Node:Convenience Vars, Next:Registers, Previous:Value History, Up:Data

### 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 (see Registers). (Value history references, in contrast, are numbers preceded by `$`. See 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 example:

         set $foo = *object_ptr

would save in `$foo` the value contained in the object pointed to by `object_ptr`.

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 (see Examining 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 `$__` is automatically set by the `x` command to the value found in the last address examined. Its type is chosen to match the format in which the data was printed.
`$_exitcode`
  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.