How can i detect “out of bound error” in C program with GDB?

Question

I wrote this program in C, adding an intentional error on purpose.

The program calculates the sum of 5 numbers entered by the user, and displays the result on the screen.

I compiled it with "gcc -Wall -Wextra -Werror -ansi -pedantic -g" and works fine.

But it has an error.

In the last repetition of the cycle, the program evaluates a[N], which is not defined!

I'd like to know how to spot this kind of error using GDB

When i use " set check range on " i get this messange "warning: the current range check setting does not match the language." and nothing happens...

This is the code to debug:

#define N 5
#include <stdio.h>


void read(float*);


int main(void) {

    float a[N], s;

    int i;

    printf("Enter %d numbers: ", N);
    read(a);

    i = -1; 
    s = 0;

    while (i != N) {
        i = i + 1;
        s = s+a[i];
    }

    printf("The sum is : %.2f \n", s);

    return 0;
}


void read(float*a) {

    int n = 0;

    while (n!=N) { 
        scanf("%f",&a[n]);
        n++;
    }

}

Answer 1

I think this is your problem:

while (i != N) {
    i = i + 1;
    s = s+a[i];
}

N is defined as 5 , so when i is 4, the condition is true. i is then incremented to 5, and s += a[i]; is executed. Just use a for loop instead, or use do {} while :

for (i=0;i<N;++i)
    s += a[i];
//or
i = 0;
do {
   s += a[i];
} while (++i != N);

Either way. Personally, I find the for loop more readable

---

To answer your question (using gdb):

• You've compiled using the -g flag, so run `gdb compiled_file_name
• In gdb, set a break-point in the while loop ( b <line-nr> [condition] )
• start the program ( run )
• use step or next to step through the code
• use pi to check the value of i every time you hit the while condition, and every time you use i as offset ( a[i] )

For more details, docs for gdb are available. It takes some time, but it's well worth it. gdb is an excellent debugger

Answer 2

The answer to this particular error is that the loop increments i and then accesses a at index i, without an intervening check. So when i equals N - 1 when it starts the loop, it's incremented to N and used in the array.

In general, gcc's -fsanitize=bounds option should be helpful for these errors.