Re: pure string functions

[Andreas Jaeger <aj at suse dot de>]

>>>>> Andi Kleen writes:

Andi> Andreas Jaeger <aj@suse.de> writes:
>> 
>> Some of common examples of pure functions are @code{strlen} or @code{memcmp}.
>> Interesting non-pure functions are functions with infinite loops or those
>> depending on volatile memory or other system resource, that may change between
>> two consetuctive calls (such as @code{feof} in multithreding environment).


Andi> So you are sure gcc correctly handles things like this in all cases with
Andi> pure strlen: 

According to the documentation: Yes. ;-)

Andi> func()
Andi> {
Andi>         char bla[10] = "blub"; 
Andi>         int a,b;
Andi>         a = strlen(bla);
Andi>         // do something with a
Andi>         func2(bla); 
Andi>         b = strlen(bla); 
Andi>         // do something with b 
Andi> } 

Andi> func2(char *bla)
Andi> {
Andi>         bla[1] = 0; 
Andi> }

Andi> ? 

Andi> My understanding of pure functions was always that they are only
Andi> allowed to depend on argument values, but not on values pointers
Andi> point to.

That's a const function - a pure function is allowed to read memory -
but is not allowed to change memory!

Andreas
-- 
 Andreas Jaeger
  SuSE Labs aj@suse.de
   private aj@arthur.rhein-neckar.de