Section 6.4 Arrays: Out of Bounds

Adapted from: “Beej’s Guide to C Programming” by Brian (Beej Jorgensen) Hall: Beej’s Guide to C Programming: 6.4 Out of Bounds

Brian (Beej Jorgensen) Hall Website

Program for Demonstrating Out of Bounds Array Access¶

#include <stdio.h>

int main(void)
{
    int i;
    int a[5] = {22, 37, 3490, 18, 95};

    for (i = 0; i < 10; i++) {  // BAD NEWS: printing too many elements!
        printf("%d\n", a[i]);
    }
}

Explanation of the Above Code¶

This C code demonstrates an example of accessing an array out of bounds, which is a common programming error. Here’s a breakdown:

Code Explanation:¶

#include <stdio.h>

Includes the standard input/output library, which provides the printf function for printing to the console.

int main(void)

The main function is the entry point of the program.

int i;
int a[5] = {22, 37, 3490, 18, 95};

int i;: Declares a variable i to be used as a loop counter.
int a[5] = {22, 37, 3490, 18, 95};: Declares an integer array a of size 5 and initializes it with the values {22, 37, 3490, 18, 95}.

for (i = 0; i < 10; i++) {  // BAD NEWS: printing too many elements!
    printf("%d\n", a[i]);
}

A for loop is used to iterate through the array a.
- i = 0: The loop starts with i set to 0.
- i < 10: The loop continues as long as i is less than 10.
- i++: Increments i by 1 after each iteration.
Inside the loop, printf("%d\n", a[i]); prints the value of the i-th element of the array a followed by a newline (\n).

Key Issue:¶

The array a is declared with a size of 5, meaning it has valid indices from 0 to 4.
The loop iterates from 0 to 9, attempting to access elements beyond the bounds of the array (a[5] to a[9]).
Accessing out-of-bounds elements results in undefined behavior in C. This could lead to:
- Printing garbage values.
- Crashing the program.
- Overwriting memory, causing unpredictable behavior.

Output:¶

The first 5 iterations (i = 0 to 4) will print the initialized values of the array:
```
22
37
3490
18
95
```
The remaining iterations (i = 5 to 9) will likely print garbage values or cause a crash, depending on the memory layout.

Lesson:¶

Always ensure that array indices are within bounds to avoid undefined behavior.

Compile and Run Code¶

Use Python to Change to Working Directory¶

import os
root_dir = os.getcwd()

code_dir = root_dir + "/" + "C_Code"

os.chdir(code_dir)

build_command = os.system("gcc -o section_6_4_out_of_bounds section_6_4_out_of_bounds.c")

exec_status = os.system("./section_6_4_out_of_bounds")