Header File Macros

[psukphranee]

I am having trouble understanding how these macros I see in header files work. I looked back into my texts and understand basic macro operations but I do not understand this.

I understand line 8: replace NORETURN with __attribute__ ((__noreturn__)).

But line 10 just defines the word.

Then what is the point of NORETURN at the end of a function argument list?

This is an excerpt from Linux Programming Interface by Michael Kerrisk
page 52. Header file is error_functions.h

#ifndef ERROR_FUNCTIONS_H
#define ERROR_FUNCTIONS_H

void errMsg(const char *format, ...);

#ifdef __GNUC__

#define NORETURN __attribute__ ((__noreturn__))
#else
#define NORETURN
#endif

void errExit(const char *format, ...) NORETURN;

void errExit(const char *format, ...) NORETURN;
void err_exit(const char *format, ...) NORETURN;
....
void fatal(const char *format, ...) NORETURN;

...........

[vart]

if you compiling with gcc - the __GNUC__ will be defined and you will get your __attribute__ ((__noreturn__)) added to each function in place of NORETURN

if you compiling with any other compiler, where the __GNUC__ is not defined - NORETURN will be empty string and preprocessor will just remove is from the source code

[oogabooga]

Since the identifier NORETURN is used in the program, it must be defined, otherwise it's a syntax error. So in your example, if __GNUC__ is defined, then NORETURN is defined as __attribute__ ((__noreturn__)), giving

void errExit(const char *format, ...) __attribute__ ((__noreturn__));
etc.

whereas if __GNUC__ is not defined, NORETURN is defined as nothing, giving

void errExit(const char *format, ...) ;
etc.

[psukphranee]

ahh . thank you guys!