evalutation order for logical operators

[gallows]

Hi,

I wondering if there is a standard specification for evalutation of
logical expressions. For instance:

if (p != NULL && strlen(p) > x )
do_something();

If p is actually NULL, should this code (built with a generic
compiler) crash?
Considering that the first element is FALSE, evalutation of strlen()
is useless, so it (theoretically) should be skipped...
My compiler (gcc) works as I expected, is it a standard behaviour?

Thanks,

s.

 

[pete]

Hi,

I wondering if there is a standard specification for evalutation of
logical expressions.

N869
6.5.13 Logical AND operator
[#4] Unlike the bitwise binary & operator, the && operator
guarantees left-to-right evaluation; there is a sequence
point after the evaluation of the first operand. If the
first operand compares equal to 0, the second operand is not
evaluated.

For instance:

if (p != NULL && strlen(p) > x )
do_something();

If p is actually NULL, should this code (built with a generic
compiler) crash?
No.

Considering that the first element is FALSE, evalutation of strlen()
is useless, so it (theoretically) should be skipped...
My compiler (gcc) works as I expected, is it a standard behaviour?

Yes.

 

[Peter Nilsson]

Hi,

I wondering if there is a standard specification for evalutation of
logical expressions. For instance:

if (p != NULL && strlen(p) > x )
do_something();

This is a FAQ...

http://c-faq.com/expr/shortcircuit.html

 

[gallows]

This is a FAQ...

http://c-faq.com/expr/shortcircuit.html

Ouch, I'm sorry.
Thanks!