Receiving a Signal and RealTime Signal

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Receiving a Signaland Real-Time Signal

• prepared by He, Xiaosu

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Contents

• Remembering Signal
• roles of signal
• pending signal
• performed actions of signal
• Receiving Signal
• do_signal ( ) function
• system calls related to signal handling
• Real Time Signal

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Remembering Signal

• Two main purposes of signal
• -making a process aware that a specific event
  has occurred
• -forcing a process to execute a signal
  handler function
• included in its code
• Pending signal
• -signals that have been sent but not yet
  received
• Three ways in which a process can respond
• -explicitly ignore the signal
• -execute the default action associated with
  the signal
• -catch the signal by invoking a
  corresponding signalhandler
• function

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Receiving Signal

• One phase related to signal transmission
• three ways in which the kernel forces the
  destination process to react to the signal
• by changing its execution state
• by starting the execution of a specified
  signal handler
• or both
• Notice
• signals are consumable resources
• each signal sent received no more than once

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do_signal ( ) function

• Motive
• in order to handle the nonblocked pending
  signals
• Two parameters
• -regs address of the stack area with User
  Mode
• register contents of the current
  process
• -oldset address of a variable with the bit
  mask
• array of blocked signals (
  Null, when
• invoked from ret_from_intr( ) )

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do_signal ( ) function

• Starting by checking
• whether the interrupt occurred while the
  process was run-
• ning in User Mode
• dequeue_signal ( )
• -heart of the do_signal ( ) function
• -repeated invoking of dequeue_signal ( )
  until no more nonblocked pending signal
• -its return code ( stored in the signr local
  variable )
• value 0 -- all pending signals have
  been handled
• value ! 0 -- a pending signal is
  waiting to be handled

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do_signal ( ) function

• notify_parent ( ) and schedule ( )
• -when the current receiver process is being
  monitored by
• some other process.
• -to make the monitoring process aware of the
  signal handling
• Loading the ka local variable
• -with the address of the k_sigaction data
  structure
• -perform three kinds of actions depending on
  the contents
• ignoring the signal
• executing a default action
• executing a signal handler

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Three Kinds of Actions

• do_signal ( )
• Ignoring the signal
• No
  Yes
• Executing the
• default action
• No Yes
• Executing the
• signal handler

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Executing a Default Action

• Depending on the signal type
• -abort the process is destroyed
• -dump the process is destroyed and a core
  file is created
• -ignore the signal is ignored
• -stop the process is stopped, put in a
  TASK_STOPPED state
• -continue if the process is stopped
  (TASK_STOPPED), it is put into the TASK_RUNNING
  state

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Catching the Signal

• User Mode Kernel
  Mode

Normal program flow Signal
handler
return code on the stack
do_signal ( ) handle_signal ( )
setup_frame ( ) system_call ( )
sys_sigreturn ( )
restore_sigcontext ( )
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Setting up the Frame

• setup_frame ( )
• -four parameters
• sig -- signal number
• ka -- address of the k_sigaction
  table associates with the signal
• oldset -- address of a bit mask array
  of blocked signals
• regs -- address in the Kernel Mode
  stack area where the User
• Mode register contents
  have been saved
• -a frame
• data structure pushed onto the User
  Mode stack by this function
• contains the information needed to
  handle the signal and to
• ensure the correct return to the
  sys_sigreturn ( ) function
• a sigframe table

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Setting up the Frame

• setup_frame ( ) function
• -starts by invoking get_sigframe ( ) to
  compute the first memory location of the frame
• -the returned address is verified by means of
  the access_ok macro
• if valid -- the function repeatedly
  invokes __put_user ( ) to fill all
• the fields of the
  frame
• -modifies the regs area of the Kernel Mode
  stack
• -terminates by resetting the segmentation
  registers saved on the Kernel Mode stack to their
  default value
• setup_rt_frame ( ) function
• an extended frame ( stored in the rt_sigframe
  data
• structure ) on the User Mode stack

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System Calls related to Signal Handling

• The kill ( ) system call to send signals
• kill ( pid, sig )
• sys_kill ( ) ( service routine of kill ( ) )
• 
  send_sig_info ( )
• kill_something_info ( )
• 
  kill_pg_info ( )
• 
  
• 
  send_sig_info ( )
• -meanings of pid parameter due to its
  numerical value

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System Calls related to Signal Handling

• The sigaction ( ) system call to specify an
  action for a signal
• sigaction ( sig, act, oact)
• sys_sigaction ( ) ( service routine of
  sigaction ( ) )
• do_sigaction ( )
• -three parameters
• -pay attention to the sys_sigaction ( )
  function

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System Calls related to Signal Handling

• Other system calls
• -sigpending ( )
• allows a process to examine the set of pending
  blocked signals
• applies only the standard signals
• the corresponding service routine sys_sigpending
  ( ) acts on a single parameter set
• -sigprocmask ( )
• allows processes to modify the set of blocked
  signals
• applies only the standard signals
• the corresponding service routine sys_sigprocmask
  ( )
• acts on three parameters oset, set, how

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System Calls related to Signal handling

• -sigsuspend ( )
• suspends the process
• the process will wake up only when a nonignored,
• nonblocked signal is sent to it.
• the corresponding service routine sys_sigsuspend
  ( )
• sys_sigsuspend ( ) invokes the do_signal ( )
  function to
• receive the signal that has woken up the
  process, when
• the process that issued the sigsuspend ( )
  system call is
• executed again.

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Real Time Signals

• Signal numbers 32-63
• Difference with respect to standard signals
• real-time signals of the same kind may be
  queued to ensure that multiple signals sent will
  be received
• Linux kernel doesnt use real-time signals
• the queue of real-time signals
• a list of signal_queue elements

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Real Time Signals

• Process discriptor
• has two specific fields
• sigqueue points to the first element
  of the queue
• sigqueue_tail points to the next
  field of the last
• element of
  the queue
• Sending
• send_sig_info ( ) No. gt 31 ? inserts the
  signal in the queue
• Receiving
• dequeue_signal ( ) No. gt 31 ? extracts the
  element from the
• 
  queue

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Real Time Signals

• System Calls for Real-Time Signals
• rt_sigaction ( )
• rt_sigpending ( )
• rt_sigprocmask ( )
• rt_sigsuspend ( )
• rt_sigqueueinfo ( )
• sends a real-time signal to add to the
  real-time signal queue
• destination process
• rt_sigtimedwait ( )
• the process remains suspended only for a
  fixed time interval

similar to those used for standard signals
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Question
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