Convenience Vars

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Convenience variables

GDB provides "convenience variables" that you can use within GDB to
hold on to a value and refer to it later.  These variables exist
entirely within GDB; they are not part of your program, and setting a
convenience variable has no direct effect on further execution of your
program.  That is why you can use them freely.
   Convenience variables are prefixed with `$'.  Any name preceded by
`$' can be used for a convenience variable, unless it is one of the
predefined machine-specific register names (*note Registers:
Registers.).  (Value history references, in contrast, are _numbers_
preceded by `$'.  *Note Value history: Value History.)
   You can save a value in a convenience variable with an assignment
expression, just as you would set a variable in your program.  For
     set $foo = *object_ptr
would save in `$foo' the value contained in the object pointed to by
   Using a convenience variable for the first time creates it, but its
value is `void' until you assign a new value.  You can alter the value
with another assignment at any time.
   Convenience variables have no fixed types.  You can assign a
convenience variable any type of value, including structures and
arrays, even if that variable already has a value of a different type.
The convenience variable, when used as an expression, has the type of
its current value.
`show convenience'
     Print a list of convenience variables used so far, and their
     values.  Abbreviated `show conv'.
   One of the ways to use a convenience variable is as a counter to be
incremented or a pointer to be advanced.  For example, to print a field
from successive elements of an array of structures:
     set $i = 0
     print bar[$i++]->contents
Repeat that command by typing <RET>.
   Some convenience variables are created automatically by GDB and given
values likely to be useful.
     The variable `$_' is automatically set by the `x' command to the
     last address examined (*note Examining memory: Memory.).  Other
     commands which provide a default address for `x' to examine also
     set `$_' to that address; these commands include `info line' and
     `info breakpoint'.  The type of `$_' is `void *' except when set
     by the `x' command, in which case it is a pointer to the type of
     The variable `$__' is automatically set by the `x' command to the
     value found in the last address examined.  Its type is chosen to
     match the format in which the data was printed.
     The variable `$_exitcode' is automatically set to the exit code
     when the program being debugged terminates.
   On HP-UX systems, if you refer to a function or variable name that
begins with a dollar sign, GDB searches for a user or system name
first, before it searches for a convenience variable.