Structure Compatibility

[Vijay Kumar R Zanvar]

I have the following question:

1. According to 6.2.7P1 of Std, are the
following structures compatible?

(Assume you compile with proper options)

/* compatible.c */
#include <stdio.h>

struct A
{
int a;
char b;
};

int
main ( void )
{
struct A a = { 5, 'a' };
extern struct A b;
b = a;
printf ( "%d %c\n", b.a, b.b );
return 0;
}

/* compatible2.c */
struct A
{
int a;
char b;
};

Thanks

 

[Mark A. Odell]

I have the following question:

1. According to 6.2.7P1 of Std, are the
following structures compatible?

(Assume you compile with proper options)

/* compatible.c */
#include <stdio.h>

struct A
{
int a;
char b;
};

int
main ( void )
{
struct A a = { 5, 'a' };
extern struct A b;

Wouldn't this be a name space colision? I don't understand your use of
"compatible".

 

[Sheldon Simms]

/* compatible.c */
#include <stdio.h>

struct A
{
int a;
char b;
};

int
main ( void )
{
struct A a = { 5, 'a' };
extern struct A b;

This means "declare b as a variable with external linkage of type
struct A". The struct A being referred to here is the struct A at
the beginning of file compatible.c

What you are specifying with the storage class specifier here is
that b is not a local variable in main. You are telling the
compiler to use some other b. Since there is no "other" b
defined in this code, it won't compile.

In the Standard, 6.2.2 says:
4 For an identifier declared with the storage-class specifier
extern ... If no prior declaration is visible ... then the
identifier has external linkage.