Kernel Header Files: ANSI Function Prototypes

Summary of Change

Kernel header files are included by code written in K&R C (compatibility mode), as well as code written in ANSI C. Compile time errors will result if ANSI function prototypes are made visible to K&R C code; the compatibility mode C compiler does not recognize ANSI function prototypes. To prevent such errors, header files use the __() macro to declare function prototypes. The macro is defined in the header file stdsyms.h as follows:

#if defined(_PROTOTYPES)
#define __(arg) arg
#else
#define __(arg) ()
#endif

To illustrate the usage of the __() macro, consider the function prototype of printf():

extern int printf __((const char *fmt, ...));

This is equivalent to declaring printf() as:

#ifdef _PROTOTYPES
extern int printf (const char *fmt, ...);
#else
extern int printf ();
#endif

Use of the __() macro can also be found in structure declarations that contain function pointers. Consider the drv_ops structure in the header file conf.h:

typedef struct drv_ops {
        int (*d_open) __((dev_t dev, int oflags, intptr_t dummy, int mode));
.
.
.
} drv_ops_t;

Many, but not all, header files contain ANSI function prototypes; many of these use the __() macro. ANSI function prototypes of many HP-UX kernel interfaces have been incorporated in header files appropriate to the kernel interface. For example, the following HP-UX kernel interfaces that specify the buf structure are declared in the header file buf.h:

extern void biodone __((struct buf *));
extern int  biowait __((struct buf *));

A new header file, kern_svcs.h, has been added to HP-UX 10.30. This header file contains ANSI function prototypes of commonly used HP-UX kernel interfaces for which an appropriate header file does not exist.

The kern_svcs.h header file defines some function prototypes, such as formal descriptions for function types and arguments. This header file is included by selected kernel header files so that other headers do not need to include it. For example, wsio.h is a key header file for WSIO drivers and it includes kern_svcs.h for the benefit of all WSIO drivers. Drivers that include wsio.h are, therefore, not required to explicitly include kern_svcs.h.

Impact

Code that is compiled with _KERNEL defined is impacted because function prototypes in the kernel header files are made visible to the compiler via #ifdef _KERNEL. Only code intended to be linked with the kernel should define _KERNEL.

The C compiler displays warning messages where arguments passed to, or use of values returned from, functions are not compatible with the specified function prototypes. In some cases, compiler errors might be generated. You are expected to correct code that result in compiler-generated warnings and errors.