gdb: Assignment
17.1 Assignment to Variables
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To alter the value of a variable, evaluate an assignment expression.
Expressions Expressions. For example,
print x=4
stores the value 4 into the variable 'x', and then prints the value of
the assignment expression (which is 4). Using GDB with Different
Languages Languages, for more information on operators in supported
languages.
If you are not interested in seeing the value of the assignment, use
the 'set' command instead of the 'print' command. 'set' is really the
same as 'print' except that the expression's value is not printed and is
not put in the value history (Value History Value History.). The
expression is evaluated only for its effects.
If the beginning of the argument string of the 'set' command appears
identical to a 'set' subcommand, use the 'set variable' command instead
of just 'set'. This command is identical to 'set' except for its lack
of subcommands. For example, if your program has a variable 'width',
you get an error if you try to set a new value with just 'set width=13',
because GDB has the command 'set width':
(gdb) whatis width
type = double
(gdb) p width
$4 = 13
(gdb) set width=47
Invalid syntax in expression.
The invalid expression, of course, is '=47'. In order to actually set
the program's variable 'width', use
(gdb) set var width=47
Because the 'set' command has many subcommands that can conflict with
the names of program variables, it is a good idea to use the 'set
variable' command instead of just 'set'. For example, if your program
has a variable 'g', you run into problems if you try to set a new value
with just 'set g=4', because GDB has the command 'set gnutarget',
abbreviated 'set g':
(gdb) whatis g
type = double
(gdb) p g
$1 = 1
(gdb) set g=4
(gdb) p g
$2 = 1
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/smith/cc_progs/a.out
"/home/smith/cc_progs/a.out": can't open to read symbols:
Invalid bfd target.
(gdb) show g
The current BFD target is "=4".
The program variable 'g' did not change, and you silently set the
'gnutarget' to an invalid value. In order to set the variable 'g', use
(gdb) set var g=4
GDB allows more implicit conversions in assignments than C; you can
freely store an integer value into a pointer variable or vice versa, and
you can convert any structure to any other structure that is the same
length or shorter.
To store values into arbitrary places in memory, use the '{...}'
construct to generate a value of specified type at a specified address
(Expressions Expressions.). For example, '{int}0x83040' refers
to memory location '0x83040' as an integer (which implies a certain size
and representation in memory), and
set {int}0x83040 = 4
stores the value 4 into that memory location.
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