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 (refer to Section 10.10 Registers). (Value history references, in contrast, are numbers preceded by $. Refer to Section 10.8 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.
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
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 (refer to Section 10.5 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.
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.