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Debugging with GDB

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8.9 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 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 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 section 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.


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