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 section Registers). (Value history references, in contrast, are numbers preceded by `$'. See section 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
Using a convenience variable for the first time creates it, but its
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.
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.
$_is automatically set by the
xcommand to the last address examined (see section Examining memory). Other commands which provide a default address for
xto examine also set
$_to that address; these commands include
info breakpoint. The type of
void *except when set by the
xcommand, in which case it is a pointer to the type of
$__is automatically set by the
xcommand to the value found in the last address examined. Its type is chosen to match the format in which the data was printed.
$_exitcodeis automatically set to the exit code when the program being debugged terminates.
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