memcmp (const void *a1, const void *a2, size_t size)

the above function - memcmp receiving a1 as pointer to void which is 'const' this means content of the a1 can not be changed.

Question:

const void *a1;

We can not dereference a1 this must be compiler error because its pointing to an address which is void then why memcmp should receive a1 as "const void*" instead of "void *".

You cannot deference a void pointer, the results are undefined (compiler may complain or program will seg fault) and so it is not done.

A reasonable implementation of memcmp must effectively cast the pointer to const char* before using it.

A reasonable caller of memcmp might have cast the pointer from const XXX* for some type XXX before passing the pointer to memcmp.

If memcmp took the pointer as void* then the caller would be casting from const XXX* to void*, which would appear to be a bug.

What is your real question? Eg:
Q: What 'const' means in 'const void *a1'?
A: memcmp won't change the memory pointed by a1
Q: What 'void *' means?
A: Generic pointer.

Because it does not modify contents of the memory pointed by a1.

That isn't what it means at all.

It means that the POINTER cannot be modified, not what it points to.

Yes, const void *a1 - can be cast to another pointer internal implementation of memcmp function.

My question was, why memcmp or for that reason any C function which receives void *arg as argument if i add 'const' qualifier - what is the use.

The use of const is to indicate a value that is not to be changed by that function.

I believe it is only a documentation aid. It makes sure that the programmer knows the parameters are not modified.

> > const void *a

> It means that the POINTER cannot be modified, not what it points to.

Wrong. Actually, it means a pointer pointing to constant data. Please read the forgotten manual, or perform tests.

>> My question was, why memcmp or for that reason any
>> C function which receives void *arg as argument if
>> i add 'const' qualifier - what is the use.

> I believe it is only a documentation aid. It makes
> sure that the programmer knows the parameters are
> not modified.

It is not 'only' a documentation aid. As already mentioned, const void *a1 means
a1 is a pointer to constant data. The compiler can use this information to do useful things such as being clever about optimising the code. And if you provided your own definition of memcmp which did modify a1, then the compiler could tell you that you had made a mistake. It's much better to be told about such mistakes at compile time than to have to try to find them when the program misbehaves at run time.

For comparison, observe that memcpy is declared as memcp(void *dest, const void *src, size_t n) because the copy process does modify the contents pointed at by dest pointer.

what was not mentioned, actually memcmp does not take care about the meaning of the content. it has [got] two memory addresses, and compares the content byte by byte (compares size number of bytes). So you can say, the type of the data is irrelevant, and void * means exactly that: a pointer to something, whatever, without knowing/handling its type, structure. const means the function will not change the content of that data.

From reading http://www.lix.polytechnique.fr/ I deduce the following:
As for the keyword void, that would indicate that the function makes no assumption about the type of data the pointer is referencing. The function would make an appropriate internal cast to allow byte by byte comparisons. (For speed purposes it might actually do a 32-bit word by 32-bit word comparison then compare the 4 bytes in the word that doesn't match).

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That's interesting, I didn't know the order mattered, but then I never use these.

thanks for correcting me.

I was wrong.

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I always use the following hint from Wikipedia on how C/C++ handle the const keyword: