .BR FUTEX_WAKE
  65 operation on one of the threads waiting on the futex.
  66 .PP
  67 The
  68 .BR get_robust_list ()
  69 system call returns the head of the robust futex list of the thread
  70 whose thread ID is specified in
  71 .IR pid .
  72 If
  73 .I pid
  74 is 0,
  75 the head of the list for the calling thread is returned.
  76 The list head is stored in the location pointed to by
  77 .IR head_ptr .
  78 The size of the object pointed to by
  79 .I **head_ptr
  80 is stored in
  81 .IR len_ptr .
  82 .PP
  83 Permission to employ
  84 .BR get_robust_list ()
  85 is governed by a ptrace access mode
  86 .B PTRACE_MODE_READ_REALCREDS
  87 check; see
  88 .BR ptrace (2).
  89 .PP
  90 The
  91 .BR set_robust_list ()
  92 system call requests the kernel to record the head of the list of
  93 robust futexes owned by the calling thread.
  94 The
  95 .I head
  96 argument is the list head to record.
  97 The
  98 .I len
  99 argument should be
 100 .IR sizeof(*head) .
 101 .SH RETURN VALUE
 102 The
 103 .BR set_robust_list ()
 104 and
 105 .BR get_robust_list ()
 106 system calls return zero when the operation is successful,
 107 an error code otherwise.
 108 .SH ERRORS
 109 The
 110 .BR set_robust_list ()
 111 system call can fail with the following error:
 112 .TP
 113 .B EINVAL
 114 .I len
 115 does not equal
 116 .IR "sizeof(struct\ robust_list_head)" .
 117 .PP
 118 The
 119 .BR get_robust_list ()
 120 system call can fail with the following errors:
 121 .TP
 122 .B EPERM
 123 The calling process does not have permission to see the robust futex list of
 124 the thread with the thread ID
 125 .IR pid ,
 126 and does not have the
 127 .BR CAP_SYS_PTRACE
 128 capability.
 129 .TP
 130 .B ESRCH
 131 No thread with the thread ID
 132 .I pid
 133 could be found.
 134 .TP
 135 .B EFAULT
 136 The head of the robust futex list can't be stored at the location
 137 .IR head .
 138 .SH VERSIONS
 139 These system calls were added in Linux 2.6.17.
 140 .SH NOTES
 141 These system calls are not needed by normal applications.
 142 No support for them is provided in glibc.
 143 In the unlikely event that you want to call them directly, use
 144 .BR syscall (2).
 145 .PP
 146 A thread can have only one robust futex list;
 147 therefore applications that wish
 148 to use this functionality should use the robust mutexes provided by glibc.
 149 .PP
 150 In the initial implementation,
 151 a thread waiting on a futex was notified that the owner had died
 152 only if the owner terminated.
 153 Starting with Linux 2.6.28,
 154 .\" commit 8141c7f3e7aee618312fa1c15109e1219de784a7
 155 notification was extended to include the case where the owner performs an
 156 .BR execve (2).
 157 .PP
 158 The thread IDs mentioned in the main text are
 159 .I kernel
 160 thread IDs of the kind returned by
 161 .BR clone (2)
 162 and
 163 .BR gettid (2).
 164 .SH SEE ALSO
 165 .BR futex (2),
 166 .BR pthread_mutexattr_setrobust (3)
 167 .PP
 168 .IR Documentation/robust-futexes.txt
 169 and
 170 .IR Documentation/robust-futex-ABI.txt
 171 in the Linux kernel source tree
 172 .\" http://lwn.net/Articles/172149/