The assign and the set variable Commands

Use the assign (dbx) and the set variable (gdb) commands to change the value associated with a variable, memory address, or expression that is accessible according to the scope and visibility rules of the language. The expression can be any expression that is valid in the current context.

DBX Mode

modifying_command

        : assign target = expression

        | patch target = expression

target

        : unary_expression

GDB Mode

modifying_command

        : set [ variable ] expression

The set variable command evaluates the specified expression. If expression includes assignment operator, it executes like all other operators. This is the way to change memory.

The only difference between the set variable and the print commands is printing the value - the set variable does not print anything.

Note:

The variable can be omitted if the beginning of expression does not confuse the debugger, for example, does not look like a valid subcommand for the set command.

The following example shows how to deposit the value 5 into the data member _data of a C++ object:

DBX Mode

(idb) print node->_data

2

(idb) assign node->_data = 5

(idb) print node->_data

5

GDB Mode

(idb) print node->_data

$2 = 2

(idb) set variable node->_data = 5

(idb) print node->_data

$3 = 5

The following example shows how to change the value associated with a variable and the value associated with an expression:

DBX Mode

(idb) print *node

class CompoundNode {

  _fdata = 12.3450003;

  _data = 5;                      // class IntNode

  _nextNode = 0x0;                // class IntNode::Node

}

(idb) assign node->_data = -32

(idb) assign node->_fdata = 3.14 * 4.5

(idb) assign node->_nextNode = _firstNode

(idb) print *node

class CompoundNode {

  _fdata = 14.1300001;

  _data = -32;                    // class IntNode

  _nextNode = 0x805e5e0;          // class IntNode::Node

}

GDB Mode

(idb) print *node

$6 = {<IntNode> = {<Node> = {_nextNode = 0x0}, _data = 5}, _fdata = 12.345}

(idb) set variable node->_data = -32

(idb) set variable node->_fdata = 3.14 * 4.5

(idb) set variable node->_nextNode = _firstNode

(idb) print *node

$7 = {<IntNode> = {<Node> = {_nextNode = 0x805e5e0}, _data = -32}, _fdata = 14.13}

For C++, use the assign (dbx) and the set variable (gdb) commands to modify static and object data members in a class, and variables declared as reference types, type const, or type static. You cannot change the address referred to by a reference type, but you can change the value at that address.

assign [classname::]member = ["filename"] `expression

assign [object.]member = ["filename"] `expression

Note:

Do not use the assign (dbx) and the set variable (gdb) commands to change the PC.  When you change the PC, no adjustment to the contents of registers or memory is made. Because most instructions change registers or memory in ways that can impact the meaning of the application, changing the PC is very likely to cause your application to do incorrect calculations and arrive at the wrong answer. Access violations and other errors and signals may result from changing the value in the PC.