Setting a return value in debugger in C in order to find input arguments-CodePudding

Is there a way in C (using a debugger) to set the return value and find out what arguments would return that value? For example, if I have a function that takes in three integers, does something inside the function, and returns an integer, is there a way for me to set the return value as 0, and find out which integers when inputted give that return value?

I tried using an online GDB debugger, as well as the gdbgui debugger (from Linux). I understand how the debugger works and have been setting breakpoints but was wondering if there is an easy way to do this.

Wording of question: "Using a source-level debugger, determine for what values of arguments the function returns a zero."

CodePudding user response：

The assignment doesn't mean you should figure out which magic command inverts a function and paste it to the debugger window. There is no such command¹.

The assignment as I understand it means you should investigate what the function does, understand its logic, and derive the inverse function, using the debugger as a tool that helps you observe behaviour of the code.

¹ That can be used in practice, at any rate. There is a finite number of machine integers. You could write a script that tries every possible input, but you may or may not live long enough to see it finished running.

CodePudding user response：

Since this operation is about learning the gdb debugger ...

Set a breakpoint on the return statement of the function. In the example below, this is line 13, so we want b 13.
Only execute the breakpoint if the return value is 0. We want: cond 1 ret == 0
Print the argument values at the breakpoint. We use commands 1 to set gdb commands to be executed at the breakpoint. We want p a1, p a2, and p a3 to print the arguments. And, then, we want c to continue execution after the breakpoint.

Here is a simple sample program:

#include <stdio.h>

#ifndef LIM
#define LIM     5
#endif

int
func(int a1,int a2,int a3)
{

    int ret = a1   a2   a3;

    return ret;
}

int
main(void)
{
    int sum = 0;

    for (int a1 = -LIM;  a1 <= LIM;    a1) {
        for (int a2 = -LIM;  a2 <= LIM;    a2) {
            for (int a3 = -LIM;  a3 <= LIM;    a3)
                sum  = func(a1,a2,a3);
        }
    }

    printf("%d\n",sum);
}

We build the above program with (e.g.):

cc -g -o program program.c

In the above program, the return for func is line 13.

Here are the gdb commands we'd use. In this example, we put them in a file (e.g. cmds), but they could just as easily be entered at the command prompt:

b 13
cond 1 ret == 0
commands 1
echo "a1 is "
p a1
echo "a2 is "
p a2
echo "a3 is "
p a3
c
end
run
quit

We invoke gdb with the command:

gdb -q -x cmds ./program > out

Here is the output (e.g. out):

Reading symbols from ./program...done.
Breakpoint 1 at 0x401143: file program.c, line 13.

Breakpoint 1, func (a1=-5, a2=0, a3=5) at program.c:13
13      return ret;
"a1 is "$1 = -5
"a2 is "$2 = 0
"a3 is "$3 = 5

Breakpoint 1, func (a1=-5, a2=1, a3=4) at program.c:13
13      return ret;
"a1 is "$4 = -5
"a2 is "$5 = 1
"a3 is "$6 = 4

Breakpoint 1, func (a1=-5, a2=2, a3=3) at program.c:13
13      return ret;
"a1 is "$7 = -5
"a2 is "$8 = 2
"a3 is "$9 = 3

Breakpoint 1, func (a1=-5, a2=3, a3=2) at program.c:13
13      return ret;
"a1 is "$10 = -5
"a2 is "$11 = 3
"a3 is "$12 = 2

Breakpoint 1, func (a1=-5, a2=4, a3=1) at program.c:13
13      return ret;
"a1 is "$13 = -5
"a2 is "$14 = 4
"a3 is "$15 = 1

Breakpoint 1, func (a1=-5, a2=5, a3=0) at program.c:13
13      return ret;
"a1 is "$16 = -5
"a2 is "$17 = 5
"a3 is "$18 = 0

Breakpoint 1, func (a1=-4, a2=-1, a3=5) at program.c:13
13      return ret;
"a1 is "$19 = -4
"a2 is "$20 = -1
"a3 is "$21 = 5

Breakpoint 1, func (a1=-4, a2=0, a3=4) at program.c:13
13      return ret;
"a1 is "$22 = -4
"a2 is "$23 = 0
"a3 is "$24 = 4

