6 Values

6.1 Values

GDB uses struct value, or values, as an internal abstraction for the representation of a variety of inferior objects and GDB convenience objects.

Values have an associated struct type, that describes a virtual view of the raw data or object stored in or accessed through the value.

A value is in addition discriminated by its lvalue-ness, given its enum lval_type enumeration type:

not_lval

This value is not an lval. It can’t be assigned to.

lval_memory

This value represents an object in memory.

lval_register

This value represents an object that lives in a register.

lval_internalvar

Represents the value of an internal variable.

lval_internalvar_component

Represents part of a GDB internal variable. E.g., a structure field.

lval_computed

These are “computed” values. They allow creating specialized value objects for specific purposes, all abstracted away from the core value support code. The creator of such a value writes specialized functions to handle the reading and writing to/from the value’s backend data, and optionally, a “copy operator” and a “destructor”.

Pointers to these functions are stored in a struct lval_funcs instance (declared in value.h), and passed to the allocate_computed_value function, as in the example below.

static void
nil_value_read (struct value *v)
{
  /* This callback reads data from some backend, and stores it in V.
     In this case, we always read null data.  You'll want to fill in
     something more interesting.  */

  memset (value_contents_all_raw (v),
          value_offset (v),
          TYPE_LENGTH (value_type (v)));
}

static void
nil_value_write (struct value *v, struct value *fromval)
{
  /* Takes the data from FROMVAL and stores it in the backend of V.  */

  to_oblivion (value_contents_all_raw (fromval),
               value_offset (v),
               TYPE_LENGTH (value_type (fromval)));
}

static struct lval_funcs nil_value_funcs =
  {
    nil_value_read,
    nil_value_write
  };

struct value *
make_nil_value (void)
{
   struct type *type;
   struct value *v;

   type = make_nils_type ();
   v = allocate_computed_value (type, &nil_value_funcs, NULL);

   return v;
}

See the implementation of the $_siginfo convenience variable in infrun.c as a real example use of lval_computed.