C function seemingly not defined anywhere!

Question

I'm looking at the vim source code, specifically the file normal.c, and I see this function nv_operator being used, but it's not defined anywhere (I grepped the entire src directory)

It's only declared as:

static void nv_operator __ARGS((cmdarg_T *cap));

I've looked up the definition of __ARGS but it's just ... nothing (pretty much)
in vim.h:

#define __ARGS(x) x

So what could be going on? Is this some kind of C technique to create a dummy function or something?

Answer 1

It's simply a forward declaration, so that the function is known to the C compiler (and can be used (called from other functions)) before it's actually defined (in line 8247). The actual formatting of the definition (which includes newlines) makes it hard to grep for it's existence.

Don't get distracted by the __ARGS macro. It's only a compatibility macro for the different function declaration syntaxes of K&R C vs. ANSI C.

In ANSI C a function declaration must look like this:

int getopt(int, char * const *, const char *);

In the (older) Kernighan and Ritchie C http://en.wikipedia.org/wiki/C_(programming_language)#K.26R_C

int getopt();

Answer 2

There is a definition present here:

/*
 * Handle an operator command.
 * The actual work is done by do_pending_operator().
 */
    static void
nv_operator(cap)
    cmdarg_T    *cap;
....

That style of definition is using an identifier list for its parameters. The style is deprecated (obsolescent) but can still be used in C. The identifiers are named in the parameter list, and their type are named in declarations that immediately follow the function declarator but precede the functions body.

The __ARGS macro is there to handle compilers that don't know about prototypes for functions (the other form to declare parameters - with type and name combined directly in the function parameter list). It would then just emit no parameters at all in declarations, i think.

Update: See this code in vim.h:

#if defined(MACOS) && (defined(__MRC__) || defined(__SC__))
   /* Apple's Compilers support prototypes */
# define __ARGS(x) x
#endif
#ifndef __ARGS
# if defined(__STDC__) || defined(__GNUC__) || defined(WIN3264)
#  define __ARGS(x) x
# else
#  define __ARGS(x) ()
# endif
#endif

Answer 3

Its hard to find because of how it is defined:

nv_operator(cap)

appears on a line by itself.

Answer 4

I am not too sure what is going on, but here are some hints to help you in your search:

First of all, the __ARGS macro seems to be there because there may be versions of C where you shouldn't include the args in the declaration of the functions (Notice that the macro is defined differently depending on other preprocessor symbols... the comments say it).

Secondly, searching for the function nv_operator may not be good enough. The function might be generated by macros and such, so you can't search for an explicit definition.... for example, maybe the "nv" prefix is added by the preprocessor.

Hope this helps.