gdb.info: Convenience Vars
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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.
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 $i = 0
Repeat that command by typing <RET>.
Some convenience variables are created automatically by GDB and given
values likely to be useful.
The variable `$_' 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.
Created Mon Nov 8 17:42:37 2004 on tillpc with info_to_html version 0.9.6.