Re: Undefine a library function

[Tim Prince <timothyprince@sbcglobal.net>]

Niklaus wrote:
> On 12/7/06, Tim Prince <timothyprince@sbcglobal.net> wrote:
>> Niklaus wrote:
>> > Hi,
>> > Like the #undef for macros , do we have a way for undefining a
>> > library function like say memset. Do we have any way(like linker) so
>> > that my function memset(with different arguments) are used everywhere
>> > instead of library function memset. One way would be to rename my
>> > function memset to mymemset or #define it . But i want to know whether
>> > there is any hack or anything so that the library is included but the
>> > memset used is mine instead of the library version.
>> >
>>
>> Do you have an example where #undef doesn't accomplish what you want?
>> Evidently, many standard C functions will have macro replacements in the
>> standard headers used in your implementation.  C standard requires
>> ability to put those aside with #undef and to have an underlying
>> separate library implementation, which you could attempt to preempt with
>> your own version.
>> It's common practice for compilers to #define memset() to a special
>> library version, but not with changes in the meaning of arguments.  If
>> your own version is not functionally compatible with the standard
>> version, you are inviting trouble by using the standard name.
> 
> Yes here is some code.
> #include<stdio.h>
> #include<string.h>
> 
> int L[10][10];
> 
> #undef memset
> /* if i include string.h it is compilation error, If i don't include i
> get warning saying conflicting
> types in library function . One way would be #define memset to
> mymemset or some other function but can it not be done any other way
> */
> void memset(void *mem,int c, int len);
> void memset(void *mem,int c, int len)
> {
> 
>     int *ptr=mem;
>     while(len--)
>         *ptr++=c;
> 
>     return;
> }
> 
> 
> int main()
> {
>     int n=10,i,j,k,ll=-200;
>     memset(L,2,100);
> }
> 
> 
You could #include <string.h> but you would have to make yours use the 
same data types, with parens around memset:
void (memset)(const void *mem, int c, size_t len){
	char *ptr = mem;
...

Of course, you would optimize by setting a value of the widest native 
data type (128 bit on most current CPUs) to a string of characters of 
value c, but you must take care of the possible remainder values at each 
end.  In addition
//off topic you would need to set a non-cached mode, where available

I don't see a purpose in your strange combination of K&R and standard C 
definition, plus some stuff of your own. If you do mean to set an array 
of ints, you shouldn't name it memset(), and there's a good chance your 
compiler would do better with a plain for loop.
Nor do I see the point of those who say any C implementation where there 
is a difference between size_t and int is broken, nor am I interested in 
discussion of it.