GCC -Warray-bounds comple flag

[golden flume]

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[acoustic token]

use const int input = 200

to make that a compile-time constant

and also consider using the address sanitizer for dynamic checking instead

[stoic ledge]

Thay is still an out of bounds read?

;compile -Warray-bounds=2 -O2 -Werror

int main(void) {
    int arr[2] = {1,2};
    const int input = 200;

    int a = arr[200];
}

[silk finch]

Compilation successful

No output.

ZeroPointer#6105 | 44ms | c | x86-64 gcc 12.2 | godbolt.org

[buoyant minnow]

;compile -Warray-bounds -O2

int main(void) {
    int arr[2] = {1,2};
    const int input = 200;

    int a = arr[200];
    return a; // use `a` to prevent optimization
}

[silk finch]

Compiler Output

<source>: In function 'int main()':
<source>:5:20: warning: array subscript 200 is above array bounds of 'int [2]' [-Warray-bounds]
    5 |     int a = arr[200];
      |             ~~~~~~~^
<source>:2:9: note: while referencing 'arr'
    2 |     int arr[2] = {1,2};
      |         ^~~

Jérôme#1807 | 45ms | c++ | x86-64 gcc 12.2 | godbolt.org

[buoyant minnow]

Because you're not using a afterwards, so optimization probably just removes this "useless" instruction.

Without optimization, this instruction is kept.

[golden flume]

Thank you and let us know if you have any more questions!

[buoyant minnow]

You can observe the assembly differences here: https://godbolt.org/z/esEEnW8vv