Re: [xml] [offtopic] evaluation order of logical operators

[Andreas Stricker <andreas stricker fela ch>]

Volker Grabsch wrote:
> >  for(int i=0, e=xpo->nodesetval->nodeNr; i<e; ++i) {
> >    xmlChar* p_val = xmlGetProp(xpo->nodesetval->nodeTab[i], "id");
> >    if(p_val && 0==strcmp("4", p_val)) {
>
> Please note that in C the order of execution of
>
>     p_val
>
> and
>
>     0==strcmp("4", p_val)
>
> is not specified! 

For a short time you put me off my stride. I do this all the time.
Then I found the following statement in ISO C99 [1]:

  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 ïrst operand. If the ïrst operand compares equal
     to 0, the second operand is not evaluated.

> Some compilers could generate code that
> checks the second condition first, which could lead to a
> segmentation fault or similar.

This may be true. Fortunately I newer saw such a compiler.


Cheers, Andy

[1] http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf