Once you think you have found an error in your program, you might want to
find out for certain whether correcting the apparent error would lead to
correct results in the rest of the run. You can find the answer by
experiment, using the gdb features for altering execution of the
program.
For example, you can store new values into variables or memory
locations, give your program a signal, restart it at a different
address, or even return prematurely from a function.
16.1. Assignment to variables
To alter the value of a variable, evaluate an assignment expression. Refer to Section 10.1 Expressions. For example,
stores the value 4 into the variable x, and then prints the
value of the assignment expression (which is 4). Refer to Chapter 14 Using gdb with Different 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 (refer to Section 10.8 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
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 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
(refer to Section 10.1 Expressions). For example, {int}0x83040 refers
to memory location 0x83040 as an integer (which implies a certain size
and representation in memory), and
stores the value 4 into that memory location.