Breakpoint 1, func (a1=-4, a2=1, a3=3) at program.c:13
13      return ret;
"a1 is "$25 = -4
"a2 is "$26 = 1
"a3 is "$27 = 3

Breakpoint 1, func (a1=-4, a2=2, a3=2) at program.c:13
13      return ret;
"a1 is "$28 = -4
"a2 is "$29 = 2
"a3 is "$30 = 2

Breakpoint 1, func (a1=-4, a2=3, a3=1) at program.c:13
13      return ret;
"a1 is "$31 = -4
"a2 is "$32 = 3
"a3 is "$33 = 1

Breakpoint 1, func (a1=-4, a2=4, a3=0) at program.c:13
13      return ret;
"a1 is "$34 = -4
"a2 is "$35 = 4
"a3 is "$36 = 0

Breakpoint 1, func (a1=-4, a2=5, a3=-1) at program.c:13
13      return ret;
"a1 is "$37 = -4
"a2 is "$38 = 5
"a3 is "$39 = -1

Breakpoint 1, func (a1=-3, a2=-2, a3=5) at program.c:13
13      return ret;
"a1 is "$40 = -3
"a2 is "$41 = -2
"a3 is "$42 = 5

Breakpoint 1, func (a1=-3, a2=-1, a3=4) at program.c:13
13      return ret;
"a1 is "$43 = -3
"a2 is "$44 = -1
"a3 is "$45 = 4

Breakpoint 1, func (a1=-3, a2=0, a3=3) at program.c:13
13      return ret;
"a1 is "$46 = -3
"a2 is "$47 = 0
"a3 is "$48 = 3

Breakpoint 1, func (a1=-3, a2=1, a3=2) at program.c:13
13      return ret;
"a1 is "$49 = -3
"a2 is "$50 = 1
"a3 is "$51 = 2

Breakpoint 1, func (a1=-3, a2=2, a3=1) at program.c:13
13      return ret;
"a1 is "$52 = -3
"a2 is "$53 = 2
"a3 is "$54 = 1

Breakpoint 1, func (a1=-3, a2=3, a3=0) at program.c:13
13      return ret;
"a1 is "$55 = -3
"a2 is "$56 = 3
"a3 is "$57 = 0

Breakpoint 1, func (a1=-3, a2=4, a3=-1) at program.c:13
13      return ret;
"a1 is "$58 = -3
"a2 is "$59 = 4
"a3 is "$60 = -1

Breakpoint 1, func (a1=-3, a2=5, a3=-2) at program.c:13
13      return ret;
"a1 is "$61 = -3
"a2 is "$62 = 5
"a3 is "$63 = -2

Breakpoint 1, func (a1=-2, a2=-3, a3=5) at program.c:13
13      return ret;
"a1 is "$64 = -2
"a2 is "$65 = -3
"a3 is "$66 = 5

Breakpoint 1, func (a1=-2, a2=-2, a3=4) at program.c:13
13      return ret;
"a1 is "$67 = -2
"a2 is "$68 = -2
"a3 is "$69 = 4

Breakpoint 1, func (a1=-2, a2=-1, a3=3) at program.c:13
13      return ret;
"a1 is "$70 = -2
"a2 is "$71 = -1
"a3 is "$72 = 3

Breakpoint 1, func (a1=-2, a2=0, a3=2) at program.c:13
13      return ret;
"a1 is "$73 = -2
"a2 is "$74 = 0
"a3 is "$75 = 2

Breakpoint 1, func (a1=-2, a2=1, a3=1) at program.c:13
13      return ret;
"a1 is "$76 = -2
"a2 is "$77 = 1
"a3 is "$78 = 1

Breakpoint 1, func (a1=-2, a2=2, a3=0) at program.c:13
13      return ret;
"a1 is "$79 = -2
"a2 is "$80 = 2
"a3 is "$81 = 0

Breakpoint 1, func (a1=-2, a2=3, a3=-1) at program.c:13
13      return ret;
"a1 is "$82 = -2
"a2 is "$83 = 3
"a3 is "$84 = -1

Breakpoint 1, func (a1=-2, a2=4, a3=-2) at program.c:13
13      return ret;
"a1 is "$85 = -2
"a2 is "$86 = 4
"a3 is "$87 = -2

