Errors with arrays in GDB (written in C)

Question

I'm trying to find the memory locations of the elements of an array. The function is basically this (in C):

int function(struct MyStruct *s)
{
    char myArray[16];

    printf("\n");
    printf("\n");
    gets(myArray);
    return strcasecmp(s->a,guess);
}

According to GDB guides online I should be able to do "x myArray" or "x myArray[0]" or "p myArray" or "p myArray[0]" to see the elements or memory locations. I set up breakpoints at function and gets (and continued until the gets breakpoint), but when I tried the GDB commands, I got "no such file." I also tried "b 15" to set a breakpoint at the array and "b 16." but I still got the same error. Why is this happening and how can I fix it?

We have just fired our entire guessing department. Please post an entire transcript of your GDB session verbatim. — n. m. could be an AI
– n. m. could be an AI, Commented Apr 28, 2012 at 13:58
I'm not at the computer I was using anymore, but I basically did "break main, break function, break gets, break 15 (which didn't work), break 16 (which didn't work), run, s (until I reach the gets breakpoint), x myArray, x myArray[0], p myArray, p myArray[0]" — Sams
– Sams, Commented Apr 28, 2012 at 14:03
I was actually more interested in GDB messages. Anyway, it looks like you have a program compiled without debugging information. — n. m. could be an AI
– n. m. could be an AI, Commented Apr 28, 2012 at 14:16
If you don't have symbolic debugging information, you cannot use symbolic debugging functionality of GDB (obviously). The GDB commands you have tried do not apply. — n. m. could be an AI
– n. m. could be an AI, Commented Apr 28, 2012 at 14:44
I don't know why your teacher want you to debug a program that lacks debugging information. Perhaps you are in the assembly hacking class? — n. m. could be an AI
– n. m. could be an AI, Commented Apr 28, 2012 at 14:59

Employed Russian · Accepted Answer · 2012-04-28 17:44:12Z

3

"break main, break function, break gets, break 15 (which didn't work), break 16 (which didn't work), run, s (until I reach the gets breakpoint), x myArray, x myArray[0], p myArray, p myArray[0]"

You are doing it wrong.

When you hit the gets breakpoint, you are stopped inside gets.

In there, the myArray variable is not visible -- it's inside function, not inside gets.

You can't break 15 while inside gets either, because you don't have debug info for libc (which is where gets is implemented).

What you want to do is finish from the breakpoint in gets (which will bring you back into function). Now you should be able to look at myArray, or break 15, etc.

answered Apr 28, 2012 at 17:44

Employed Russian

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