qual-return-1.c

this is a gcc file, you can download it and learn its usetility, for further detail please look at t

/* Test for warnings for qualified function return types.  */
/* Origin: Joseph Myers <[e-mail]> */
/* { dg-do compile } */
/* { dg-options "-std=gnu99 -W" } */

/* Qualifying a function return type makes no sense.  */

const int int_fn (void); /* { dg-warning "qualifiers" "int decl" } */
const int (*int_ptr) (void); /* { dg-warning "qualifiers" "int ptr" } */
const int int_fn2 (void) { return 0; } /* { dg-warning "qualifiers" "int defn" } */

const void void_fn (void); /* { dg-warning "qualifiers" "void decl" } */
const void (*void_ptr) (void); /* { dg-warning "qualifiers" "void ptr" } */
const void void_fn2 (void) { } /* { dg-warning "qualifiers" "void defn" } */

/* "volatile void" is a GNU extension, so only warn at -pedantic.  */

volatile void vvoid_fn (void);
volatile void (*vvoid_ptr) (void);
volatile void vvoid_fn2 (void) { }

int *restrict ip_fn (void); /* { dg-warning "qualifiers" "restrict decl" } */
int *restrict (*ip_ptr) (void); /* { dg-warning "qualifiers" "restrict ptr" } */
int *restrict ip_fn2 (void) { return (int *)0; }; /* { dg-warning "qualifiers" "restrict defn" } */