Breakpoint 1, func (a1=-2, a2=5, a3=-3) at program.c:13
13      return ret;
"a1 is "$88 = -2
"a2 is "$89 = 5
"a3 is "$90 = -3

Breakpoint 1, func (a1=-1, a2=-4, a3=5) at program.c:13
13      return ret;
"a1 is "$91 = -1
"a2 is "$92 = -4
"a3 is "$93 = 5

Breakpoint 1, func (a1=-1, a2=-3, a3=4) at program.c:13
13      return ret;
"a1 is "$94 = -1
"a2 is "$95 = -3
"a3 is "$96 = 4

Breakpoint 1, func (a1=-1, a2=-2, a3=3) at program.c:13
13      return ret;
"a1 is "$97 = -1
"a2 is "$98 = -2
"a3 is "$99 = 3

Breakpoint 1, func (a1=-1, a2=-1, a3=2) at program.c:13
13      return ret;
"a1 is "$100 = -1
"a2 is "$101 = -1
"a3 is "$102 = 2

Breakpoint 1, func (a1=-1, a2=0, a3=1) at program.c:13
13      return ret;
"a1 is "$103 = -1
"a2 is "$104 = 0
"a3 is "$105 = 1

Breakpoint 1, func (a1=-1, a2=1, a3=0) at program.c:13
13      return ret;
"a1 is "$106 = -1
"a2 is "$107 = 1
"a3 is "$108 = 0

Breakpoint 1, func (a1=-1, a2=2, a3=-1) at program.c:13
13      return ret;
"a1 is "$109 = -1
"a2 is "$110 = 2
"a3 is "$111 = -1

Breakpoint 1, func (a1=-1, a2=3, a3=-2) at program.c:13
13      return ret;
"a1 is "$112 = -1
"a2 is "$113 = 3
"a3 is "$114 = -2

Breakpoint 1, func (a1=-1, a2=4, a3=-3) at program.c:13
13      return ret;
"a1 is "$115 = -1
"a2 is "$116 = 4
"a3 is "$117 = -3

Breakpoint 1, func (a1=-1, a2=5, a3=-4) at program.c:13
13      return ret;
"a1 is "$118 = -1
"a2 is "$119 = 5
"a3 is "$120 = -4

Breakpoint 1, func (a1=0, a2=-5, a3=5) at program.c:13
13      return ret;
"a1 is "$121 = 0
"a2 is "$122 = -5
"a3 is "$123 = 5

Breakpoint 1, func (a1=0, a2=-4, a3=4) at program.c:13
13      return ret;
"a1 is "$124 = 0
"a2 is "$125 = -4
"a3 is "$126 = 4

Breakpoint 1, func (a1=0, a2=-3, a3=3) at program.c:13
13      return ret;
"a1 is "$127 = 0
"a2 is "$128 = -3
"a3 is "$129 = 3

Breakpoint 1, func (a1=0, a2=-2, a3=2) at program.c:13
13      return ret;
"a1 is "$130 = 0
"a2 is "$131 = -2
"a3 is "$132 = 2

Breakpoint 1, func (a1=0, a2=-1, a3=1) at program.c:13
13      return ret;
"a1 is "$133 = 0
"a2 is "$134 = -1
"a3 is "$135 = 1

Breakpoint 1, func (a1=0, a2=0, a3=0) at program.c:13
13      return ret;
"a1 is "$136 = 0
"a2 is "$137 = 0
"a3 is "$138 = 0

Breakpoint 1, func (a1=0, a2=1, a3=-1) at program.c:13
13      return ret;
"a1 is "$139 = 0
"a2 is "$140 = 1
"a3 is "$141 = -1

Breakpoint 1, func (a1=0, a2=2, a3=-2) at program.c:13
13      return ret;
"a1 is "$142 = 0
"a2 is "$143 = 2
"a3 is "$144 = -2

Breakpoint 1, func (a1=0, a2=3, a3=-3) at program.c:13
13      return ret;
"a1 is "$145 = 0
"a2 is "$146 = 3
"a3 is "$147 = -3

Breakpoint 1, func (a1=0, a2=4, a3=-4) at program.c:13
13      return ret;
"a1 is "$148 = 0
"a2 is "$149 = 4
"a3 is "$150 = -4

