(gdb.info) Auto Display

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 Automatic display
 =================
 
    If you find that you want to print the value of an expression
 frequently (to see how it changes), you might want to add it to the
 "automatic display list" so that GDB prints its value each time your
 program stops.  Each expression added to the list is given a number to
 identify it; to remove an expression from the list, you specify that
 number.  The automatic display looks like this:
 
      2: foo = 38
      3: bar[5] = (struct hack *) 0x3804
 
 This display shows item numbers, expressions and their current values.
 As with displays you request manually using `x' or `print', you can
 specify the output format you prefer; in fact, `display' decides
 whether to use `print' or `x' depending on how elaborate your format
 specification is--it uses `x' if you specify a unit size, or one of the
 two formats (`i' and `s') that are only supported by `x'; otherwise it
 uses `print'.
 
 `display EXPR'
      Add the expression EXPR to the list of expressions to display each
      time your program stops.   Expressions Expressions.
 
      `display' does not repeat if you press <RET> again after using it.
 
 `display/FMT EXPR'
      For FMT specifying only a display format and not a size or count,
      add the expression EXPR to the auto-display list but arrange to
      display it each time in the specified format FMT.   Output
      formats Output Formats.
 
 `display/FMT ADDR'
      For FMT `i' or `s', or including a unit-size or a number of units,
      add the expression ADDR as a memory address to be examined each
      time your program stops.  Examining means in effect doing `x/FMT
      ADDR'.   Examining memory Memory.
 
    For example, `display/i $pc' can be helpful, to see the machine
 instruction about to be executed each time execution stops (`$pc' is a
 common name for the program counter;  Registers Registers.).
 
 `undisplay DNUMS...'
 `delete display DNUMS...'
      Remove item numbers DNUMS from the list of expressions to display.
 
      `undisplay' does not repeat if you press <RET> after using it.
      (Otherwise you would just get the error `No display number ...'.)
 
 `disable display DNUMS...'
      Disable the display of item numbers DNUMS.  A disabled display
      item is not printed automatically, but is not forgotten.  It may be
      enabled again later.
 
 `enable display DNUMS...'
      Enable display of item numbers DNUMS.  It becomes effective once
      again in auto display of its expression, until you specify
      otherwise.
 
 `display'
      Display the current values of the expressions on the list, just as
      is done when your program stops.
 
 `info display'
      Print the list of expressions previously set up to display
      automatically, each one with its item number, but without showing
      the values.  This includes disabled expressions, which are marked
      as such.  It also includes expressions which would not be
      displayed right now because they refer to automatic variables not
      currently available.
 
    If a display expression refers to local variables, then it does not
 make sense outside the lexical context for which it was set up.  Such an
 expression is disabled when execution enters a context where one of its
 variables is not defined.  For example, if you give the command
 `display last_char' while inside a function with an argument
 `last_char', GDB displays this argument while your program continues to
 stop inside that function.  When it stops elsewhere--where there is no
 variable `last_char'--the display is disabled automatically.  The next
 time your program stops where `last_char' is meaningful, you can enable
 the display expression once again.
 

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