Question

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What is the use of ## preprocessor in C

From Fun with Google TTS …
Anyone can shed a light on how this code is working? And in particular – what kind of notation is “c##_”? –

#define DsHook(a,b,c) if (!c##_) { 
INT_PTR* p=b+*(INT_PTR**)a;  VirtualProtect(&c##_,4,PAGE_EXECUTE_READWRITE,&no);
*(INT_PTR*)&c##_=*p;  VirtualProtect(p,4,PAGE_EXECUTE_READWRITE,&no);  *p=(INT_PTR)c;

It is unfolded in code as –

//  redirect  7th member func of IAsyncReader (SyncReadAlligned) to grab mp3 data from output pin of source filter
DsHook(reader,6,SyncReadAlligned); 

I guess, it is not very safe way of playing with memory, but it is commented by the author as “unimportant”:

Unimportant code like hook is folded in snippet form but feel free to
unfold and format the code in the way you like.

P.S. The “Fun with Google” itself if blocked already by Google (if anyone is interested to test).

Update: I think, that the short analysis would be –
To “redirect 7th member func of IAsyncReader (SyncReadAlligned) to grab mp3 data from output pin of source filter”, we change the memory protection on certain region of committed pages from “whatever it was” to PAGE_EXECUTE_READWRITE (allowing full access).

And concerning safety MSDN says –

It is best to avoid using VirtualProtect to change page protections on
memory blocks allocated by GlobalAlloc, HeapAlloc, or LocalAlloc,
because multiple memory blocks can exist on a single page.
…
When protecting a region that will be executable, the calling program
bears responsibility for ensuring cache coherency…

Answer 1

It’s a preprocessor directive meaning “concatenation”. c##_ will result in a token containing the substitued value of the argument c and the literal character _. Example:

#define foo(c) c##_

when called using foo(bar), this will expand to bar_.