Breakpoint 1, func (a1=0, a2=5, a3=-5) at program.c:13
13      return ret;
"a1 is "$151 = 0
"a2 is "$152 = 5
"a3 is "$153 = -5

Breakpoint 1, func (a1=1, a2=-5, a3=4) at program.c:13
13      return ret;
"a1 is "$154 = 1
"a2 is "$155 = -5
"a3 is "$156 = 4

Breakpoint 1, func (a1=1, a2=-4, a3=3) at program.c:13
13      return ret;
"a1 is "$157 = 1
"a2 is "$158 = -4
"a3 is "$159 = 3

Breakpoint 1, func (a1=1, a2=-3, a3=2) at program.c:13
13      return ret;
"a1 is "$160 = 1
"a2 is "$161 = -3
"a3 is "$162 = 2

Breakpoint 1, func (a1=1, a2=-2, a3=1) at program.c:13
13      return ret;
"a1 is "$163 = 1
"a2 is "$164 = -2
"a3 is "$165 = 1

Breakpoint 1, func (a1=1, a2=-1, a3=0) at program.c:13
13      return ret;
"a1 is "$166 = 1
"a2 is "$167 = -1
"a3 is "$168 = 0

Breakpoint 1, func (a1=1, a2=0, a3=-1) at program.c:13
13      return ret;
"a1 is "$169 = 1
"a2 is "$170 = 0
"a3 is "$171 = -1

Breakpoint 1, func (a1=1, a2=1, a3=-2) at program.c:13
13      return ret;
"a1 is "$172 = 1
"a2 is "$173 = 1
"a3 is "$174 = -2

Breakpoint 1, func (a1=1, a2=2, a3=-3) at program.c:13
13      return ret;
"a1 is "$175 = 1
"a2 is "$176 = 2
"a3 is "$177 = -3

Breakpoint 1, func (a1=1, a2=3, a3=-4) at program.c:13
13      return ret;
"a1 is "$178 = 1
"a2 is "$179 = 3
"a3 is "$180 = -4

Breakpoint 1, func (a1=1, a2=4, a3=-5) at program.c:13
13      return ret;
"a1 is "$181 = 1
"a2 is "$182 = 4
"a3 is "$183 = -5

Breakpoint 1, func (a1=2, a2=-5, a3=3) at program.c:13
13      return ret;
"a1 is "$184 = 2
"a2 is "$185 = -5
"a3 is "$186 = 3

Breakpoint 1, func (a1=2, a2=-4, a3=2) at program.c:13
13      return ret;
"a1 is "$187 = 2
"a2 is "$188 = -4
"a3 is "$189 = 2

Breakpoint 1, func (a1=2, a2=-3, a3=1) at program.c:13
13      return ret;
"a1 is "$190 = 2
"a2 is "$191 = -3
"a3 is "$192 = 1

Breakpoint 1, func (a1=2, a2=-2, a3=0) at program.c:13
13      return ret;
"a1 is "$193 = 2
"a2 is "$194 = -2
"a3 is "$195 = 0

Breakpoint 1, func (a1=2, a2=-1, a3=-1) at program.c:13
13      return ret;
"a1 is "$196 = 2
"a2 is "$197 = -1
"a3 is "$198 = -1

Breakpoint 1, func (a1=2, a2=0, a3=-2) at program.c:13
13      return ret;
"a1 is "$199 = 2
"a2 is "$200 = 0
"a3 is "$201 = -2

Breakpoint 1, func (a1=2, a2=1, a3=-3) at program.c:13
13      return ret;
"a1 is "$202 = 2
"a2 is "$203 = 1
"a3 is "$204 = -3

Breakpoint 1, func (a1=2, a2=2, a3=-4) at program.c:13
13      return ret;
"a1 is "$205 = 2
"a2 is "$206 = 2
"a3 is "$207 = -4

Breakpoint 1, func (a1=2, a2=3, a3=-5) at program.c:13
13      return ret;
"a1 is "$208 = 2
"a2 is "$209 = 3
"a3 is "$210 = -5

