Accessing out of bound array in c

Question

I have seen a particular style of code that according to me is dangerous and should be avoided, but I have seen it so many times in lot of places, that I am confused whether or not I am missing something.

int a[100];

/* Some code later */

for(i =0; (i < 100 && a[i] != 0); i++)
           :
           :

In the above code it is possible that the expression (i < 100 && a[i] != 0) is evaluated from right to left, in that case when i == 100, we are going out of bounds of the array 'a'.

Can someone explain whether or not this is safe code

when it comes to `&&` if the first half is evaluated to false the 2nd part is not checked so it does evaluate from left to right http://stackoverflow.com/questions/5211961/how-does-c-handle-short-circuit-evaluation — Alexandru Barbarosie, Jun 19 '13 at 04:56

score 4 · Accepted Answer · edited May 23 '17 at 12:21

The order of evaluation or arguments is well defined for &&, it cannot be changed.
#1 coupled with short circuiting makes this perfectly safe.

References:

C++03 Standard:

Section 5: Expressions, Para 4:

except where noted [e.g. special rules for && and ||], the order of evaluation of operands of individual operators and subexpressions of individual expressions, and the order in which side effects take place, is Unspecified.

C99 Standard:

Section 6.5:

The grouping of operators and operands is indicated by the syntax.72) Except as specified later (for the function-call (), &&, ||, ?:, and comma operators), the order of evaluation of subexpressions and the order in which side effects take place are both unspecified.

Note: Emphasis mine.

score 3 · Answer 2 · answered Jun 19 '13 at 04:46

3

I think you can count on short-circuit evaluation not to access a[i] when i < 100 is false.

answered Jun 19 '13 at 04:46

Brian Cain

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score 2 · Answer 3 · answered Jun 19 '13 at 04:47

2

It is not possible that (i < 100 && a[i] != 0) is evaluated from right to left. && and || are explicitly defined as evaluating their left argument first, and their right argument only if necessary.

answered Jun 19 '13 at 04:47

rici

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score 1 · Answer 4 · answered Jun 19 '13 at 04:55

"In the above code it is possible that the expression (i < 100 && a[i] != 0) is evaluated from right to left..."

No, it is not possible, neither in C nor in C++. In both languages this expression is guaranteed to be evaluated from left to right. And if the first part is false, the second part is guaranteed not to be evaluated.

So, the code is perfectly safe from out-of-bounds access.

score 0 · Answer 5 · answered Jun 19 '13 at 04:54

0

&& and || are evaluated left to right as every one said. You might want to consider following link for further information

http://en.cppreference.com/w/cpp/language/operator_precedence

answered Jun 19 '13 at 04:54

SureshS

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Accessing out of bound array in c

5 Answers5