Why does lldb debugger assign a value when i set a watchpoint?

December 7, 2021

In the following C code:

#include <stdio.h>

int main(void)
{
    unsigned i = 1;
    while (i)
    {
        i <<= 1;
    }
    return 0;
}

I first compile with debugging info as followed: gcc -g single-step.c
Then I do lldb a.out. I want to see what i is doing, so i do b main and run, after which i set my watchpoint for i: watchpoint set variable i. When I do that however, lldb seems to assign a value to variable i, which it shouldn’t
Watchpoint created: Watchpoint 1: addr = 0x16fdff498 size = 4 state = enabled type = w declare @ '/Users/d1ff1cult/Documents/KUL 2021-2022/IW/oefenzitting-c/les8-debugging/examples/single-step.c:5' watchpoint spec = 'i' new value: 53472
It gives i a seemingly completely random value, so what am I doing wrong?
I use CLion to write my code, but this happens in the CLion terminal as well as my macOS proper terminal. I am also using an M1 Mac.
Thanks in advance!

>Solution :

Eugene is correct.

On my system, when I run your program, I get an initial value of 0 at the watchpoint.

I changed the program to put your code in a function (e.g.) orig.

I created a function fill that loops on filling in its stack frame an int array of length 100.

From main, I call fill and then orig with b orig. Then, doing the watchpoint command, I get a value of 100.

So, your watchpoint is displaying whatever happens to be in the stack frame for what i is assigned to. That is, the uninitialized value.

Here is the test program I created:

#include <stdio.h>
#include <stdlib.h>

int
orig(void)
{
    unsigned i = 1;

    while (i) {
        i <<= 1;
    }

    return 0;
}

void
fill(void)
{
    unsigned x[100];

    for (int i = 0;  i < 100;  ++i)
        x[i] = rand();
}

int
main(void)
{

    fill();
    orig();

    return 0;
}

Here is the debug output:

> lldb ./fix1
(lldb) target create "./fix1"
Current executable set to './fix1' (x86_64).
(lldb) b main
Breakpoint 1: where = fix1`main + 4 at fix1.c:29, address = 0x000000000040117b
(lldb) b orig
Breakpoint 2: where = fix1`orig + 4 at fix1.c:7, address = 0x000000000040112a
(lldb) run
Process 193289 launched: '/tmp/watch/fix1' (x86_64)
Process 193289 stopped
* thread #1, name = 'fix1', stop reason = breakpoint 1.1
    frame #0: 0x000000000040117b fix1`main at fix1.c:29
   26   main(void)
   27   {
   28   
-> 29       fill();
   30       orig();
   31   
   32       return 0;
(lldb) c
Process 193289 resuming
Process 193289 stopped
* thread #1, name = 'fix1', stop reason = breakpoint 2.1
    frame #0: 0x000000000040112a fix1`orig at fix1.c:7
   4    int
   5    orig(void)
   6    {
-> 7        unsigned i = 1;
   8    
   9        while (i) {
   10           i <<= 1;
(lldb) watchpoint set variable i
Watchpoint created: Watchpoint 1: addr = 0x7fffffffdd6c size = 4 state = enabled type = w
    declare @ '/tmp/watch/fix1.c:7'
    watchpoint spec = 'i'
    new value: 100