Breakpoint 1, func (a1=3, a2=-5, a3=2) at program.c:13
13      return ret;
"a1 is "$211 = 3
"a2 is "$212 = -5
"a3 is "$213 = 2

Breakpoint 1, func (a1=3, a2=-4, a3=1) at program.c:13
13      return ret;
"a1 is "$214 = 3
"a2 is "$215 = -4
"a3 is "$216 = 1

Breakpoint 1, func (a1=3, a2=-3, a3=0) at program.c:13
13      return ret;
"a1 is "$217 = 3
"a2 is "$218 = -3
"a3 is "$219 = 0

Breakpoint 1, func (a1=3, a2=-2, a3=-1) at program.c:13
13      return ret;
"a1 is "$220 = 3
"a2 is "$221 = -2
"a3 is "$222 = -1

Breakpoint 1, func (a1=3, a2=-1, a3=-2) at program.c:13
13      return ret;
"a1 is "$223 = 3
"a2 is "$224 = -1
"a3 is "$225 = -2

Breakpoint 1, func (a1=3, a2=0, a3=-3) at program.c:13
13      return ret;
"a1 is "$226 = 3
"a2 is "$227 = 0
"a3 is "$228 = -3

Breakpoint 1, func (a1=3, a2=1, a3=-4) at program.c:13
13      return ret;
"a1 is "$229 = 3
"a2 is "$230 = 1
"a3 is "$231 = -4

Breakpoint 1, func (a1=3, a2=2, a3=-5) at program.c:13
13      return ret;
"a1 is "$232 = 3
"a2 is "$233 = 2
"a3 is "$234 = -5

Breakpoint 1, func (a1=4, a2=-5, a3=1) at program.c:13
13      return ret;
"a1 is "$235 = 4
"a2 is "$236 = -5
"a3 is "$237 = 1

Breakpoint 1, func (a1=4, a2=-4, a3=0) at program.c:13
13      return ret;
"a1 is "$238 = 4
"a2 is "$239 = -4
"a3 is "$240 = 0

Breakpoint 1, func (a1=4, a2=-3, a3=-1) at program.c:13
13      return ret;
"a1 is "$241 = 4
"a2 is "$242 = -3
"a3 is "$243 = -1

Breakpoint 1, func (a1=4, a2=-2, a3=-2) at program.c:13
13      return ret;
"a1 is "$244 = 4
"a2 is "$245 = -2
"a3 is "$246 = -2

Breakpoint 1, func (a1=4, a2=-1, a3=-3) at program.c:13
13      return ret;
"a1 is "$247 = 4
"a2 is "$248 = -1
"a3 is "$249 = -3

Breakpoint 1, func (a1=4, a2=0, a3=-4) at program.c:13
13      return ret;
"a1 is "$250 = 4
"a2 is "$251 = 0
"a3 is "$252 = -4

Breakpoint 1, func (a1=4, a2=1, a3=-5) at program.c:13
13      return ret;
"a1 is "$253 = 4
"a2 is "$254 = 1
"a3 is "$255 = -5

Breakpoint 1, func (a1=5, a2=-5, a3=0) at program.c:13
13      return ret;
"a1 is "$256 = 5
"a2 is "$257 = -5
"a3 is "$258 = 0

Breakpoint 1, func (a1=5, a2=-4, a3=-1) at program.c:13
13      return ret;
"a1 is "$259 = 5
"a2 is "$260 = -4
"a3 is "$261 = -1

Breakpoint 1, func (a1=5, a2=-3, a3=-2) at program.c:13
13      return ret;
"a1 is "$262 = 5
"a2 is "$263 = -3
"a3 is "$264 = -2

Breakpoint 1, func (a1=5, a2=-2, a3=-3) at program.c:13
13      return ret;
"a1 is "$265 = 5
"a2 is "$266 = -2
"a3 is "$267 = -3

Breakpoint 1, func (a1=5, a2=-1, a3=-4) at program.c:13
13      return ret;
"a1 is "$268 = 5
"a2 is "$269 = -1
"a3 is "$270 = -4

Breakpoint 1, func (a1=5, a2=0, a3=-5) at program.c:13
13      return ret;
"a1 is "$271 = 5
"a2 is "$272 = 0
"a3 is "$273 = -5
0
[Inferior 1 (process 1816035) exited normally]