illumos gate

Why it would be better to include stdio_tag.h instead of stdio.h to get the FILE?

BTW, I do not see the FILE defined in stdio_tag.h.

The reason for stdio_tag.h is for libraries that want to define a function as accepting an arg of type FILE, i.e. for an output formatting function. In the rpc/xdr.h case we want FILE for xdrstdio_create.

The way to do this without forcing a stdio.h include on programs including rpc/xdr.h is either of:

1: Anonymous struct:

struct __FILE;
extern void   xdrstdio_create(XDR *, struct __FILE *, const enum xdr_op);

2: Use stdio_tag.h

#include <stdio_tag.h>
extern void   xdrstdio_create(XDR *, struct __FILE *, const enum xdr_op);

Lots of places in our system use the first method.
The second method is more standard (I think).

Here's a little C module demonstrating the two methods. (cc -c demo.c)

typedef struct _XDR XDR;
enum xdr_op { a, b };

#if 0 /* either of these work */
struct __FILE;
#else
#include <stdio_tag.h>
#endif

extern void   xdrstdio_create(XDR *, struct __FILE *, const enum xdr_op);

void
xdrstdio_create(XDR *x, struct __FILE *f, const enum xdr_op op)
{
}